The Angular Router enables navigation from one view to the next as users perform application tasks.

This guide covers the router's primary features, illustrating them through the evolution of a small application that you can run live in the browser.

To see the URL changes in the browser address bar of the live example, open it again in the Plunker editor by clicking the icon in the upper right, then pop out the preview window by clicking the blue 'X' button in the upper right corner.

Overview

The browser is a familiar model of application navigation:

• Enter a URL in the address bar and the browser navigates to a corresponding page.
• Click links on the page and the browser navigates to a new page.
• Click the browser's back and forward buttons and the browser navigates backward and forward through the history of pages you've seen.

The Angular Router ("the router") borrows from this model. It can interpret a browser URL as an instruction to navigate to a client-generated view. It can pass optional parameters along to the supporting view component that help it decide what specific content to present. You can bind the router to links on a page and it will navigate to the appropriate application view when the user clicks a link. You can navigate imperatively when the user clicks a button, selects from a drop box, or in response to some other stimulus from any source. And the router logs activity in the browser's history journal so the back and forward buttons work as well.

Contents

• The Basics
  • <base href>
  • Router imports
  • Configuration
  • Router outlet
  • Router links
  • Router state
  • Summary
• The sample application
• Milestone 1: Getting started with the router
  • Setting the base href
  • Importing from the router library
  • Define routes
  • The AppComponent shell
  • RouterOutlet
  • RouterLink binding
  • RouterLinkActive binding
  • Wildcard route
  • The default route to heroes
• Milestone 2: Routing module
  • Refactor the routing configuration into a routing module
  • Do you need a Routing Module?
• Milestone 3: Heroes feature
  • Add heroes functionality
  • Hero feature routing requirements
  • Hero feature route configuration
  • Add the routing module to the HeroesModule
  • Remove duplicate hero routes
  • Import hero module into AppModule
  • Module import order matters
  • Route Definition with a parameter
  • Navigate to hero detail imperatively
  • Setting the route parameters in the list view
  • ActivatedRoute: the one-stop-shop for route information
  • Observable params and component reuse
  • Snapshot: the no-observable alternative
  • Navigating back to the list component
  • Route Parameters: Required or optional?
  • Heroes list: optionally selecting a hero
  • Route parameters in the ActivatedRoute service
  • Adding animations to the routed component
  • Milestone 3 wrap up
• Milestone 4: Crisis center feature
  • A crisis center with child routes
  • Child routing component
  • Child route configuration
  • Import crisis center module into the AppModule routes
  • Relative navigation
  • Navigate to crisis detail with a relative URL
  • Displaying multiple routes in named outlets
  • Secondary routes
  • Add a secondary route
  • Secondary route navigation: merging routes during navigation
  • Clearing secondary routes
• Milestone 5: Route guards
  • CanActivate: requiring authentication
  • Component-less route: grouping routes without a component
  • Guard the admin feature
  • Teach AuthGuard to authenticate
  • Add the login component
  • CanActivateChild: guarding child routes
  • CanDeactivate: handling unsaved changes
  • Cancel and save
  • Resolve: pre-fetching component data
  • Fetch data before navigating
  • Query parameters and fragments
• Milestone 6: Asynchronous routing
  • Lazy loading route configuration
  • CanLoad Guard: guarding unauthorized loading of feature modules
  • Preloading: background loading of feature areas
  • How preloading works
  • Lazy load the crisis center
  • CanLoad blocks preload
  • Custom Preloading Strategy
  • Inspect the router's configuration
  • Wrap up and final app
• Appendices
  • Appendix: link parameters array
  • Appendix: LocationStrategy and browser URL styles

The Basics

This guide proceeds in phases, marked by milestones, starting from a simple two-pager and building toward a modular, multi-view design with child routes.

An introduction to a few core router concepts will help orient you to the details that follow.

<base href>

Most routing applications should add a <base> element to the index.html as the first child in the <head> tag to tell the router how to compose navigation URLs.

If the app folder is the application root, as it is for the sample application, set the href value exactly as shown here.

Router imports

The Angular Router is an optional service that presents a particular component view for a given URL. It is not part of the Angular core. It is in its own library package, @angular/router. Import what you need from it as you would from any other Angular package.

Configuration

A routed Angular application has one singleton instance of the Router service. When the browser's URL changes, that router looks for a corresponding Route from which it can determine the component to display.

A router has no routes until you configure it. The following example creates four route definitions, configures the router via the RouterModule.forRoot method, and adds the result to the AppModule's imports array.

The appRoutes array of routes describes how to navigate. Pass it to the RouterModule.forRoot method in the module imports to configure the router.

Each Route maps a URL path to a component. There are no leading slashes in the path. The router parses and builds the final URL for you, allowing you to use both relative and absolute paths when navigating between application views.

The :id in the first route is a token for a route parameter. In a URL such as /hero/42, "42" is the value of the id parameter. The corresponding HeroDetailComponent will use that value to find and present the hero whose id is 42. You'll learn more about route parameters later in this guide.

The data property in the third route is a place to store arbitrary data associated with this specific route. The data property is accessible within each activated route. Use it to store items such as page titles, breadcrumb text, and other read-only, static data. You'll use the resolve guard to retrieve dynamic data later in the guide.

The empty path in the fourth route represents the default path for the application, the place to go when the path in the URL is empty, as it typically is at the start. This default route redirects to the route for the /heroes URL and, therefore, will display the HeroesListComponent.

The ** path in the last route is a wildcard. The router will select this route if the requested URL doesn't match any paths for routes defined earlier in the configuration. This is useful for displaying a "404 - Not Found" page or redirecting to another route.

The order of the routes in the configuration matters and this is by design. The router uses a first-match wins strategy when matching routes, so more specific routes should be placed above less specific routes. In the configuration above, routes with a static path are listed first, followed by an empty path route, that matches the default route. The wildcard route comes last because it matches every URL and should be selected only if no other routes are matched first.

Router outlet

Given this configuration, when the browser URL for this application becomes /heroes, the router matches that URL to the route path /heroes and displays the HeroListComponent after a RouterOutlet that you've placed in the host view's HTML.

Router links

Now you have routes configured and a place to render them, but how do you navigate? The URL could arrive directly from the browser address bar. But most of the time you navigate as a result of some user action such as the click of an anchor tag.

Consider the following template:

The RouterLink directives on the anchor tags give the router control over those elements. The navigation paths are fixed, so you can assign a string to the routerLink (a "one-time" binding).

Had the navigation path been more dynamic, you could have bound to a template expression that returned an array of route link parameters (the link parameters array). The router resolves that array into a complete URL.

The RouterLinkActive directive on each anchor tag helps visually distinguish the anchor for the currently selected "active" route. The router adds the active CSS class to the element when the associated RouterLink becomes active. You can add this directive to the anchor or to its parent element.

Router state

After the end of each successful navigation lifecycle, the router builds a tree of ActivatedRoute objects that make up the current state of the router. You can access the current RouterState from anywhere in the application using the Router service and the routerState property.

Each ActivatedRoute in the RouterState provides methods to traverse up and down the route tree to get information from parent, child and sibling routes.

Summary

The application has a configured router. The shell component has a RouterOutlet where it can display views produced by the router. It has RouterLinks that users can click to navigate via the router.

Here are the key Router terms and their meanings:

The sample application

This guide describes development of a multi-page routed sample application. Along the way, it highlights design decisions and describes key features of the router such as:

• Organizing the application features into modules.
• Navigating to a component (Heroes link to "Heroes List").
• Including a route parameter (passing the Hero id while routing to the "Hero Detail").
• Child routes (the Crisis Center has its own routes).
• The CanActivate guard (checking route access).
• The CanActivateChild guard (checking child route access).
• The CanDeactivate guard (ask permission to discard unsaved changes).
• The Resolve guard (pre-fetching route data).
• Lazy loading feature modules.
• The CanLoad guard (check before loading feature module assets).

The guide proceeds as a sequence of milestones as if you were building the app step-by-step. But, it is not a tutorial and it glosses over details of Angular application construction that are more thoroughly covered elsewhere in the documentation.

The full source for the final version of the app can be seen and downloaded from the .

The sample application in action

Imagine an application that helps the Hero Employment Agency run its business. Heroes need work and the agency finds crises for them to solve.

The application has three main feature areas:

1. A Crisis Center for maintaining the list of crises for assignment to heroes.
2. A Heroes area for maintaining the list of heroes employed by the agency.
3. An Admin area to manage the list of crises and heroes.

Try it by clicking on this live example link.

Once the app warms up, you'll see a row of navigation buttons and the Heroes view with its list of heroes.

Select one hero and the app takes you to a hero editing screen.

Alter the name. Click the "Back" button and the app returns to the heroes list which displays the changed hero name. Notice that the name change took effect immediately.

Had you clicked the browser's back button instead of the "Back" button, the app would have returned you to the heroes list as well. Angular app navigation updates the browser history as normal web navigation does.

Now click the Crisis Center link for a list of ongoing crises.

Select a crisis and the application takes you to a crisis editing screen. The Crisis Detail appears in a child view on the same page, beneath the list.

Alter the name of a crisis. Notice that the corresponding name in the crisis list does not change.

Unlike Hero Detail, which updates as you type, Crisis Detail changes are temporary until you either save or discard them by pressing the "Save" or "Cancel" buttons. Both buttons navigate back to the Crisis Center and its list of crises.

Do not click either button yet. Click the browser back button or the "Heroes" link instead.

Up pops a dialog box.

You can say "OK" and lose your changes or click "Cancel" and continue editing.

Behind this behavior is the router's CanDeactivate guard. The guard gives you a chance to clean-up or ask the user's permission before navigating away from the current view.

The Admin and Login buttons illustrate other router capabilities to be covered later in the guide. This short introduction will do for now.

Proceed to the first application milestone.

Milestone 1: Getting started with the router

Begin with a simple version of the app that navigates between two empty views.

Set the <base href>

The router uses the browser's history.pushState for navigation. Thanks to pushState, you can make in-app URL paths look the way you want them to look, e.g. localhost:3000/crisis-center. The in-app URLs can be indistinguishable from server URLs.

Modern HTML5 browsers were the first to support pushState which is why many people refer to these URLs as "HTML5 style" URLs.

You must add a <base href> element to the app's index.html for pushState routing to work. The browser uses the <base href> value to prefix relative URLs when referencing CSS files, scripts, and images.

Add the <base> element just after the <head> tag. If the app folder is the application root, as it is for this application, set the href value in index.html exactly as shown here.

Importing from the router library

Begin by importing some symbols from the router library. The Router is in its own @angular/router package. It's not part of the Angular core. The router is an optional service because not all applications need routing and, depending on your requirements, you may need a different routing library.

You teach the router how to navigate by configuring it with routes.

Define routes

A router must be configured with a list of route definitions.

The first configuration defines an array of two routes with simple paths leading to the CrisisListComponent and HeroListComponent.

Each definition translates to a Route object which has two things: a path, the URL path segment for this route; and a component, the component associated with this route.

The router draws upon its registry of definitions when the browser URL changes or when application code tells the router to navigate along a route path.

In simpler terms, you might say this of the first route:

• When the browser's location URL changes to match the path segment /crisis-center, then the router activates an instance of the CrisisListComponent and displays its view.

• When the application requests navigation to the path /crisis-center, the router activates an instance of CrisisListComponent, displays its view, and updates the browser's address location and history with the URL for that path.

Here is the first configuration. Pass the array of routes, appRoutes, to the RouterModule.forRoot method. It returns a module, containing the configured Router service provider, plus other providers that the routing library requires. Once the application is bootstrapped, the Router performs the initial navigation based on the current browser URL.

Providing the RouterModule in the AppModule makes the Router available everywhere in the application.

The AppComponent shell

The root AppComponent is the application shell. It has a title, a navigation bar with two links, and a router outlet where the router swaps views on and off the page. Here's what you get:

The corresponding component template looks like this:

RouterOutlet

The RouterOutlet is a directive from the router library that marks the spot in the template where the router should display the views for that outlet.

RouterLink binding

Above the outlet, within the anchor tags, you see attribute bindings to the RouterLink directive that look like routerLink="...".

The links in this example each have a string path, the path of a route that you configured earlier. There are no route parameters yet.

You can also add more contextual information to the RouterLink by providing query string parameters or a URL fragment for jumping to different areas on the page. Query string parameters are provided through the [queryParams] binding which takes an object (e.g. { name: 'value' }), while the URL fragment takes a single value bound to the [fragment] input binding.

RouterLinkActive binding

On each anchor tag, you also see property bindings to the RouterLinkActive directive that look like routerLinkActive="...".

The template expression to the right of the equals (=) contains a space-delimited string of CSS classes that the Router will add when this link is active (and remove when the link is inactive). You can also set the RouterLinkActive directive to a string of classes such as [routerLinkActive]="active fluffy" or bind it to a component property that returns such a string.

The RouterLinkActive directive toggles css classes for active RouterLinks based on the current RouterState. This cascades down through each level of the route tree, so parent and child router links can be active at the same time. To override this behavior, you can bind to the [routerLinkActiveOptions] input binding with the { exact: true } expression. By using { exact: true }, a given RouterLink will only be active if its URL is an exact match to the current URL.

Router directives

RouterLink, RouterLinkActive and RouterOutlet are directives provided by the Angular RouterModule package. They are readily available for you to use in the template.

The current state of app.component.ts looks like this:

Wildcard route

You've created two routes in the app so far, one to /crisis-center and the other to /heroes. Any other URL causes the router to throw an error and crash the app.

Add a wildcard route to intercept invalid URLs and handle them gracefully. A wildcard route has a path consisting of two asterisks. It matches every URL. The router will select this route if it can't match a route earlier in the configuration. A wildcard route can navigate to a custom "404 Not Found" component or redirect to an existing route.

To test this feature, add a button with a RouterLink to the HeroListComponent template and set the link to "/sidekicks".

The application will fail if the user clicks that button because you haven't defined a "/sidekicks" route yet.

Instead of adding the "/sidekicks" route, define a wildcard route instead and have it navigate to a simple PageNotFoundComponent.

Create the PageNotFoundComponent to display when users visit invalid URLs.

As with the other components, add the PageNotFoundComponent to the AppModule declarations.

Now when the user visits /sidekicks, or any other invalid URL, the browser displays "Page not found". The browser address bar continues to point to the invalid URL.

The default route to heroes

When the application launches, the initial URL in the browser bar is something like:

That doesn't match any of the configured routes which means that the application won't display any component when it's launched. The user must click one of the links to trigger a navigation and display a component.

It would be nicer if the application had a default route that displayed the list of heroes immediately, just as it will when the user clicks the "Heroes" link or pastes localhost:3000/heroes into the address bar.

Redirecting routes

The preferred solution is to add a redirect route that translates the initial relative URL ('') to the desired default path (/heroes). The browser address bar shows .../heroes as if you'd navigated there directly.

Add the default route somewhere above the wildcard route. It's just above the wildcard route in the following excerpt showing the complete appRoutes for this milestone.

A redirect route requires a pathMatch property to tell the router how to match a URL to the path of a route. The router throws an error if you don't. In this app, the router should select the route to the HeroListComponent only when the entire URL matches '', so set the pathMatch value to 'full'.

Basics wrap up

You've got a very basic navigating app, one that can switch between two views when the user clicks a link.

You've learned how to do the following:

• Load the router library.
• Add a nav bar to the shell template with anchor tags, routerLink and routerLinkActive directives.
• Add a router-outlet to the shell template where views will be displayed.
• Configure the router module with RouterModule.forRoot.
• Set the router to compose HTML5 browser URLs.
• handle invalid routes with a wildcard route.
• navigate to the default route when the app launches with an empty path.

The rest of the starter app is mundane, with little interest from a router perspective. Here are the details for readers inclined to build the sample through to this milestone.

The starter app's structure looks like this:

Here are the files discussed in this milestone.

Milestone 2: Routing module

In the initial route configuration, you provided a simple setup with two routes used to configure the application for routing. This is perfectly fine for simple routing. As the application grows and you make use of more Router features, such as guards, resolvers, and child routing, you'll naturally want to refactor the routing configuration into its own file. We recommend moving the routing information into a special-purpose module called a Routing Module.

The Routing Module has several characteristics:

• Separates routing concerns from other application concerns.
• Provides a module to replace or remove when testing the application.
• Provides a well-known location for routing service providers including guards and resolvers.
• Does not declare components.

Refactor the routing configuration into a routing module

Create a file named app-routing.module.ts in the /app folder to contain the routing module.

Import the CrisisListComponent and the HeroListComponent components just like you did in the app.module.ts. Then move the Router imports and routing configuration, including RouterModule.forRoot, into this routing module.

Following convention, add a class name AppRoutingModule and export it so you can import it later in AppModule.

Finally, re-export the Angular RouterModule by adding it to the module exports array. By re-exporting the RouterModule here and importing AppRoutingModule in AppModule, the components declared in AppModule will have access to router directives such as RouterLink and RouterOutlet.

After these steps, the file should look like this.

Next, update the app.module.ts file, first importing the newly created AppRoutingModulefrom app-routing.module.ts, then replacing RouterModule.forRoot in the imports array with the AppRoutingModule.

The application continues to work just the same, and you can use AppRoutingModule as the central place to maintain future routing configuration.

Do you need a Routing Module?

The Routing Module replaces the routing configuration in the root or feature module. Either configure routes in the Routing Module or within the module itself but not in both.

The Routing Module is a design choice whose value is most obvious when the configuration is complex and includes specialized guard and resolver services. It can seem like overkill when the actual configuration is dead simple.

Some developers skip the Routing Module (for example, AppRoutingModule) when the configuration is simple and merge the routing configuration directly into the companion module (for example, AppModule).

Choose one pattern or the other and follow that pattern consistently.

Most developers should always implement a Routing Module for the sake of consistency. It keeps the code clean when configuration becomes complex. It makes testing the feature module easier. Its existence calls attention to the fact that a module is routed. It is where developers expect to find and expand routing configuration.

Milestone 3: Heroes feature

You've seen how to navigate using the RouterLink directive. Now you'll learn the following:

• Organize the app and routes into feature areas using modules.
• Navigate imperatively from one component to another.
• Pass required and optional information in route parameters.

This example recreates the heroes feature in the "Services" episode of the Tour of Heroes tutorial, and you'll be copying much of the code from the .

Here's how the user will experience this version of the app:

A typical application has multiple feature areas, each dedicated to a particular business purpose.

While you could continue to add files to the src/app/ folder, that is unrealistic and ultimately not maintainable. Most developers prefer to put each feature area in its own folder.

You are about to break up the app into different feature modules, each with its own concerns. Then you'll import into the main module and navigate among them.

Add heroes functionality

Follow these steps:

• Create the src/app/heroes folder; you'll be adding files implementing hero management there.
• Delete the placeholder hero-list.component.ts that's in the app folder.
• Create a new hero-list.component.ts under src/app/heroes.
• Copy into it the contents of the app.component.ts from the "Services" tutorial.
• Make a few minor but necessary changes:
  • Delete the selector (routed components don't need them).
  • Delete the <h1>.
  • Relabel the <h2> to <h2>HEROES</h2>.
  • Delete the <hero-detail> at the bottom of the template.
  • Rename the AppComponent class to HeroListComponent.
• Copy the hero-detail.component.ts and the hero.service.ts files into the heroes subfolder.
• Create a (pre-routing) heroes.module.ts in the heroes folder that looks like this:

When you're done, you'll have these hero management files:

Hero feature routing requirements

The heroes feature has two interacting components, the hero list and the hero detail. The list view is self-sufficient; you navigate to it, it gets a list of heroes and displays them.

The detail view is different. It displays a particular hero. It can't know which hero to show on its own. That information must come from outside.

When the user selects a hero from the list, the app should navigate to the detail view and show that hero. You tell the detail view which hero to display by including the selected hero's id in the route URL.

Hero feature route configuration

Create a new heroes-routing.module.ts in the heroes folder using the same techniques you learned while creating the AppRoutingModule.

Import the hero components from their new locations in the src/app/heroes/ folder, define the two hero routes, and export the HeroRoutingModule class.

Now that you have routes for the Heroes module, register them with the Router via the RouterModule almost as you did in the AppRoutingModule.

There is a small but critical difference. In the AppRoutingModule, you used the static RouterModule.forRoot method to register the routes and application level service providers. In a feature module you use the static forChild method.

Add the routing module to the HeroesModule

Add the HeroRoutingModule to the HeroModule just as you added AppRoutingModule to the AppModule.

Open heroes.module.ts. Import the HeroRoutingModule token from heroes-routing.module.ts and add it to the imports array of the HeroesModule. The finished HeroesModule looks like this:

Remove duplicate hero routes

The hero routes are currently defined in two places: in the HeroesRoutingModule, by way of the HeroesModule, and in the AppRoutingModule.

Routes provided by feature modules are combined together into their imported module's routes by the router. This allows you to continue defining the feature module routes without modifying the main route configuration.

But you don't want to define the same routes twice. Remove the HeroListComponent import and the /heroes route from the app-routing.module.ts.

Leave the default and the wildcard routes! These are concerns at the top level of the application itself.

Import hero module into AppModule

The heroes feature module is ready, but the application doesn't know about the HeroesModule yet. Open app.module.ts and revise it as follows.

Import the HeroesModule and add it to the imports array in the @NgModule metadata of the AppModule.

Remove the HeroListComponent from the AppModule's declarations because it's now provided by the HeroesModule. This is important. There can be only one owner for a declared component. In this case, the Heroes module is the owner of the Heroes components and is making them available to components in the AppModule via the HeroesModule.

As a result, the AppModule no longer has specific knowledge of the hero feature, its components, or its route details. You can evolve the hero feature with more components and different routes. That's a key benefit of creating a separate module for each feature area.

After these steps, the AppModule should look like this:

Module import order matters

Look at the module imports array. Notice that the AppRoutingModule is last. Most importantly, it comes after the HeroesModule.

The order of route configuration matters. The router accepts the first route that matches a navigation request path.

When all routes were in one AppRoutingModule, you put the default and wildcard routes last, after the /heroes route, so that the router had a chance to match a URL to the /heroes route before hitting the wildcard route and navigating to "Page not found".

The routes are no longer in one file. They are distributed across two modules, AppRoutingModule and HeroesRoutingModule.

Each routing module augments the route configuration in the order of import. If you list AppRoutingModule first, the wildcard route will be registered before the hero routes. The wildcard route — which matches every URL — will intercept the attempt to navigate to a hero route.

Route definition with a parameter

Return to the HeroesRoutingModule and look at the route definitions again. The route to HeroDetailComponent has a twist.

Notice the :id token in the path. That creates a slot in the path for a Route Parameter. In this case, the router will insert the id of a hero into that slot.

If you tell the router to navigate to the detail component and display "Magneta", you expect a hero id to appear in the browser URL like this:

If a user enters that URL into the browser address bar, the router should recognize the pattern and go to the same "Magneta" detail view.

Navigate to hero detail imperatively

Users will not navigate to the detail component by clicking a link so you won't add a new RouterLink anchor tag to the shell.

Instead, when the user clicks a hero in the list, you'll ask the router to navigate to the hero detail view for the selected hero.

Start in the HeroListComponent. Revise its constructor so that it acquires the Router and the HeroService by dependency injection:

Make the following few changes to the component's template:

The template defines an *ngFor repeater such as you've seen before. There's a (click) event binding to the component's onSelect method which you implement as follows:

The component's onSelect calls the router's navigate method with a link parameters array. You can use this same syntax in a RouterLink if you decide later to navigate in HTML template rather than in component code.

Setting the route parameters in the list view

After navigating to the HeroDetailComponent, you expect to see the details of the selected hero. You need two pieces of information: the routing path to the component and the hero's id.

Accordingly, the link parameters array has two items: the routing path and a route parameter that specifies the id of the selected hero.

The router composes the destination URL from the array like this: localhost:3000/hero/15.

ActivatedRoute: the one-stop-shop for route information

The route path and parameters are available through an injected router service called the ActivatedRoute. It has a great deal of useful information including:

Import the Router, ActivatedRoute, and Params tokens from the router package.

Import the switchMap operator because you need it later to process the Observable route parameters.

As usual, you write a constructor that asks Angular to inject services that the component requires and reference them as private variables.

Later, in the ngOnInit method, you use the ActivatedRoute service to retrieve the parameters for the route, pull the hero id from the parameters and retrieve the hero to display.

Since the parameters are provided as an Observable, you use the switchMap operator to provide them for the id parameter by name and tell the HeroService to fetch the hero with that id.

The switchMap operator allows you to perform an action with the current value of the Observable, and map it to a new Observable. As with many rxjs operators, switchMap handles an Observable as well as a Promise to retrieve the value they emit.

The switchMap operator will also cancel any in-flight requests if the user re-navigates to the route while still retrieving a hero.

Use the subscribe method to detect id changes and to (re)set the retrieved Hero.

Observable params and component reuse

In this example, you retrieve the route params from an Observable. That implies that the route params can change during the lifetime of this component.

They might. By default, the router re-uses a component instance when it re-navigates to the same component type without visiting a different component first. The route parameters could change each time.

Suppose a parent component navigation bar had "forward" and "back" buttons that scrolled through the list of heroes. Each click navigated imperatively to the HeroDetailComponent with the next or previous id.

You don't want the router to remove the current HeroDetailComponent instance from the DOM only to re-create it for the next id. That could be visibly jarring. Better to simply re-use the same component instance and update the parameter.

Unfortunately, ngOnInit is only called once per component instantiation. You need a way to detect when the route parameters change from within the same instance. The observable params property handles that beautifully.

Snapshot: the no-observable alternative

This application won't re-use the HeroDetailComponent. The user always returns to the hero list to select another hero to view. There's no way to navigate from one hero detail to another hero detail without visiting the list component in between. Therefore, the router creates a new HeroDetailComponent instance every time.

When you know for certain that a HeroDetailComponent instance will never, never, ever be re-used, you can simplify the code with the snapshot.

The route.snapshot provides the initial value of the route parameters. You can access the parameters directly without subscribing or adding observable operators. It's much simpler to write and read:

Navigating back to the list component

The HeroDetailComponent has a "Back" button wired to its gotoHeroes method that navigates imperatively back to the HeroListComponent.

The router navigate method takes the same one-item link parameters array that you can bind to a [routerLink] directive. It holds the path to the HeroListComponent:

Route Parameters: Required or optional?

Use route parameters to specify a required parameter value within the route URL as you do when navigating to the HeroDetailComponent in order to view the hero with id 15:

You can also add optional information to a route request. For example, when returning to the heroes list from the hero detail view, it would be nice if the viewed hero was preselected in the list.

You'll implement this feature in a moment by including the viewed hero's id in the URL as an optional parameter when returning from the HeroDetailComponent.

Optional information takes other forms. Search criteria are often loosely structured, e.g., name='wind*'. Multiple values are common—after='12/31/2015' & before='1/1/2017'—in no particular order—before='1/1/2017' & after='12/31/2015'— in a variety of formats—during='currentYear'.

These kinds of parameters don't fit easily in a URL path. Even if you could define a suitable URL token scheme, doing so greatly complicates the pattern matching required to translate an incoming URL to a named route.

Optional parameters are the ideal vehicle for conveying arbitrarily complex information during navigation. Optional parameters aren't involved in pattern matching and afford flexibility of expression.

The router supports navigation with optional parameters as well as required route parameters. Define optional parameters in a separate object after you define the required route parameters.

In general, prefer a required route parameter when the value is mandatory (for example, if necessary to distinguish one route path from another); prefer an optional parameter when the value is optional, complex, and/or multivariate.

Heroes list: optionally selecting a hero

When navigating to the HeroDetailComponent you specified the required id of the hero-to-edit in the route parameter and made it the second item of the link parameters array.

The router embedded the id value in the navigation URL because you had defined it as a route parameter with an :id placeholder token in the route path:

When the user clicks the back button, the HeroDetailComponent constructs another link parameters array which it uses to navigate back to the HeroListComponent.

This array lacks a route parameter because you had no reason to send information to the HeroListComponent.

Now you have a reason. You'd like to send the id of the current hero with the navigation request so that the HeroListComponent can highlight that hero in its list. This is a nice-to-have feature; the list will display perfectly well without it.

Send the id with an object that contains an optional id parameter. For demonstration purposes, there's an extra junk parameter (foo) in the object that the HeroListComponent should ignore. Here's the revised navigation statement:

The application still works. Clicking "back" returns to the hero list view.

Look at the browser address bar.

To see the URL changes in the browser address bar of the live example, open it again in the Plunker editor by clicking the icon in the upper right, then pop out the preview window by clicking the blue 'X' button in the upper right corner.

It should look something like this, depending on where you run it:

The id value appears in the URL as (;id=15;foo=foo), not in the URL path. The path for the "Heroes" route doesn't have an :id token.

The optional route parameters are not separated by "?" and "&" as they would be in the URL query string. They are separated by semicolons ";" This is matrix URL notation — something you may not have seen before.

Route parameters in the ActivatedRoute service

The list of heroes is unchanged. No hero row is highlighted.

The HeroListComponent isn't expecting any parameters at all and wouldn't know what to do with them. You can change that.

Previously, when navigating from the HeroListComponent to the HeroDetailComponent, you subscribed to the route params Observable and made it available to the HeroDetailComponent in the ActivatedRoute service. You injected that service in the constructor of the HeroDetailComponent.

This time you'll be navigating in the opposite direction, from the HeroDetailComponent to the HeroListComponent.

First you extend the router import statement to include the ActivatedRoute service symbol:

Import the switchMap operator to perform an operation on the Observable of route parameters.

Then you inject the ActivatedRoute in the HeroListComponent constructor.

The ActivatedRoute.params property is an Observable of route parameters. The params emits new id values when the user navigates to the component. In ngOnInit you subscribe to those values, set the selectedId, and get the heroes.

Add an isSelected method that returns true when a hero's id matches the selected id.

Finally, update the template with a class binding to that isSelected method. The binding adds the selected CSS class when the method returns true and removes it when false. Look for it within the repeated <li> tag as shown here:

When the user navigates from the heroes list to the "Magneta" hero and back, "Magneta" appears selected:

The optional foo route parameter is harmless and continues to be ignored.

Adding animations to the routed component

The heroes feature module is almost complete, but what is a feature without some smooth transitions?

This section shows you how to add some animations to the HeroDetailComponent.

Create an animations.ts file in the root src/app/ folder. The contents look like this:

This file does the following:

• Imports the animation symbols that build the animation triggers, control state, and manage transitions between states.

• Exports a constant named slideInDownAnimation set to an animation trigger named routeAnimation; animated components will refer to this name.

• Specifies the wildcard state , *, that matches any animation state that the route component is in.

• Defines two transitions, one to ease the component in from the left of the screen as it enters the application view (:enter), the other to animate the component down as it leaves the application view (:leave).

You could create more triggers with different transitions for other route components. This trigger is sufficient for the current milestone.

Back in the HeroDetailComponent, import the slideInDownAnimation from './animations.ts. Add the HostBinding decorator to the imports from @angular/core; you'll need it in a moment.

Add an animations array to the @Component metadata's that contains the slideInDownAnimation.

Then add three @HostBinding properties to the class to set the animation and styles for the route component's element.

The '@routeAnimation' passed to the first @HostBinding matches the name of the slideInDownAnimation trigger, routeAnimation. Set the routeAnimation property to true because you only care about the :enter and :leave states.

The other two @HostBinding properties style the display and position of the component.

The HeroDetailComponent will ease in from the left when routed to and will slide down when navigating away.

Milestone 3 wrap up

You've learned how to do the following:

• Organize the app into feature areas.
• Navigate imperatively from one component to another.
• Pass information along in route parameters and subscribe to them in the component.
• Import the feature area NgModule into the AppModule.
• Apply animations to the route component.

After these changes, the folder structure looks like this:

Milestone 4: Crisis center feature

It's time to add real features to the app's current placeholder crisis center.

Begin by imitating the heroes feature:

• Delete the placeholder crisis center file.
• Create an app/crisis-center folder.
• Copy the files from app/heroes into the new crisis center folder.
• In the new files, change every mention of "hero" to "crisis", and "heroes" to "crises".

You'll turn the CrisisService into a purveyor of mock crises instead of mock heroes:

The resulting crisis center is a foundation for introducing a new concept—child routing. You can leave Heroes in its current state as a contrast with the Crisis Center and decide later if the differences are worthwhile.

A crisis center with child routes

This section shows you how to organize the crisis center to conform to the following recommended pattern for Angular applications:

• Each feature area resides in its own folder.
• Each feature has its own Angular feature module.
• Each area has its own area root component.
• Each area root component has its own router outlet and child routes.
• Feature area routes rarely (if ever) cross with routes of other features.

If your app had many feature areas, the app component trees might look like this:

Child routing component

Add the following crisis-center.component.ts to the crisis-center folder:

The CrisisCenterComponent has the following in common with the AppComponent:

• It is the root of the crisis center area, just as AppComponent is the root of the entire application.
• It is a shell for the crisis management feature area, just as the AppComponent is a shell to manage the high-level workflow.

Like most shells, the CrisisCenterComponent class is very simple, simpler even than AppComponent: it has no business logic, and its template has no links, just a title and <router-outlet> for the crisis center child views.

Unlike AppComponent, and most other components, it lacks a selector. It doesn't need one since you don't embed this component in a parent template, instead you use the router to navigate to it.

Child route configuration

The CrisisCenterComponent is a routing component like the AppComponent. It has its own RouterOutlet and its own child routes.

Add the following crisis-center-home.component.ts to the crisis-center folder.

Create a crisis-center-routing.module.ts file as you did the heroes-routing.module.ts file. This time, you define child routes within the parent crisis-center route.

Notice that the parent crisis-center route has a children property with a single route containing the CrisisListComponent. The CrisisListComponent route also has a children array with two routes.

These two routes navigate to the crisis center child components, CrisisCenterHomeComponent and CrisisDetailComponent, respectively.

There are important differences in the way the router treats these child routes.

The router displays the components of these routes in the RouterOutlet of the CrisisCenterComponent, not in the RouterOutlet of the AppComponent shell.

The CrisisListComponent contains the crisis list and a RouterOutlet to display the Crisis Center Home and Crisis Detail route components.

The Crisis Detail route is a child of the Crisis List. Since the router reuses components by default, the Crisis Detail component will be re-used as you select different crises. In contrast, back in the Hero Detail route, the component was recreated each time you selected a different hero.

At the top level, paths that begin with / refer to the root of the application. But child routes extend the path of the parent route. With each step down the route tree, you add a slash followed by the route path, unless the path is empty.

Apply that logic to navigation within the crisis center for which the parent path is /crisis-center.

• To navigate to the CrisisCenterHomeComponent, the full URL is /crisis-center (/crisis-center + '' + '').

• To navigate to the CrisisDetailComponent for a crisis with id=2, the full URL is /crisis-center/2 (/crisis-center + '' + '/2').

The absolute URL for the latter example, including the localhost origin, is

Here's the complete crisis-center-routing.module.ts file with its imports.

Import crisis center module into the AppModule routes

As with the HeroesModule, you must add the CrisisCenterModule to the imports array of the AppModule before the AppRoutingModule:

Remove the initial crisis center route from the app-routing.module.ts. The feature routes are now provided by the HeroesModule and the CrisisCenter modules.

The app-routing.module.ts file retains the top-level application routes such as the default and wildcard routes.

Relative navigation

While building out the crisis center feature, you navigated to the crisis detail route using an absolute path that begins with a slash.

The router matches such absolute paths to routes starting from the top of the route configuration.

You could continue to use absolute paths like this to navigate inside the Crisis Center feature, but that pins the links to the parent routing structure. If you changed the parent /crisis-center path, you would have to change the link parameters array.

You can free the links from this dependency by defining paths that are relative to the current URL segment. Navigation within the feature area remains intact even if you change the parent route path to the feature.

Here's an example:

To navigate a relative path with the Router.navigate method, you must supply the ActivatedRoute to give the router knowledge of where you are in the current route tree.

After the link parameters array, add an object with a relativeTo property set to the ActivatedRoute. The router then calculates the target URL based on the active route's location.

Navigate to crisis detail with a relative URL

Update the Crisis List onSelect method to use relative navigation so you don't have to start from the top of the route configuration.

You've already injected the ActivatedRoute that you need to compose the relative navigation path.

When you visit the Crisis Center, the ancestor path is /crisis-center, so you only need to add the id of the Crisis Center to the existing path.

If you were using a RouterLink to navigate instead of the Router service, you'd use the same link parameters array, but you wouldn't provide the object with the relativeTo property. The ActivatedRoute is implicit in a RouterLink directive.

Update the gotoCrises method of the CrisisDetailComponent to navigate back to the Crisis Center list using relative path navigation.

Notice that the path goes up a level using the ../ syntax. If the current crisis id is 3, the resulting path back to the crisis list is /crisis-center/;id=3;foo=foo.

Displaying multiple routes in named outlets

You decide to give users a way to contact the crisis center. When a user clicks a "Contact" button, you want to display a message in a popup view.

The popup should stay open, even when switching between pages in the application, until the user closes it by sending the message or canceling. Clearly you can't put the popup in the same outlet as the other pages.

Until now, you've defined a single outlet and you've nested child routes under that outlet to group routes together. The router only supports one primary unnamed outlet per template.

A template can also have any number of named outlets. Each named outlet has its own set of routes with their own components. Multiple outlets can be displaying different content, determined by different routes, all at the same time.

Add an outlet named "popup" in the AppComponent, directly below the unnamed outlet.

That's where a popup will go, once you learn how to route a popup component to it.

Secondary routes

Named outlets are the targets of secondary routes.

Secondary routes look like primary routes and you configure them the same way. They differ in a few key respects.

• They are independent of each other.
• They work in combination with other routes.
• They are displayed in named outlets.

Create a new component named ComposeMessageComponent in src/app/compose-message.component.ts. It displays a simple form with a header, an input box for the message, and two buttons, "Send" and "Cancel".

Here's the component and its template:

It looks about the same as any other component you've seen in this guide. There are two noteworthy differences.

Note that the send() method simulates latency by waiting a second before "sending" the message and closing the popup.

The closePopup() method closes the popup view by navigating to the popup outlet with a null. That's a peculiarity covered below.

As with other application components, you add the ComposeMessageComponent to the declarations of an NgModule. Do so in the AppModule.

Add a secondary route

Open the AppRoutingModule and add a new compose route to the appRoutes.

The path and component properties should be familiar. There's a new property, outlet, set to 'popup'. This route now targets the popup outlet and the ComposeMessageComponent will display there.

The user needs a way to open the popup. Open the AppComponent and add a "Contact" link.

Although the compose route is pinned to the "popup" outlet, that's not sufficient for wiring the route to a RouterLink directive. You have to specify the named outlet in a link parameters array and bind it to the RouterLink with a property binding.

The link parameters array contains an object with a single outlets property whose value is another object keyed by one (or more) outlet names. In this case there is only the "popup" outlet property and its value is another link parameters array that specifies the compose route.

You are in effect saying, when the user clicks this link, display the component associated with the compose route in the popup outlet.

Secondary route navigation: merging routes during navigation

Navigate to the Crisis Center and click "Contact". you should see something like the following URL in the browser address bar.

The interesting part of the URL follows the ...:

• The crisis-center is the primary navigation.
• Parentheses surround the secondary route.
• The secondary route consists of an outlet name (popup), a colon separator, and the secondary route path (compose).

Click the Heroes link and look at the URL again.

The primary navigation part has changed; the secondary route is the same.

The router is keeping track of two separate branches in a navigation tree and generating a representation of that tree in the URL.

You can add many more outlets and routes, at the top level and in nested levels, creating a navigation tree with many branches. The router will generate the URL to go with it.

You can tell the router to navigate an entire tree at once by filling out the outlets object mentioned above. Then pass that object inside a link parameters array to the router.navigate method.

Experiment with these possibilities at your leisure.

Clearing secondary routes

As you've learned, a component in an outlet persists until you navigate away to a new component. Secondary outlets are no different in this regard.

Each secondary outlet has its own navigation, independent of the navigation driving the primary outlet. Changing a current route that displays in the primary outlet has no effect on the popup outlet. That's why the popup stays visible as you navigate among the crises and heroes.

Clicking the "send" or "cancel" buttons does clear the popup view. To see how, look at the closePopup() method again:

It navigates imperatively with the Router.navigate() method, passing in a link parameters array.

Like the array bound to the Contact RouterLink in the AppComponent, this one includes an object with an outlets property. The outlets property value is another object with outlet names for keys. The only named outlet is 'popup'.

This time, the value of 'popup' is null. That's not a route, but it is a legitimate value. Setting the popup RouterOutlet to null clears the outlet and removes the secondary popup route from the current URL.

Milestone 5: Route guards

At the moment, any user can navigate anywhere in the application anytime. That's not always the right thing to do.

• Perhaps the user is not authorized to navigate to the target component.
• Maybe the user must login (authenticate) first.
• Maybe you should fetch some data before you display the target component.
• You might want to save pending changes before leaving a component.
• You might ask the user if it's OK to discard pending changes rather than save them.

You can add guards to the route configuration to handle these scenarios.

A guard's return value controls the router's behavior:

• If it returns true, the navigation process continues.
• If it returns false, the navigation process stops and the user stays put.

The guard might return its boolean answer synchronously. But in many cases, the guard can't produce an answer synchronously. The guard could ask the user a question, save changes to the server, or fetch fresh data. These are all asynchronous operations.

Accordingly, a routing guard can return an Observable<boolean> or a Promise<boolean> and the router will wait for the observable to resolve to true or false.

The router supports multiple kinds of guards:

1. CanActivate to mediate navigation to a route.

2. CanActivateChild() to mediate navigation to a child route.

3. CanDeactivate to mediate navigation away from the current route.

4. Resolve to perform route data retrieval before route activation.

5. CanLoad to mediate navigation to a feature module loaded asynchronously.

You can have multiple guards at every level of a routing hierarchy. The router checks the CanDeactivate() and CanActivateChild() guards first, from the deepest child route to the top. Then it checks the CanActivate() guards from the top down to the deepest child route. If the feature module is loaded asynchronously, the CanLoad() guard is checked before the module is loaded. If any guard returns false, pending guards that have not completed will be canceled, and the entire navigation is canceled.

There are several examples over the next few sections.

CanActivate: requiring authentication

Applications often restrict access to a feature area based on who the user is. You could permit access only to authenticated users or to users with a specific role. You might block or limit access until the user's account is activated.

The CanActivate guard is the tool to manage these navigation business rules.

Add an admin feature module

In this next section, you'll extend the crisis center with some new administrative features. Those features aren't defined yet. But you can start by adding a new feature module named AdminModule.

Create an admin folder with a feature module file, a routing configuration file, and supporting components.

The admin feature file structure looks like this:

The admin feature module contains the AdminComponent used for routing within the feature module, a dashboard route and two unfinished components to manage crises and heroes.

The initial admin routing configuration:

Component-less route: grouping routes without a component

Looking at the child route under the AdminComponent, there is a path and a children property but it's not using a component. You haven't made a mistake in the configuration. You've defined a component-less route.

The goal is to group the Crisis Center management routes under the admin path. You don't need a component to do it. A component-less route makes it easier to guard child routes.

Next, import the AdminModule into app.module.ts and add it to the imports array to register the admin routes.

Add an "Admin" link to the AppComponent shell so that users can get to this feature.

Guard the admin feature

Currently every route within the Crisis Center is open to everyone. The new admin feature should be accessible only to authenticated users.

You could hide the link until the user logs in. But that's tricky and difficult to maintain.

Instead you'll write a CanActivate() guard to redirect anonymous users to the login page when they try to enter the admin area.

This is a general purpose guard—you can imagine other features that require authenticated users—so you create an auth-guard.service.ts in the application root folder.

At the moment you're interested in seeing how guards work so the first version does nothing useful. It simply logs to console and returns true immediately, allowing navigation to proceed:

Next, open admin-routing.module.ts, import the AuthGuard class, and update the admin route with a CanActivate() guard property that references it:

The admin feature is now protected by the guard, albeit protected poorly.

Teach AuthGuard to authenticate

Make the AuthGuard at least pretend to authenticate.

The AuthGuard should call an application service that can login a user and retain information about the current user. Here's a demo AuthService:

Although it doesn't actually log in, it has what you need for this discussion. It has an isLoggedIn flag to tell you whether the user is authenticated. Its login method simulates an API call to an external service by returning an Observable that resolves successfully after a short pause. The redirectUrl property will store the attempted URL so you can navigate to it after authenticating.

Revise the AuthGuard to call it.

Notice that you inject the AuthService and the Router in the constructor. You haven't provided the AuthService yet but it's good to know that you can inject helpful services into routing guards.

This guard returns a synchronous boolean result. If the user is logged in, it returns true and the navigation continues.

The ActivatedRouteSnapshot contains the future route that will be activated and the RouterStateSnapshot contains the future RouterState of the application, should you pass through the guard check.

If the user is not logged in, you store the attempted URL the user came from using the RouterStateSnapshot.url and tell the router to navigate to a login page—a page you haven't created yet. This secondary navigation automatically cancels the current navigation; checkLogin() returns false just to be clear about that.

Add the LoginComponent

You need a LoginComponent for the user to log in to the app. After logging in, you'll redirect to the stored URL if available, or use the default URL. There is nothing new about this component or the way you wire it into the router configuration.

Register a /login route in the login-routing.module.ts and add the necessary providers to the providers array. In app.module.ts, import the LoginComponent and add it to the AppModule declarations. Import and add the LoginRoutingModule to the AppModule imports as well.

CanActivateChild: guarding child routes

You can also protect child routes with the CanActivateChild guard. The CanActivateChild guard is similar to the CanActivate guard. The key difference is that it runs before any child route is activated.

You protected the admin feature module from unauthorized access. You should also protect child routes within the feature module.

Extend the AuthGuard to protect when navigating between the admin routes. Open auth-guard.service.ts and add the CanActivateChild interface to the imported tokens from the router package.

Next, implement the CanActivateChild method which takes the same arguments as the CanActivate method: an ActivatedRouteSnapshot and RouterStateSnapshot. The CanActivateChild method can return an Observable<boolean> or Promise<boolean> for async checks and a boolean for sync checks. This one returns a boolean:

Add the same AuthGuard to the component-less admin route to protect all other child routes at one time instead of adding the AuthGuard to each route individually.

CanDeactivate: handling unsaved changes

Back in the "Heroes" workflow, the app accepts every change to a hero immediately without hesitation or validation.

In the real world, you might have to accumulate the users changes. You might have to validate across fields. You might have to validate on the server. You might have to hold changes in a pending state until the user confirms them as a group or cancels and reverts all changes.

What do you do about unapproved, unsaved changes when the user navigates away? You can't just leave and risk losing the user's changes; that would be a terrible experience.

It's better to pause and let the user decide what to do. If the user cancels, you'll stay put and allow more changes. If the user approves, the app can save.

You still might delay navigation until the save succeeds. If you let the user move to the next screen immediately and the save were to fail (perhaps the data are ruled invalid), you would lose the context of the error.

You can't block while waiting for the server—that's not possible in a browser. You need to stop the navigation while you wait, asynchronously, for the server to return with its answer.

You need the CanDeactivate guard.

Cancel and save

The sample application doesn't talk to a server. Fortunately, you have another way to demonstrate an asynchronous router hook.

Users update crisis information in the CrisisDetailComponent. Unlike the HeroDetailComponent, the user changes do not update the crisis entity immediately. Instead, the app updates the entity when the user presses the Save button and discards the changes when the user presses the Cancel button.

Both buttons navigate back to the crisis list after save or cancel.

What if the user tries to navigate away without saving or canceling? The user could push the browser back button or click the heroes link. Both actions trigger a navigation. Should the app save or cancel automatically?

This demo does neither. Instead, it asks the user to make that choice explicitly in a confirmation dialog box that waits asynchronously for the user's answer.

The DialogService, provided in the AppModule for app-wide use, does the asking.

It returns a promise that resolves when the user eventually decides what to do: either to discard changes and navigate away (true) or to preserve the pending changes and stay in the crisis editor (false).

Create a guard that checks for the presence of a canDeactivate method in a component—any component. The CrisisDetailComponent will have this method. But the guard doesn't have to know that. The guard shouldn't know the details of any component's deactivation method. It need only detect that the component has a canDeactivate() method and call it. This approach makes the guard reusable.

Alternatively, you could make a component-specific CanDeactivate guard for the CrisisDetailComponent. The canDeactivate() method provides you with the current instance of the component, the current ActivatedRoute, and RouterStateSnapshot in case you needed to access some external information. This would be useful if you only wanted to use this guard for this component and needed to get the component's properties or confirm whether the router should allow navigation away from it.

Looking back at the CrisisDetailComponent, it implements the confirmation workflow for unsaved changes.

Notice that the canDeactivate method can return synchronously; it returns true immediately if there is no crisis or there are no pending changes. But it can also return a Promise or an Observable and the router will wait for that to resolve to truthy (navigate) or falsy (stay put).

Add the Guard to the crisis detail route in crisis-center-routing.module.ts using the canDeactivate array.

Add the Guard to the main AppRoutingModule providers array so the Router can inject it during the navigation process.

Now you have given the user a safeguard against unsaved changes.

Resolve: pre-fetching component data

In the Hero Detail and Crisis Detail, the app waited until the route was activated to fetch the respective hero or crisis.

This worked well, but there's a better way. If you were using a real world API, there might be some delay before the data to display is returned from the server. You don't want to display a blank component while waiting for the data.

It's preferable to pre-fetch data from the server so it's ready the moment the route is activated. This also allows you to handle errors before routing to the component. There's no point in navigating to a crisis detail for an id that doesn't have a record. It'd be better to send the user back to the Crisis List that shows only valid crisis centers.

In summary, you want to delay rendering the routed component until all necessary data have been fetched.

You need a resolver.

Fetch data before navigating

At the moment, the CrisisDetailComponent retrieves the selected crisis. If the crisis is not found, it navigates back to the crisis list view.

The experience might be better if all of this were handled first, before the route is activated. A CrisisDetailResolver service could retrieve a Crisis or navigate away if the Crisis does not exist before activating the route and creating the CrisisDetailComponent.

Create the crisis-detail-resolver.service.ts file within the Crisis Center feature area.

Take the relevant parts of the crisis retrieval logic in CrisisDetailComponent.ngOnInit and move them into the CrisisDetailResolver. Import the Crisis model, CrisisService, and the Router so you can navigate elsewhere if you can't fetch the crisis.

Be explicit. Implement the Resolve interface with a type of Crisis.

Inject the CrisisService and Router and implement the resolve() method. That method could return a Promise, an Observable, or a synchronous return value.

The CrisisService.getCrisis method returns a promise. Return that promise to prevent the route from loading until the data is fetched. If it doesn't return a valid Crisis, navigate the user back to the CrisisListComponent, canceling the previous in-flight navigation to the CrisisDetailComponent.

Import this resolver in the crisis-center-routing.module.ts and add a resolve object to the CrisisDetailComponent route configuration.

Remember to add the CrisisDetailResolver service to the CrisisCenterRoutingModule's providers array.

The CrisisDetailComponent should no longer fetch the crisis. Update the CrisisDetailComponent to get the crisis from the ActivatedRoute.data.crisis property instead; that's where you said it should be when you re-configured the route. It will be there when the CrisisDetailComponent ask for it.

Two critical points

1. The router's Resolve interface is optional. The CrisisDetailResolver doesn't inherit from a base class. The router looks for that method and calls it if found.

2. Rely on the router to call the resolver. Don't worry about all the ways that the user could navigate away. That's the router's job. Write this class and let the router take it from there.

The relevant Crisis Center code for this milestone follows.

Query parameters and fragments

In the route parameters example, you only dealt with parameters specific to the route, but what if you wanted optional parameters available to all routes? This is where query parameters come into play.

Fragments refer to certain elements on the page identified with an id attribute.

Update the AuthGuard to provide a session_id query that will remain after navigating to another route.

Add an anchor element so you can jump to a certain point on the page.

Add the NavigationExtras object to the router.navigate method that navigates you to the /login route.

You can also preserve query parameters and fragments across navigations without having to provide them again when navigating. In the LoginComponent, you'll add an object as the second argument in the router.navigate function and provide the preserveQueryParams and preserveFragment to pass along the current query parameters and fragment to the next route.

Since you'll be navigating to the Admin Dashboard route after logging in, you'll update it to handle the query parameters and fragment.

Query Parameters and Fragments are also available through the ActivatedRoute service. Just like route parameters, the query parameters and fragments are provided as an Observable. The updated Crisis Admin component feeds the Observable directly into the template using the AsyncPipe.

To see the URL changes in the browser address bar of the live example, open it again in the Plunker editor by clicking the icon in the upper right, then pop out the preview window by clicking the blue 'X' button in the upper right corner.

Now, you can click on the Admin button, which takes you to the Login page with the provided query params and fragment. After you click the login button, notice that you have been redirected to the Admin Dashboard page with the query params and fragment still intact.

You can use these persistent bits of information for things that need to be provided across pages like authentication tokens or session ids.

Milestone 6: Asynchronous routing

As you've worked through the milestones, the application has naturally gotten larger. As you continue to build out feature areas, the overall application size will continue to grow. At some point you'll reach a tipping point where the application takes long time to load.

How do you combat this problem? With asynchronous routing, which loads feature modules lazily, on request. Lazy loading has multiple benefits.

• You can load feature areas only when requested by the user.
• You can speed up load time for users that only visit certain areas of the application.
• You can continue expanding lazy loaded feature areas without increasing the size of the initial load bundle.

You're already made part way there.
By organizing the application into modules—AppModule, HeroesModule, AdminModule and CrisisCenterModule—you have natural candidates for lazy loading.

Some modules, like AppModule, must be loaded from the start. But others can and should be lazy loaded. The AdminModule, for example, is needed by a few authorized users, so you should only load it when requested by the right people.

Lazy Loading route configuration

Change the admin path in the admin-routing.module.ts from 'admin' to an empty string, '', the empty path.

The Router supports empty path routes; use them to group routes together without adding any additional path segments to the URL. Users will still visit /admin and the AdminComponent still serves as the Routing Component containing child routes.

Open the AppRoutingModule and add a new admin route to its appRoutes array.

Give it a loadChildren property (not a children property!), set to the address of the AdminModule. The address is the AdminModule file location (relative to the app root), followed by a # separator, followed by the name of the exported module class, AdminModule.

When the router navigates to this route, it uses the loadChildren string to dynamically load the AdminModule. Then it adds the AdminModule routes to its current route configuration. Finally, it loads the requested route to the destination admin component.

The lazy loading and re-configuration happen just once, when the route is first requested; the module and routes are available immediately for subsequent requests.

Take the final step and detach the admin feature set from the main application. The root AppModule must neither load nor reference the AdminModule or its files.

In app.module.ts, remove the AdminModule import statement from the top of the file and remove the AdminModule from the Angular module's imports array.

CanLoad Guard: guarding unauthorized loading of feature modules

You're already protecting the AdminModule with a CanActivate guard that prevents unauthorized users from accessing the admin feature area. It redirects to the login page if the user is not authorized.

But the router is still loading the AdminModule even if the user can't visit any of its components. Ideally, you'd only load the AdminModule if the user is logged in.

Add a CanLoad guard that only loads the AdminModule once the user is logged in and attempts to access the admin feature area.

The existing AuthGuard already has the essential logic in its checkLogin() method to support the CanLoad guard.

Open auth-guard.service.ts. Import the CanLoad interface from @angular/router. Add it to the AuthGuard class's implements list. Then implement canLoad as follows:

The router sets the canLoad() method's route parameter to the intended destination URL. The checkLogin() method redirects to that URL once the user has logged in.

Now import the AuthGuard into the AppRoutingModule and add the AuthGuard to the canLoad array for the admin route. The completed admin route looks like this:

Preloading: background loading of feature areas

You've learned how to load modules on-demand. You can also load modules asynchronously with preloading.

This may seem like what the app has been doing all along. Not quite. The AppModule is loaded when the application starts; that's eager loading. Now the AdminModule loads only when the user clicks on a link; that's lazy loading.

Preloading is something in between. Consider the Crisis Center. It isn't the first view that a user sees.
By default, the Heroes are the first view. For the smallest initial payload and fastest launch time, you should eagerly load the AppModule and the HeroesModule.

You could lazy load the Crisis Center. But you're almost certain that the user will visit the Crisis Center within minutes of launching the app. Ideally, the app would launch with just the AppModule and the HeroesModule loaded and then, almost immediately, load the CrisisCenterModule in the background. By the time the user navigates to the Crisis Center, its module will have been loaded and ready to go.

That's preloading.

How preloading works

After each successful navigation, the router looks in its configuration for an unloaded module that it can preload. Whether it preloads a module, and which modules it preloads, depends upon the preload strategy.

The Router offers two preloading strategies out of the box:

• No preloading at all which is the default. Lazy loaded feature areas are still loaded on demand.
• Preloading of all lazy loaded feature areas.

Out of the box, the router either never preloads, or preloads every lazy load module. The Router also supports custom preloading strategies for fine control over which modules to preload and when.

In this next section, you'll update the CrisisCenterModule to load lazily by default and use the PreloadAllModules strategy to load it (and all other lazy loaded modules) as soon as possible.

Lazy load the crisis center

Update the route configuration to lazy load the CrisisCenterModule. Take the same steps you used to configure AdminModule for lazy load.

1. Change the crisis-center path in the CrisisCenterRoutingModule to an empty string.

2. Add a crisis-center route to the AppRoutingModule.

3. Set the loadChildren string to load the CrisisCenterModule.

4. Remove all mention of the CrisisCenterModule from app.module.ts.

Here are the updated modules before enabling preload:

You could try this now and confirm that the CrisisCenterModule loads after you click the "Crisis Center" button.

To enable preloading of all lazy loaded modules, import the PreloadAllModules token from the Angular router package.

The second argument in the RouterModule.forRoot method takes an object for additional configuration options. The preloadingStrategy is one of those options. Add the PreloadAllModules token to the forRoot call:

This tells the Router preloader to immediately load all lazy loaded routes (routes with a loadChildren property).

When you visit http://localhost:3000, the /heroes route loads immediately upon launch and the router starts loading the CrisisCenterModule right after the HeroesModule loads.

Surprisingly, the AdminModule does not preload. Something is blocking it.

CanLoad blocks preload

The PreloadAllModules strategy does not load feature areas protected by a CanLoad guard. This is by design.

You added a CanLoad guard to the route in the AdminModule a few steps back to block loading of that module until the user is authorized. That CanLoad guard takes precedence over the preload strategy.

If you want to preload a module and guard against unauthorized access, drop the canLoad guard and rely on the CanActivate guard alone.

Custom Preloading Strategy

Preloading every lazy loaded modules works well in many situations, but it isn't always the right choice, especially on mobile devices and over low bandwidth connections. You may choose to preload only certain feature modules, based on user metrics and other business and technical factors.

You can control what and how the router preloads with a custom preloading strategy.

In this section, you'll add a custom strategy that only preloads routes whose data.preload flag is set to true. Recall that you can add anything to the data property of a route.

Set the data.preload flag in the crisis-center route in the AppRoutingModule.

Add a new file to the project called selective-preloading-strategy.ts and define a SelectivePreloadingStrategy service class as follows:

SelectivePreloadingStrategy implements the PreloadingStrategy, which has one method, preload.

The router calls the preload method with two arguments:

1. The route to consider.
2. A loader function that can load the routed module asynchronously.

An implementation of preloadmust return an Observable. If the route should preload, it returns the observable returned by calling the loader function. If the route should not preload, it returns an Observable of null.

In this sample, the preload method loads the route if the route's data.preload flag is truthy.

It also has a side-effect. SelectivePreloadingStrategy logs the path of a selected route in its public preloadedModules array.

Shortly, you'll extend the AdminDashboardComponent to inject this service and display its preloadedModules array.

But first, make a few changes to the AppRoutingModule.

1. Import SelectivePreloadingStrategy into AppRoutingModule.
2. Replace the PreloadAllModules strategy in the call to forRoot with this SelectivePreloadingStrategy.
3. Add the SelectivePreloadingStrategy strategy to the AppRoutingModule providers array so it can be injected elsewhere in the app.

Now edit the AdminDashboardComponent to display the log of preloaded routes.

1. Import the SelectivePreloadingStrategy (it's a service).
2. Inject it into the dashboard's constructor.
3. Update the template to display the strategy service's preloadedModules array.

When you're done it looks like this.

Once the application loads the initial route, the CrisisCenterModule is preloaded. Verify this by logging in to the Admin feature area and noting that the crisis-center is listed in the Preloaded Modules. It's also logged to the browser's console.

Inspect the router's configuration

You put a lot of effort into configuring the router in several routing module files and were careful to list them in the proper order. Are routes actually evaluated as you planned? How is the router really configured?

You can inspect the router's current configuration any time by injecting it and examining its config property. For example, update the AppModule as follows and look in the browser console window to see the finished route configuration.

Wrap up and final app

You've covered a lot of ground in this guide and the application is too big to reprint here. Please visit the where you can download the final source code.

Appendices

The balance of this guide is a set of appendices that elaborate some of the points you covered quickly above.

The appendix material isn't essential. Continued reading is for the curious.

Appendix: link parameters array

A link parameters array holds the following ingredients for router navigation:

• The path of the route to the destination component.
• Required and optional route parameters that go into the route URL.

You can bind the RouterLink directive to such an array like this:

You've written a two element array when specifying a route parameter like this:

You can provide optional route parameters in an object like this:

These three examples cover the need for an app with one level routing. The moment you add a child router, such as the crisis center, you create new link array possibilities.

Recall that you specified a default child route for the crisis center so this simple RouterLink is fine.

Parse it out.

• The first item in the array identifies the parent route (/crisis-center).
• There are no parameters for this parent route so you're done with it.
• There is no default for the child route so you need to pick one.
• You're navigating to the CrisisListComponent, whose route path is /, but you don't need to explicitly add the slash.
• Voilà! ['/crisis-center'].

Take it a step further. Consider the following router link that navigates from the root of the application down to the Dragon Crisis:

• The first item in the array identifies the parent route (/crisis-center).
• There are no parameters for this parent route so you're done with it.
• The second item identifies the child route details about a particular crisis (/:id).
• The details child route requires an id route parameter.
• You added the id of the Dragon Crisis as the second item in the array (1).
• The resulting path is /crisis-center/1.

If you wanted to, you could redefine the AppComponent template with Crisis Center routes exclusively:

In sum, you can write applications with one, two or more levels of routing. The link parameters array affords the flexibility to represent any routing depth and any legal sequence of route paths, (required) router parameters, and (optional) route parameter objects.

Appendix: LocationStrategy and browser URL styles

When the router navigates to a new component view, it updates the browser's location and history with a URL for that view. This is a strictly local URL. The browser shouldn't send this URL to the server and should not reload the page.

Modern HTML5 browsers support history.pushState, a technique that changes a browser's location and history without triggering a server page request. The router can compose a "natural" URL that is indistinguishable from one that would otherwise require a page load.

Here's the Crisis Center URL in this "HTML5 pushState" style:

Older browsers send page requests to the server when the location URL changes unless the change occurs after a "#" (called the "hash"). Routers can take advantage of this exception by composing in-application route URLs with hashes. Here's a "hash URL" that routes to the Crisis Center.

The router supports both styles with two LocationStrategy providers:

1. PathLocationStrategy—the default "HTML5 pushState" style.
2. HashLocationStrategy—the "hash URL" style.

The RouterModule.forRoot function sets the LocationStrategy to the PathLocationStrategy, making it the default strategy. You can switch to the HashLocationStrategy with an override during the bootstrapping process if you prefer it.

Which strategy is best?

You must choose a strategy and you need to make the right call early in the project. It won't be easy to change later once the application is in production and there are lots of application URL references in the wild.

Almost all Angular projects should use the default HTML5 style. It produces URLs that are easier for users to understand. And it preserves the option to do server-side rendering later.

Rendering critical pages on the server is a technique that can greatly improve perceived responsiveness when the app first loads. An app that would otherwise take ten or more seconds to start could be rendered on the server and delivered to the user's device in less than a second.

This option is only available if application URLs look like normal web URLs without hashes (#) in the middle.

Stick with the default unless you have a compelling reason to resort to hash routes.

HTML5 URLs and the <base href>

While the router uses the HTML5 pushState style by default, you must configure that strategy with a base href.

The preferred way to configure the strategy is to add a <base href> element tag in the <head> of the index.html.

Without that tag, the browser may not be able to load resources (images, CSS, scripts) when "deep linking" into the app. Bad things could happen when someone pastes an application link into the browser's address bar or clicks such a link in an email.

Some developers may not be able to add the <base> element, perhaps because they don't have access to <head> or the index.html.

Those developers may still use HTML5 URLs by taking two remedial steps:

1. Provide the router with an appropriate APP_BASE_HREF value.
2. Use root URLs for all web resources: CSS, images, scripts, and template HTML files.

HashLocationStrategy

You can go old-school with the HashLocationStrategy by providing the useHash: true in an object as the second argument of the RouterModule.forRoot in the AppModule.