component-router.ngdoc

@ngdoc overview
@name Component Router
@sortOrder 306
@description

# Component Router

This guide describes the new Component Router for AngularJS 1.5.

<div class="alert alert-info">
  If you are looking for information about the old router for AngularJS 1.4 and
  earlier have a look at the {@link ngRoute} module.

  If you are looking for information about the Component Router for Angular 2 then
  check out the [Angular 2 Router Guide](https://angular.io/docs/ts/latest/guide/router.html).
</div>

## Overview

Here is a table of the main concepts used in the Component Router.

| Concept               | Description |
| ----------------------|-------------------------------------------------------------------------------------- |
| Router                | Displays the Routing Components for the active Route. Manages navigation from one component to the next. |
| RootRouter            | The top level Router that interacts with the current URL location |
| RouteConfig           | Configures a Router with RouteDefinitions, each mapping a URL path to a component. |
| Routing Component     | An Angular component with a RouteConfig and an associated Router. |
| RouteDefinition       | Defines how the router should navigate to a component based on a URL pattern. |
| ngOutlet              | The directive (`<ng-outlet>`) that marks where the router should display a view. |
| ngLink                | The directive (`ng-link="..."`) for binding a clickable HTML element to a route, via a Link Paramaters Array. |
| Link Parameters Array | An array that the router inteprets into a routing instruction. We can bind a RouterLink to that array or pass the array as an argument to the Router.navigate method. |


## Component-based Applications

It is recommended to develop AngularJS applications as a hierarchy of Components. Each Component
is an isolated part of the application, which is responsible for its own user interface and has
a well defined programmatic interface to the Component that contains it. Take a look at the
{@link guide/component component guide} for more information.

![Component Based Architecture](img/guide/component-based-architecture.svg)


## URLs and Navigation

In most applications, users navigate from one view to the next as they perform application tasks.
The browser provides a familiar model of application navigation. We enter a URL in the address bar
or click on a link and the browser navigates to a new page. We click the browser's back and forward
buttons and the browser navigates backward and forward through the history of pages we've seen.

We understand that each view corresponds to a particular URL. In a Component-based application,
each of these views is implemented by one or more Components.


## Component Routes

**How do we choose which Components to display given a particular URL?**

When using the Component Router, each **Component** in the application can have a **Router** associated
with it. This **Router** contains a mapping of URL segments to child **Components**.

```js
$routeConfig: [
  { path: '/a/b/c', component: 'someComponent' }, ...
]
```

This means that for a given URL the **Router** will render an associated child **Component**.


## Outlets

**How do we know where to render a child Component?**

Each **Routing Component**, needs to have a template that contains one or more **Outlets**, which is
where its child **Components** are rendered. We specify the **Outlet** in the template using the
{@link ngOutlet `<ng-outlet>`} directive.

```html
<ng-outlet></ng-outlet>
```

*In the future `ng-outlet` will be able to render different child **Components** for a given **Route**
by specifying a `name` attribute.*


## Root Router and Component

**How does the Component Router know which Component to render first?**

All Component Router applications must contain a top level **Routing Component**, which is associated with
a top level **Root Router**.

The **Root Router** is the starting point for all navigation. You can access this **Router** by injecting the
`$rootRouter` service.

We define the top level **Root Component** by providing a value for the {@link $routerRootComponent} service.

```js
myModule.value('$routerRootComponent', 'myApp');
```

Here we have specified that the **Root Component** is the component directive with the name `myApp`.

Remember to instantiate this **Root Component** in our `index.html` file.

```html
<my-app></my-app>
```

## Route Matching

When we navigate to any given URL, the {@link $rootRouter} matches its **Route Config** against the URL.
If a **Route Definition** in the **Route Config** recognizes a part of the URL then the **Component**
associated with the **Route Definition** is instantiated and rendered in the **Outlet**.

If the new **Component** contains routes of its own then a new **Router ({@link ChildRouter})** is created for
this **Routing Component**.

The {@link ChildRouter} for the new **Routing Component** then attempts to match its **Route Config** against
the parts of the URL that have not already been matched by the previous **Router**.

This process continues until we run out of **Routing Components** or consume the entire URL.

![Routed Components](img/guide/component-routes.svg)

In the previous diagram, we can see that the URL `/heros/4` has been matched against the `App`, `Heroes` and
`HeroDetail` **Routing Components**. The **Routers** for each of the **Routing Components** consumed a part
of the URL: "/", "/heroes" and "/4" respectively.

The result is that we end up with a hierarchy of **Routing Components** rendered in **Outlets**, via the
{@link ngOutlet} directive, in each **Routing Component's** template, as you can see in the following diagram.

![Component Hierarchy](img/guide/component-hierarchy.svg)


# Example Heroes App

You can see the complete application running below.

<example name="componentRouter" module="app" fixBase="true">

  <file name="index.html">
    <h1 class="title">Component Router</h1>
    <app></app>

    <!-- Load up the router library - normally you might use npm and host it locally -->
    <script src="https://npmcdn.com/@angular/router@0.2.0/angular1/angular_1_router.js"></script>
  </file>

  <file name="app.js">
    angular.module('app', ['ngComponentRouter', 'heroes', 'crisis-center'])

    .config(function($locationProvider) {
      $locationProvider.html5Mode(true);
    })

    .value('$routerRootComponent', 'app')

    .component('app', {
      template:
        '<nav>\n' +
        '  <a ng-link="[\'CrisisCenter\']">Crisis Center</a>\n' +
        '  <a ng-link="[\'Heroes\']">Heroes</a>\n' +
        '</nav>\n' +
        '<ng-outlet></ng-outlet>\n',
      $routeConfig: [
        {path: '/crisis-center/...', name: 'CrisisCenter', component: 'crisisCenter', useAsDefault: true},
        {path: '/heroes/...', name: 'Heroes', component: 'heroes' }
      ]
    });
  </file>

  <file name="heroes.js">
    angular.module('heroes', [])
      .service('heroService', HeroService)

      .component('heroes', {
        template: '<h2>Heroes</h2><ng-outlet></ng-outlet>',
        $routeConfig: [
          {path: '/',    name: 'HeroList',   component: 'heroList', useAsDefault: true},
          {path: '/:id', name: 'HeroDetail', component: 'heroDetail'}
        ]
      })

      .component('heroList', {
        template:
          '<div ng-repeat="hero in $ctrl.heroes" ' +
          '     ng-class="{ selected: $ctrl.isSelected(hero) }">\n' +
            '<a ng-link="[\'HeroDetail\', {id: hero.id}]">{{hero.name}}</a>\n' +
          '</div>',
        controller: HeroListComponent
      })

      .component('heroDetail', {
        template:
          '<div ng-if="$ctrl.hero">\n' +
          '  <h3>"{{$ctrl.hero.name}}"</h3>\n' +
          '  <div>\n' +
          '    <label>Id: </label>{{$ctrl.hero.id}}</div>\n' +
          '  <div>\n' +
          '    <label>Name: </label>\n' +
          '    <input ng-model="$ctrl.hero.name" placeholder="name"/>\n' +
          '  </div>\n' +
          '  <button ng-click="$ctrl.gotoHeroes()">Back</button>\n' +
          '</div>\n',
        bindings: { $router: '<' },
        controller: HeroDetailComponent
      });


    function HeroService($q) {
      var heroesPromise = $q.when([
        { id: 11, name: 'Mr. Nice' },
        { id: 12, name: 'Narco' },
        { id: 13, name: 'Bombasto' },
        { id: 14, name: 'Celeritas' },
        { id: 15, name: 'Magneta' },
        { id: 16, name: 'RubberMan' }
      ]);

      this.getHeroes = function() {
        return heroesPromise;
      };

      this.getHero = function(id) {
        return heroesPromise.then(function(heroes) {
          for(var i=0; i<heroes.length; i++) {
            if ( heroes[i].id == id) return heroes[i];
          }
        });
      };
    }

    function HeroListComponent(heroService) {
      var selectedId = null;
      var $ctrl = this;

      this.$routerOnActivate = function(next) {
        // Load up the heroes for this view
        heroService.getHeroes().then(function(heroes) {
          $ctrl.heroes = heroes;
          selectedId = next.params.id;
        });
      };

      this.isSelected = function(hero) {
        return (hero.id == selectedId);
      };
    }

    function HeroDetailComponent(heroService) {
      var $ctrl = this;

      this.$routerOnActivate = function(next) {
        // Get the hero identified by the route parameter
        var id = next.params.id;
        heroService.getHero(id).then(function(hero) {
          $ctrl.hero = hero;
        });
      };

      this.gotoHeroes = function() {
        var heroId = this.hero && this.hero.id;
        this.$router.navigate(['HeroList', {id: heroId}]);
      };
    }
  </file>

  <file name="crisis.js">
    angular.module('crisis-center', ['dialog'])
      .service('crisisService', CrisisService)

      .component('crisisCenter', {
        template: '<h2>Crisis Center</h2><ng-outlet></ng-outlet>',
        $routeConfig: [
          {path:'/',    name: 'CrisisList',   component: 'crisisList', useAsDefault: true},
          {path:'/:id', name: 'CrisisDetail', component: 'crisisDetail'}
        ]
      })

      .component('crisisList', {
        template:
          '<ul>\n' +
          '  <li ng-repeat="crisis in $ctrl.crises"\n' +
          '    ng-class="{ selected: $ctrl.isSelected(crisis) }"\n' +
          '    ng-click="$ctrl.onSelect(crisis)">\n' +
          '    <span class="badge">{{crisis.id}}</span> {{crisis.name}}\n' +
          '  </li>\n' +
          '</ul>\n',
        bindings: { $router: '<' },
        controller: CrisisListComponent,
        $canActivate: function($nextInstruction, $prevInstruction) {
          console.log('$canActivate', arguments);
        }
      })

      .component('crisisDetail', {
        templateUrl: 'crisisDetail.html',
        bindings: { $router: '<' },
        controller: CrisisDetailComponent
      });


    function CrisisService($q) {
      var crisesPromise = $q.when([
        {id: 1, name: 'Princess Held Captive'},
        {id: 2, name: 'Dragon Burning Cities'},
        {id: 3, name: 'Giant Asteroid Heading For Earth'},
        {id: 4, name: 'Release Deadline Looms'}
      ]);

      this.getCrises = function() {
        return crisesPromise;
      };

      this.getCrisis = function(id) {
        return crisesPromise.then(function(crises) {
          for(var i=0; i<crises.length; i++) {
            if ( crises[i].id == id) return crises[i];
          }
        });
      };
    }

    function CrisisListComponent(crisisService) {
      var selectedId = null;
      var ctrl = this;

      this.$routerOnActivate = function(next) {
        console.log('$routerOnActivate', this, arguments);
        // Load up the crises for this view
        crisisService.getCrises().then(function(crises) {
          ctrl.crises = crises;
          selectedId = next.params.id;
        });
      };

      this.isSelected = function(crisis) {
        return (crisis.id == selectedId);
      };

      this.onSelect = function(crisis) {
        this.$router.navigate(['CrisisDetail', { id: crisis.id }]);
      };
    }

    function CrisisDetailComponent(crisisService, dialogService) {
      var ctrl = this;
      this.$routerOnActivate = function(next) {
        // Get the crisis identified by the route parameter
        var id = next.params.id;
        crisisService.getCrisis(id).then(function(crisis) {
          if (crisis) {
            ctrl.editName = crisis.name;
            ctrl.crisis = crisis;
          } else { // id not found
            ctrl.gotoCrises();
          }
        });
      };

      this.$routerCanDeactivate = function() {
        // Allow synchronous navigation (`true`) if no crisis or the crisis is unchanged.
        if (!this.crisis || this.crisis.name === this.editName) {
          return true;
        }
        // Otherwise ask the user with the dialog service and return its
        // promise which resolves to true or false when the user decides
        return dialogService.confirm('Discard changes?');
      };

      this.cancel = function() {
        ctrl.editName = ctrl.crisis.name;
        ctrl.gotoCrises();
      };

      this.save = function() {
        ctrl.crisis.name = ctrl.editName;
        ctrl.gotoCrises();
      };

      this.gotoCrises = function() {
        var crisisId = ctrl.crisis && ctrl.crisis.id;
        // Pass along the hero id if available
        // so that the CrisisListComponent can select that hero.
        this.$router.navigate(['CrisisList', {id: crisisId}]);
      };
    }
  </file>

  <file name="crisisDetail.html">
    <div ng-if="$ctrl.crisis">
      <h3>"{{$ctrl.editName}}"</h3>
      <div>
        <label>Id: </label>{{$ctrl.crisis.id}}</div>
      <div>
        <label>Name: </label>
        <input ng-model="$ctrl.editName" placeholder="name"/>
      </div>
      <button ng-click="$ctrl.save()">Save</button>
      <button ng-click="$ctrl.cancel()">Cancel</button>
    </div>
  </file>

  <file name="dialog.js">
    angular.module('dialog', [])

    .service('dialogService', DialogService);

    function DialogService($q) {
      this.confirm = function(message) {
        return $q.when(window.confirm(message || 'Is it OK?'));
      };
    }
  </file>

  <file name="styles.css">
    h1 {color: #369; font-family: Arial, Helvetica, sans-serif; font-size: 250%;}
    h2 { color: #369; font-family: Arial, Helvetica, sans-serif;  }
    h3 { color: #444; font-weight: lighter; }
    body { margin: 2em; }
    body, input[text], button { color: #888; font-family: Cambria, Georgia; }
    button {padding: 0.2em; font-size: 14px}

    ul {list-style-type: none; margin-left: 1em; padding: 0; width: 20em;}

    li { cursor: pointer; position: relative; left: 0; transition: all 0.2s ease; }
    li:hover {color: #369; background-color: #EEE; left: .2em;}

    /* route-link anchor tags */
    a {padding: 5px; text-decoration: none; font-family: Arial, Helvetica, sans-serif; }
    a:visited, a:link {color: #444;}
    a:hover {color: white; background-color: #1171a3; }
    a.router-link-active {color: white; background-color: #52b9e9; }

    .selected { background-color: #EEE; color: #369; }

    .badge {
      font-size: small;
      color: white;
      padding: 0.1em 0.7em;
      background-color: #369;
      line-height: 1em;
      position: relative;
      left: -1px;
      top: -1px;
    }

    crisis-detail input {
      width: 20em;
    }
  </file>

</example>


# Getting Started

In the following sections we will step through building this application. The finished application has views
to display list and detail views of Heroes and Crises.

## Install the libraries

It is easier to use npm to install the **Component Router** module. For this guide we will also install
AngularJS itself via npm:

```bash
npm init
npm install angular@1.5.x --save
npm install @angular/router@0.2.0 --save
```


## Load the scripts

Just like any Angular application, we load the JavaScript files into our `index.html`:

```html
  <script src="/node_modules/angular/angular.js"></script>
  <script src="/node_modules/@angular/router/angular1/angular_1_router.js"></script>
  <script src="/app/app.js"></script>
```


## Create the `app` module

In the app.js file, create the main application module `app` which depends on the `ngComponentRouter`
module, which is provided by the **Component Router** script.

```js
angular.module('app', ['ngComponentRouter'])
```

We must choose what **Location Mode** the **Router** should use. We are going to use HTML5 mode locations,
so that we will not have hash-based paths. We must rely on the browser to provide `pushState` support,
which is true for most modern browsers. See {@link $locationProvider#html5Mode} for more information.

<div class="alert alert-info">
  Using HTML5 mode means that we can have clean URLs for our application routes. However, HTML5 mode does require that our
  web server, which hosts the application, understands that it must respond with the index.html file for
  requests to URLs that represent all our application routes. We are going to use the `lite-server` web server
  to do this for us.
</div>

```js
.config(function($locationProvider) {
  $locationProvider.html5Mode(true);
})
```

Configure the top level routed `App` Component.

```js
.value('$routerRootComponent', 'app')
```

Create a very simple App Component to test that the application is working.

We are using the Angular 1.5 {@link $compileProvider#component `.component()`} helper method to create
all the **Components** in our application. It is perfectly suited to this task.

```js
.component('app', {
  template: 'It worked!'
});
```

Add a `<base>` element to the head of our index.html.
Remember that we have chosen to use HTML5 mode for the `$location` service. This means that our HTML
must have a base URL.

```html
<head>
  <base href="/">
  ...
```

## Bootstrap AngularJS

Bootstrap the Angular application and add the top level App Component.

```html
<body ng-app="app">
    <h1 class="title">Component Router</h1>
    <app></app>
  </body>
```


# Implementing the AppComponent

In the previous section we have created a single top level **App Component**. Let's now create some more
**Routing Components** and wire up **Route Config** for those. We start with a Heroes Feature, which
will display one of two views.

* A list of Heroes that are available:

![Heroes List View](img/guide/heroes-list.png)

* A detailed view of a single Hero:

![Heroes List View](img/guide/hero-detail.png)

We are going to have a `Heroes` Component for the Heroes feature of our application, and then `HeroList`
and `HeroDetail` **Components** that will actually display the two different views.


## App Component

Configure the **App Component** with a template and **Route Config**:

```js
.component('app', {
  template:
    '<nav>\n' +
    '  <a>Crisis Center</a>\n' +
    '  <a ng-link="[\'Heroes\']">Heroes</a>\n' +
    '</nav>\n' +
    '<ng-outlet></ng-outlet>\n',
  $routeConfig: [
    {path: '/heroes/...', name: 'Heroes', component: 'heroes'},
  ]
});
```

The **App Component** has an `<ng-outlet>` directive in its template. This is where the child **Components**
of this view will be rendered.

### ngLink

We have used the `ng-link` directive to create a link to navigate to the Heroes Component. By using this
directive we don't need to know what the actual URL will be. We can let the Router generate that for us.

We have included a link to the Crisis Center but have not included the `ng-link` directive as we have not yet
implemented the CrisisCenter component.


### Non-terminal Routes

We need to tell the **Router** that the `Heroes` **Route Definition** is **non-terminal**, that it should
continue to match **Routes** in its child **Components**. We do this by adding a **continuation ellipsis
(`...`)** to the path of the Heroes Route, `/heroes/...`.
Without the **continuation ellipsis** the `HeroList` **Route** will never be matched because the Router will
stop at the `Heroes` **Routing Component** and not try to match the rest of the URL.


## Heroes Feature

Now we can implement our Heroes Feature which consists of three **Components**: `Heroes`, `HeroList` and
`HeroDetail`. The `Heroes` **Routing Component** simply provides a template containing the {@link ngOutlet}
directive and a **Route Config** that defines a set of child **Routes** which delegate through to the
`HeroList` and `HeroDetail` **Components**.

## HeroesComponent

Create a new file `heroes.js`, which defines a new Angular module for the **Components** of this feature
and registers the Heroes **Component**.

```js
angular.module('heroes', [])
  .component('heroes', {
    template: '<h2>Heroes</h2><ng-outlet></ng-outlet>',
    $routeConfig: [
      {path: '/',    name: 'HeroList',   component: 'heroList', useAsDefault: true},
      {path: '/:id', name: 'HeroDetail', component: 'heroDetail'}
    ]
  })
```

Remember to load this file in the index.html:

```html
<script src="/app/heroes.js"></script>
```

and also to add the module as a dependency of the `app` module:

```js
angular.module('app', ['ngComponentRouter', 'heroes'])
```

### Use As Default
The `useAsDefault` property on the `HeroList` **Route Definition**, indicates that if no other **Route
Definition** matches the URL, then this **Route Definition** should be used by default.

### Route Parameters
The `HeroDetail` Route has a named parameter (`id`), indicated by prefixing the URL segment with a colon,
as part of its `path` property. The **Router** will match anything in this segment and make that value
available to the HeroDetail **Component**.

### Terminal Routes
Both the Routes in the `HeroesComponent` are terminal, i.e. their routes do not end with `...`. This is
because the `HeroList` and `HeroDetail` will not contain any child routes.

### Route Names
**What is the difference between the `name` and `component` properties on a Route Definition?**

The `component` property in a **Route Definition** defines the **Component** directive that will be rendered
into the DOM via the **Outlet**. For example the `heroDetail` **Component** will be rendered into the page
where the `<ng-outlet></ng-outlet>` lives as `<hero-detail></hero-detail>`.

The `name` property is used to reference the **Route Definition** when generating URLs or navigating to
**Routes**. For example this link will `<a ng-link="['Heroes']">Heroes</a>` navigate the **Route Definition**
that has the `name` property of `"Heroes"`.


## HeroList Component

The HeroList **Component** is the first component in the application that actually contains significant
functionality. It loads up a list of heroes from a `heroService` and displays them using `ng-repeat`.
Add it to the `heroes.js` file:

```js
  .component('heroList', {
    template:
      '<div ng-repeat="hero in $ctrl.heroes">\n' +
        '<a ng-link="[\'HeroDetail\', {id: hero.id}]">{{hero.name}}</a>\n' +
      '</div>',
    controller: HeroListComponent
  })
```

The `ng-link` directive creates links to a more detailed view of each hero, via the expression
`['HeroDetail', {id: hero.id}]`. This expression is an array describing what Routes to use to generate
the link. The first item is the name of the HeroDetail **Route Definition** and the second is a parameter
object that will be available to the HeroDetail **Component**.

*The HeroDetail section below explains how to get hold of the `id` parameter of the HeroDetail Route.*

The template iterates through each `hero` object of the array in the `$ctrl.heroes` property.

*Remember that the `module.component()` helper automatically provides the **Component's Controller** as
the `$ctrl` property on the scope of the template.*


## HeroService

Our HeroService simulates requesting a list of heroes from a server. In a real application this would be
making an actual server request, perhaps over HTTP.

```js
function HeroService($q) {
  var heroesPromise = $q.when([
    { id: 11, name: 'Mr. Nice' },
    ...
  ]);

  this.getHeroes = function() {
    return heroesPromise;
  };

  this.getHero = function(id) {
    return heroesPromise.then(function(heroes) {
      for(var i=0; i<heroes.length; i++) {
        if ( heroes[i].id == id) return heroes[i];
      }
    });
  };
}
```

Note that both the `getHeroes()` and `getHero(id)` methods return a promise for the data. This is because
in real-life we would have to wait for the server to respond with the data.


## Router Lifecycle Hooks

**How do I know when my Component is active?**

To deal with initialization and tidy up of **Components** that are rendered by a **Router**, we can implement
one or more **Lifecycle Hooks** on the **Component**. These will be called at well defined points in the
lifecycle of the **Component**.

The **Lifecycle Hooks** that can be implemented as instance methods on the **Component** are as follows:

* `$routerCanReuse` : called to to determine whether a **Component** can be reused across **Route Definitions**
  that match the same type of **Component**, or whether to destroy and instantiate a new **Component** every time.
* `$routerOnActivate` / `$routeOnReuse` : called by the **Router** at the end of a successful navigation. Only
  one of `$routerOnActivate` and `$routerOnReuse` will be called depending upon the result of a call to
  `$routerCanReuse`.
* `$routerCanDeactivate` : called by the **Router** to determine if a **Component** can be removed as part of a
  navigation.
* `$routerOnDeactivate` : called by the **Router** before destroying a **Component** as part of a navigation.

We can also provide an **Injectable function** (`$routerCanActivate`) on the **Component Definition Object**,
or as a static method on the **Component**, that will determine whether this **Component** is allowed to be
activated. If any of the `$routerCan...` methods return false or a promise that resolves to false, the
navigation will be cancelled.

For our HeroList **Component** we want to load up the list of heroes when the **Component** is activated.
So we implement the `$routerOnActivate()` instance method.

```js
function HeroListComponent(heroService) {
  var $ctrl = this;
  this.$routerOnActivate = function() {
    return heroService.getHeroes().then(function(heroes) {
      $ctrl.heroes = heroes;
    });
  }
}
```

Running the application should update the browser's location to `/heroes` and display the list of heroes
returned from the `heroService`.

By returning a promise for the list of heroes from `$routerOnActivate()` we can delay the activation of the
Route until the heroes have arrived successfully. This is similar to how a `resolve` works in {@link ngRoute}.


## Route Parameters

**How do I access parameters for the current route?**

The HeroDetailComponent displays details of an individual hero. The `id` of the hero to display is passed
as part of the URL, for example **/heroes/12**.

The **Router** parses the id from the URL when it recognizes the **Route Definition** and provides it to the
**Component** as part of the parameters of the `$routerOnActivate()` hook.

```js
function HeroDetailComponent(heroService) {
  var $ctrl = this;

  this.$routerOnActivate = function(next, previous) {
    // Get the hero identified by the route parameter
    var id = next.params.id;
    return heroService.getHero(id).then(function(hero) {
      $ctrl.hero = hero;
    });
  };
```

The `$routerOnActivate(next, previous)` hook receives two parameters, which hold the `next` and `previous`
**Instruction** objects for the **Route** that is being activated.

These parameters have a property called `params` which will hold the `id` parameter extracted from the URL
by the **Router**. In this code it is used to identify a specific Hero to retrieve from the `heroService`.
This hero is then attached to the **Component** so that it can be accessed in the template.


## Access to the Current Router

**How do I get hold of the current router for my component?**

Each component has its own Router. Unlike in Angular 2, we cannot use the dependency injector to get hold of a component's Router.
We can only inject the `$rootRouter`. Instead we use the fact that the `ng-outlet` directive binds the current router to a `$router`
attribute on our component.

```html
<ng-outlet><hero-detail $router="$$router"></hero-detail></ng-outlet>
```

We can then specify a `bindings` property on our component definition to bind the current router to our component:

```js
bindings: { $router: '<' }
```

This sets up a one-way binding of the current Router to the `$router` property of our Component. The binding is available once
the component has been activated, and the `$routerOnActivate` hook is called.

As you might know from reading the {@link guide/component component guide}, the binding is actually available by the time the `$onInit`
hook is called, which is before the call to `$routerOnActivate`.

### HeroDetailComponent

The `HeroDetailComponent` displays a form that allows the Hero to be modified.

```js
  .component('heroDetail', {
    template:
      '<div ng-if="$ctrl.hero">\n' +
      '  <h3>"{{$ctrl.hero.name}}"</h3>\n' +
      '  <div>\n' +
      '    <label>Id: </label>{{$ctrl.hero.id}}</div>\n' +
      '  <div>\n' +
      '    <label>Name: </label>\n' +
      '    <input ng-model="$ctrl.hero.name" placeholder="name"/>\n' +
      '  </div>\n' +
      '  <button ng-click="$ctrl.gotoHeroes()">Back</button>\n' +
      '</div>\n',
    bindings: { $router: '<' },
    controller: HeroDetailComponent
  });
```

The template contains a button to navigate back to the HeroList. We could have styled an anchor to look
like a button and used `ng-link="['HeroList']" but here we demonstrate programmatic navigation via the
Router itself, which was made available by the binding in the **Component Definition Object**.

```js
function HeroDetailComponent(heroService) {
  ...
  this.gotoHeroes = function() {
    var heroId = this.hero && this.hero.id;
    this.$router.navigate(['HeroList']);
  };
```

Here we are asking the Router to navigate to a route defined by `['HeroList']`.
This is the same kind of array used by the `ng-link` directive.

Other options for generating this navigation are:
* manually create the URL and call `this.$router.navigateByUrl(url)` - this is discouraged because it
  couples the code of your component to the router URLs.
* generate an Instruction for a route and navigate directly with this instruction.
  ```js
  var instruction = this.$router.generate(['HeroList']);
  this.$router.navigateByInstruction(instruction);
  ```
  this form gives you the possibility of caching the instruction, but is more verbose.

### Absolute vs Relative Navigation

**Why not use `$rootRouter` to do the navigation?**

Instead of binding to the current **Router**, we can inject the `$rootRouter` into our **Component** and
use that: `$rootRouter.navigate(...)`.

The trouble with doing this is that navigation is always relative to the **Router**. So in order to navigate
to the `HeroListComponent` with the `$rootRouter`, we would have to provide a complete path of Routes:
`['App','Heroes','HeroList']`.


## Extra Parameters

We can also pass additional optional parameters to routes, which get encoded into the URL and are again
available to the `$routerOnActivate(next, previous)` hook. If we pass the current `id` from the
HeroDetailComponent back to the HeroListComponent we can use it to highlight the previously selected hero.

```js
  this.gotoHeroes = function() {
    var heroId = this.hero && this.hero.id;
    this.$router.navigate(['HeroList', {id: heroId}]);
  };
```

Then in the HeroList component we can extract this `id` in the `$routerOnActivate()` hook.

```js
function HeroListComponent(heroService) {
  var selectedId = null;
  var $ctrl = this;

  this.$routerOnActivate = function(next) {
    heroService.getHeroes().then(function(heroes) {
      $ctrl.heroes = heroes;
      selectedId = next.params.id;
    });
  };

  this.isSelected = function(hero) {
    return (hero.id == selectedId);
  };
}
```

Finally, we can use this information to higlight the current hero in the template.

```html
<div ng-repeat="hero in $ctrl.heroes"
       ng-class="{ selected: $ctrl.isSelected(hero) }">
  <a ng-link="['HeroDetail', {id: hero.id}]">{{hero.name}}</a>
</div>
```

## Crisis Center

Let's implement the Crisis Center feature, which displays a list if crises that need to be dealt with by a hero.
The detailed crisis view has an additional feature where it blocks you from navigating if you have not saved
changes to the crisis being edited.

* A list of Crises that are happening:

![Crisis List View](img/guide/crisis-list.png)

* A detailed view of a single Crisis:

![Crisis Detail View](img/guide/crisis-detail.png)


## Crisis Feature

This feature is very similar to the Heroes feature. It contains the following **Components**:

* CrisisService: contains method for getting a list of crises and an individual crisis.
* CrisisListComponent: displays the list of crises, similar to HeroListComponent.
* CrisisDetailComponent: displays a specific crisis

CrisisService and CrisisListComponent are basically the same as HeroService and HeroListComponent
respectively.

## Navigation Control Hooks

**How do I prevent navigation from occurring?**

Each **Component** can provide the `$canActivate` and `$routerCanDeactivate` **Lifecycle Hooks**. The
`$routerCanDeactivate` hook is an instance method on the **Component**. The `$canActivate` hook is used as a
static method defined on the **Component Definition Object**.

The **Router** will call these hooks to control navigation from one **Route** to another. Each of these hooks can
return a `boolean` or a Promise that will resolve to a `boolean`.

During a navigation, some **Components** will become inactive and some will become active. Before the navigation
can complete, all the **Components** must agree that they can be deactivated or activated, respectively.

The **Router** will call the `$routerCanDeactivate` and `$canActivate` hooks, if they are provided. If any
of the hooks resolve to `false` then the navigation is cancelled.

### Dialog Box Service

We can implement a very simple dialog box that will prompt the user whether they are happy to lose changes they
have made. The result of the prompt is a promise that can be used in a `$routerCanDeactivate` hook.

```js
.service('dialogService', DialogService);

function DialogService($q) {
  this.confirm = function(message) {
    return $q.when(window.confirm(message || 'Is it OK?'));
  };
}
```

### CrisisDetailComponent

We put the template into its own file by using a `templateUrl` property in the **Component Definition
Object**:

```js
  .component('crisisDetail', {
    templateUrl: 'app/crisisDetail.html',
    bindings: { $router: '<' },
    controller: CrisisDetailComponent
  });
```

In the `$routerOnActivate` hook, we make a local copy of the `crisis.name` property to compare with the
original value so that we can determine whether the name has changed.

```js
  this.$routerOnActivate = function(next) {
    // Get the crisis identified by the route parameter
    var id = next.params.id;
    crisisService.getCrisis(id).then(function(crisis) {
      if (crisis) {
        ctrl.editName = crisis.name;  // Make a copy of the crisis name for editing
        ctrl.crisis = crisis;
      } else { // id not found
        ctrl.gotoCrises();
      }
    });
  };
```

In the `$routerCanDeactivate` we check whether the name has been modified and ask whether the user
wishes to discard the changes.

```js
  this.$routerCanDeactivate = function() {
    // Allow synchronous navigation (`true`) if no crisis or the crisis is unchanged.
    if (!this.crisis || this.crisis.name === this.editName) {
      return true;
    }
    // Otherwise ask the user with the dialog service and return its
    // promise which resolves to true or false when the user decides
    return dialogService.confirm('Discard changes?');
  };
```

You can test this check by navigating to a crisis detail page, modifying the name and then either
pressing the browser's back button to navigate back to the previous page, or by clicking on one of
the links to the Crisis Center or Heroes features.

The Save and Cancel buttons update the `editName` and/or `crisis.name` properties before navigating
to prevent the `$routerCanDeactivate` hook from displaying the dialog box.


## Summary

This guide has given an overview of the features of the Component Router and how to implement a simple
application.