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Zhicheng Wang 2016-04-15 22:32:57 +08:00
parent e42953db5c 3547e4cc15
commit cb964960ab
33 changed files with 729 additions and 36 deletions
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2
public/docs/_examples/architecture/dart/pubspec.yaml
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@ -5,7 +5,7 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:

2
public/docs/_examples/attribute-directives/dart/pubspec.yaml
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@ -5,7 +5,7 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:

2
public/docs/_examples/dependency-injection/dart/pubspec.yaml
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@ -5,7 +5,7 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:

2
public/docs/_examples/displaying-data/dart/pubspec.yaml
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@ -5,7 +5,7 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:

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public/docs/_examples/forms/dart/pubspec.yaml
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@ -5,7 +5,7 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:

2
public/docs/_examples/hierarchical-dependency-injection/dart/pubspec.yaml
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@ -5,7 +5,7 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:

2
public/docs/_examples/lifecycle-hooks/dart/pubspec.yaml
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@ -5,7 +5,7 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:

2
public/docs/_examples/pipes/dart/pubspec.yaml
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@ -5,7 +5,7 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:

2
public/docs/_examples/quickstart/dart/pubspec.yaml
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@ -7,7 +7,7 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
# #enddocregion no-rewriter
dart_to_js_script_rewriter: ^1.0.1

2
public/docs/_examples/structural-directives/dart/pubspec.yaml
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@ -5,7 +5,7 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:

2
public/docs/_examples/template-syntax/dart/pubspec.yaml
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@ -5,7 +5,7 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:

2
public/docs/_examples/toh-1/dart/pubspec.yaml
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@ -4,7 +4,7 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:

4
public/docs/_examples/toh-1/dart/web/main.dart
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@ -1,9 +1,9 @@
// #docregion pt1
import 'package:angular2/bootstrap.dart';
import 'package:angular2/platform/browser.dart';
import 'package:angular2_tour_of_heroes/app_component.dart';
main() {
bootstrap(AppComponent);
}
// #enddocregion pt1
// #enddocregion pt1

4
public/docs/_examples/toh-2/dart/pubspec.yaml
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@ -3,9 +3,9 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.1
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^0.1.0+4
dart_to_js_script_rewriter: ^1.0.1
transformers:
- angular2:
platform_directives:

4
public/docs/_examples/toh-2/dart/web/main.dart
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@ -1,9 +1,9 @@
// #docregion pt2
import 'package:angular2/bootstrap.dart';
import 'package:angular2/platform/browser.dart';
import 'package:angular2_tour_of_heroes/app_component.dart';
main() {
bootstrap(AppComponent);
}
// #enddocregion pt2
// #enddocregion pt2

2
public/docs/_examples/toh-3/dart/pubspec.yaml
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@ -3,7 +3,7 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:

4
public/docs/_examples/toh-3/dart/web/main.dart
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@ -1,9 +1,9 @@
// #docregion pt1
import 'package:angular2/bootstrap.dart';
import 'package:angular2/platform/browser.dart';
import 'package:angular2_tour_of_heroes/app_component.dart';
main() {
bootstrap(AppComponent);
}
// #enddocregion pt1
// #enddocregion pt1

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public/docs/_examples/toh-4/dart/lib/app_component.dart Normal file
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@ -0,0 +1,106 @@
// #docplaster
// #docregion
import 'dart:async';
import 'package:angular2/core.dart';
import 'hero.dart';
import 'hero_detail_component.dart';
// #docregion hero-service-import
import 'hero_service.dart';
// #enddocregion hero-service-import
@Component(
selector: 'my-app',
template: '''
<h1>{{title}}</h1>
<h2>My Heroes</h2>
<ul class="heroes">
<li *ngFor="#hero of heroes"
[class.selected]="hero == selectedHero"
(click)="onSelect(hero)">
<span class="badge">{{hero.id}}</span> {{hero.name}}
</li>
</ul>
<my-hero-detail [hero]="selectedHero"></my-hero-detail>
''',
styles: const [
'''
.selected {
background-color: #CFD8DC !important;
color: white;
}
.heroes {
margin: 0 0 2em 0;
list-style-type: none;
padding: 0;
width: 10em;
}
.heroes li {
cursor: pointer;
position: relative;
left: 0;
background-color: #EEE;
margin: .5em;
padding: .3em 0em;
height: 1.6em;
border-radius: 4px;
}
.heroes li.selected:hover {
color: white;
}
.heroes li:hover {
color: #607D8B;
background-color: #EEE;
left: .1em;
}
.heroes .text {
position: relative;
top: -3px;
}
.heroes .badge {
display: inline-block;
font-size: small;
color: white;
padding: 0.8em 0.7em 0em 0.7em;
background-color: #607D8B;
line-height: 1em;
position: relative;
left: -1px;
top: -4px;
height: 1.8em;
margin-right: .8em;
border-radius: 4px 0px 0px 4px;
}
'''
],
directives: const [
HeroDetailComponent
],
providers: const [
HeroService
])
class AppComponent implements OnInit {
String title = 'Tour of Heroes';
List<Hero> heroes;
Hero selectedHero;
final HeroService _heroService;
AppComponent(this._heroService);
// #docregion get-heroes
getHeroes() async {
heroes = await _heroService.getHeroes();
}
// #enddocregion get-heroes
ngOnInit() {
getHeroes();
}
onSelect(Hero hero) {
selectedHero = hero;
}
}

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public/docs/_examples/toh-4/dart/lib/app_component_1.dart Normal file
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@ -0,0 +1,64 @@
// #docplaster
// #docregion on-init
import 'package:angular2/core.dart';
// #enddocregion on-init
import 'hero.dart';
import 'hero_detail_component.dart';
// #docregion hero-service-import
import 'hero_service_1.dart';
// #enddocregion hero-service-import
// Testable but never shown
@Component(
selector: 'my-app',
template: '''
<div *ngFor="#hero of heroes" (click)="onSelect(hero)">
{{hero.name}}
</div>
<my-hero-detail [hero]="selectedHero"></my-hero-detail>
''',
directives: const [HeroDetailComponent],
// #docregion providers
providers: const [HeroService])
// #enddocregion providers
// #docregion on-init
class AppComponent implements OnInit {
// #enddocregion on-init
String title = 'Tour of Heroes';
// #docregion heroes-prop
List<Hero> heroes;
// #enddocregion heroes-prop
Hero selectedHero;
// #docregion new-service
HeroService heroService = new HeroService(); // don't do this
// #enddocregion new-service
// #docregion ctor
final HeroService _heroService;
AppComponent(this._heroService);
// #enddocregion ctor
// #docregion getHeroes
getHeroes() {
//#docregion get-heroes
heroes = _heroService.getHeroes();
// #enddocregion get-heroes
}
// #enddocregion getHeroes
// #docregion ng-on-init
// #docregion on-init
ngOnInit() {
// #enddocregion on-init
getHeroes();
// #docregion on-init
}
// #enddocregion on-init
// #enddocregion ng-on-init
onSelect(Hero hero) {
selectedHero = hero;
}
// #docregion on-init
}
// #enddocregion on-init

6
public/docs/_examples/toh-4/dart/lib/hero.dart Normal file
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@ -0,0 +1,6 @@
class Hero {
final int id;
String name;
Hero(this.id, this.name);
}

20
public/docs/_examples/toh-4/dart/lib/hero_detail_component.dart Normal file
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@ -0,0 +1,20 @@
import 'package:angular2/core.dart';
import 'hero.dart';
@Component(
selector: 'my-hero-detail',
template: '''
<div *ngIf="hero != null">
<h2>{{hero.name}} details!</h2>
<div><label>id: </label>{{hero.id}}</div>
<div>
<label>name: </label>
<input [(ngModel)]="hero.name" placeholder="name"/>
</div>
</div>
''',
inputs: const ['hero'])
class HeroDetailComponent {
Hero hero;
}

23
public/docs/_examples/toh-4/dart/lib/hero_service.dart Normal file
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@ -0,0 +1,23 @@
// #docplaster
// #docregion
import 'dart:async';
import 'package:angular2/core.dart';
import 'hero.dart';
import 'mock_heroes.dart';
@Injectable()
class HeroService {
//#docregion get-heroes
Future<List<Hero>> getHeroes() async => mockHeroes;
//#enddocregion get-heroes
// See the "Take it slow" appendix
//#docregion get-heroes-slowly
Future<List<Hero>> getHeroesSlowly() {
return new Future.delayed(const Duration(seconds: 2), () => mockHeroes);
}
//#enddocregion get-heroes-slowly
}
// #enddocregion

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public/docs/_examples/toh-4/dart/lib/hero_service_1.dart Normal file
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@ -0,0 +1,28 @@
// #docplaster
// #docregion final
// #docregion empty-class
import 'package:angular2/core.dart';
// #enddocregion empty-class
import 'hero.dart';
import 'mock_heroes.dart';
// #docregion getHeroes-stub
@Injectable()
class HeroService {
// #enddocregion getHeroes-stub
// #enddocregion empty-class
// #enddocregion final
/*
// #docregion getHeroes-stub
List<Hero> getHeroes() {}
// #enddocregion getHeroes-stub
*/
// #docregion final
List<Hero> getHeroes() => mockHeroes;
// #docregion empty-class
// #docregion getHeroes-stub
}
// #enddocregion getHeroes-stub
// #enddocregion empty-class
// #enddocregion final

16
public/docs/_examples/toh-4/dart/lib/mock_heroes.dart Normal file
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@ -0,0 +1,16 @@
// #docregion
import 'hero.dart';
final List<Hero> mockHeroes = [
new Hero(11, "Mr. Nice"),
new Hero(12, "Narco"),
new Hero(13, "Bombasto"),
new Hero(14, "Celeritas"),
new Hero(15, "Magneta"),
new Hero(16, "RubberMan"),
new Hero(17, "Dynama"),
new Hero(18, "Dr IQ"),
new Hero(19, "Magma"),
new Hero(20, "Tornado")
];
// #enddocregion

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public/docs/_examples/toh-4/dart/pubspec.yaml Normal file
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@ -0,0 +1,15 @@
name: angular2_tour_of_heroes
version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:
- angular2:
platform_directives:
- package:angular2/common.dart#COMMON_DIRECTIVES
platform_pipes:
- package:angular2/common.dart#COMMON_PIPES
entry_points: web/main.dart

10
public/docs/_examples/toh-4/dart/web/index.html Normal file
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@ -0,0 +1,10 @@
<!DOCTYPE html>
<html>
<head>
<script defer src="main.dart" type="application/dart"></script>
<script defer src="packages/browser/dart.js"></script>
</head>
<body>
<my-app>Loading...</my-app>
</body>
</html>

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public/docs/_examples/toh-4/dart/web/main.dart Normal file
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@ -0,0 +1,9 @@
// #docregion pt1
import 'package:angular2/platform/browser.dart';
import 'package:angular2_tour_of_heroes/app_component.dart';
main() {
bootstrap(AppComponent);
}
// #enddocregion pt1

9
public/docs/_examples/toh-4/dart/web/main_1.dart Normal file
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@ -0,0 +1,9 @@
// #docregion pt1
import 'package:angular2/platform/browser.dart';
import 'package:angular2_tour_of_heroes/app_component_1.dart';
main() {
bootstrap(AppComponent);
}
// #enddocregion pt1

8
public/docs/_examples/toh-5/dart/pubspec.yaml
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@ -5,12 +5,14 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:
- angular2:
platform_directives: package:angular2/common.dart#COMMON_DIRECTIVES
platform_pipes: package:angular2/common.dart#COMMON_PIPES
platform_directives:
- package:angular2/common.dart#COMMON_DIRECTIVES
platform_pipes:
- package:angular2/common.dart#COMMON_PIPES
entry_points: web/main.dart
- dart_to_js_script_rewriter

4
public/docs/_examples/toh-5/dart/web/index.html
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@ -3,8 +3,8 @@
<!-- #docregion head -->
<!-- #docregion base-href -->
<head>
<!-- base href="/" --> <!-- For testing using pub serve directly -->
<base href="/dart/web/"> <!-- For testing in WebStorm -->
<base href="/"> <!-- For testing using pub serve directly -->
<!-- base href="/dart/web/" --> <!-- For testing in WebStorm -->
<!-- #enddocregion base-href -->
<title>Angular 2 Tour of Heroes</title>
<meta name="viewport" content="width=device-width, initial-scale=1">

2
public/docs/_examples/user-input/dart/pubspec.yaml
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@ -5,7 +5,7 @@ version: 0.0.1
environment:
sdk: '>=1.13.0 <2.0.0'
dependencies:
angular2: 2.0.0-beta.14
angular2: 2.0.0-beta.15
browser: ^0.10.0
dart_to_js_script_rewriter: ^1.0.1
transformers:

2
public/docs/dart/latest/quickstart.jade
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@ -41,7 +41,7 @@ p.
specify the angular2 and browser packages as dependencies,
as well as the angular2 transformer.
Angular 2 is still changing, so provide an exact version:
<b>2.0.0-beta.14</b>.
<b>2.0.0-beta.15</b>.
+makeExample('quickstart/dart/pubspec.yaml', 'no-rewriter', 'pubspec.yaml')

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public/docs/dart/latest/tutorial/toh-pt4.jade
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@ -1,14 +1,399 @@
include ../_util-fns
:marked
We're working on the Dart version of this case study.
In the meantime, please see these resources:
# Services
The Tour of Heroes is evolving and we anticipate adding more components in the near future.
* [Services](/docs/ts/latest/tutorial/toh-pt4.html):
The TypeScript version of this chapter
Multiple components will need access to hero data and we don't want to copy and
paste the same code over and over.
Instead, we'll create a single reusable data service and learn to
inject it in the components that need it.
* [Dart source code](https://github.com/angular/angular.io/tree/master/public/docs/_examples/toh-5/dart):
A preliminary Dart version of the Tour of Heroes app,
featuring the hero editor, a master/detail page,
multiple components, services, and routing
Refactoring data access to a separate service keeps the component lean and focused on supporting the view.
It also makes it easier to unit test the component with a mock service.
Because data services are invariably asynchronous,
we'll finish the chapter with a promise-based version of the data service.
The complete source code for the example app in this chapter is
[in GitHub](https://github.com/angular/angular.io/tree/master/public/docs/_examples/toh-4/dart).
.l-main-section
:marked
## Where We Left Off
Before we continue with our Tour of Heroes, let’s verify we have the following structure.
If not, we’ll need to go back and follow the previous chapters.
.filetree
.file angular2_tour_of_heroes
.children
.file lib
.children
.file app_component.dart
.file hero.dart
.file hero_detail_component.dart
.file web
.children
.file index.html
.file main.dart
.file pubspec.yaml
:marked
### Keep the app compiling and running
Open a terminal/console window.
Start the Dart compiler, watch for changes, and start our server by entering the command:
code-example(format="." language="bash").
pub serve
:marked
The application runs and updates automatically as we continue to build the Tour of Heroes.
## Creating a Hero Service
Our stakeholders have shared their larger vision for our app.
They tell us they want to show the heroes in various ways on different pages.
We already can select a hero from a list.
Soon we'll add a dashboard with the top performing heroes and create a separate view for editing hero details.
All three views need hero data.
At the moment the `AppComponent` defines mock heroes for display.
We have at least two objections.
First, defining heroes is not the component's job.
Second, we can't easily share that list of heroes with other components and views.
We can refactor this hero data acquisition business to a single service that provides heroes and
share that service with all components that need heroes.
### Create the HeroService
Create a file in the `lib` folder called `hero_service.dart`.
.l-sub-section
:marked
We've adopted a convention in which we spell the name of a service in lowercase followed by `_service`.
If the service name were multi-word, we'd spell the base filename with lower underscore case (AKA "snake_case").
The `SpecialSuperHeroService` would be defined in the `special_super_hero_service.dart` file.
:marked
We name the class `HeroService`.
+makeExample('toh-4/dart/lib/hero_service_1.dart', 'empty-class', 'hero_service.dart (class)')(format=".")
:marked
### Injectable Services
Notice that we used an `@Injectable()` annotation.
.callout.is-helpful
:marked
**Don't forget the parentheses!** Neglecting them leads to an error that's difficult to diagnose.
:marked
Dart sees the `@Injectable()` annotation and emits metadata about our service,
metadata that Angular may need to inject other dependencies into this service.
The `HeroService` doesn't have any dependencies *at the moment*. Add the annotation anyway.
It is a "best practice" to apply the `@Injectable()` annotation ​*from the start*​
both for consistency and for future-proofing.
:marked
### Getting Heroes
Add a `getHeroes` method stub.
+makeExample('toh-4/dart/lib/hero_service_1.dart', 'getHeroes-stub', 'hero_service.dart (getHeroes stub)')(format=".")
:marked
We're holding back on the implementation for a moment to make an important point.
The consumer of our service doesn't know how the service gets the data.
Our `HeroService` could get `Hero` data from anywhere.
It could get the data from a web service or local storage
or from a mock data source.
That's the beauty of removing data access from the component.
We can change our minds about the implementation as often as we like,
for whatever reason, without touching any of the components that need heroes.
### Mock Heroes
We already have mock `Hero` data sitting in the `AppComponent`. It doesn't belong there. It doesn't belong *here* either.
We'll move the mock data to its own file.
Cut the the `mockHeroes` list from `app_component.dart` and paste it to a new file in the `lib` folder named `mock_heroes.dart`.
We copy the `import 'hero.dart'` statement as well because the heroes list uses the `Hero` class.
+makeExample('toh-4/dart/lib/mock_heroes.dart', null, 'mock_heroes.dart (Heroes list)')
:marked
Meanwhile, back in `app_component.dart` where we cut away the `mockHeroes` list,
we leave behind an uninitialized `heroes` property:
+makeExample('toh-4/dart/lib/app_component_1.dart', 'heroes-prop', 'app_component.dart (heroes property)')(format=".")
:marked
### Return Mocked Heroes
Back in the `HeroService` we import the mock `mockHeroes` and return it from the `getHeroes` method.
Our `HeroService` looks like this:
+makeExample('toh-4/dart/lib/hero_service_1.dart', 'final', 'hero_service.dart')(format=".")
:marked
### Use the Hero Service
We're ready to use the `HeroService` in other components starting with our `AppComponent`.
We begin, as usual, by importing the thing we want to use, the `HeroService`.
+makeExample('toh-4/dart/lib/app_component.dart', 'hero-service-import', 'app_component.dart (import HeroService)')(format=".")
:marked
Importing the service allows us to *reference* it in our code.
How should the `AppComponent` acquire a runtime concrete `HeroService` instance?
### Do we *new* the *HeroService*? No way!
We could create a new instance of the `HeroService` with "new" like this:
+makeExample('toh-4/dart/lib/app_component_1.dart', 'new-service')(format=".")
:marked
That's a bad idea for several reasons including
* Our component has to know how to create a `HeroService`.
If we ever change the `HeroService` constructor,
we'll have to find every place we create the service and fix it.
Running around patching code is error prone and adds to the test burden.
* We create a new service each time we use "new".
What if the service should cache heroes and share that cache with others?
We couldn't do that.
* We're locking the `AppComponent` into a specific implementation of the `HeroService`.
It will be hard to switch implementations for different scenarios.
Can we operate offline?
Will we need different mocked versions under test?
Not easy.
*What if ... what if ... Hey, we've got work to do!*
We get it. Really we do.
But it is so ridiculously easy to avoid these problems that there is no excuse for doing it wrong.
### Inject the *HeroService*
Three lines replace the one line of *new*:
1. We add a property.
1. We add a constructor that sets the property.
1. We add to the component's `providers` metadata.
Here are the property and the constructor:
+makeExample('toh-4/dart/lib/app_component_1.dart', 'ctor', 'app_component.dart (constructor)')(format='.')
:marked
The constructor does nothing except set the `_heroService`
property. The `HeroService` type of `_heroService`
identifies the constructor's parameter as
a `HeroService` injection site.
Now Angular will know to supply an instance of the `HeroService` when it creates a new `AppComponent`.
Angular has to get that instance from somewhere. That's the role of the Angular *Dependency Injector*.
The **Injector** has a **container** of previously created services.
Either it finds and returns a pre-existing `HeroService` from its container or it creates a new instance, adds
it to the container, and returns it to Angular.
.l-sub-section
:marked
Learn more about Dependency Injection in the [Dependency Injection](../guide/dependency-injection.html) chapter.
:marked
The *injector* does not know yet how to create a `HeroService`.
If we ran our code now, Angular would fail with an error:
code-example(format="." language="html").
EXCEPTION: No provider for HeroService! (AppComponent -> HeroService)
:marked
We have to teach the *injector* how to make a `HeroService` by registering a `HeroService` **provider**.
Do that by adding the following `providers` parameter to the bottom of the component metadata
in the `@Component` annotation.
+makeExample('toh-4/dart/lib/app_component_1.dart', 'providers', 'app_component.dart (providing HeroService)')(format=".")
:marked
The `providers` parameter tells Angular to create a fresh instance of the `HeroService` when it creates a new `AppComponent`.
The `AppComponent` can use that service to get heroes and so can every child component of its component tree.
<a id="child-component"></a>
.l-sub-section
:marked
### Services and the component tree
Recall that the `AppComponent` creates an instance of `HeroDetail` by virtue of the
`<my-hero-detail>` tag at the bottom of its template. That `HeroDetail` is a child of the `AppComponent`.
If the `HeroDetailComponent` needed its parent component's `HeroService`,
it would ask Angular to inject the service into its constructor which would look just like the one for `AppComponent`:
+makeExample('toh-4/dart/lib/app_component_1.dart', 'ctor', 'hero_detail_component.dart (constructor)')(format=".")
:marked
The `HeroDetailComponent` must *not* repeat its parent's `providers` list! Guess [why](#shadow-provider).
The `AppComponent` is the top level component of our application.
There should be only one instance of that component and only one instance of the `HeroService` in our entire app.
:marked
### *getHeroes* in the *AppComponent*
We've got the service in a `_heroService` private variable. Let's use it.
We pause to think. We can call the service and get the data in one line.
+makeExample('toh-4/dart/lib/app_component_1.dart', 'get-heroes')(format=".")
:marked
We don't really need a dedicated method to wrap one line. We write it anyway:
+makeExample('toh-4/dart/lib/app_component_1.dart', 'getHeroes', 'app_component.dart (getHeroes)')(format=".")
<a id="oninit"></a>
:marked
### The *ngOnInit* Lifecycle Hook
`AppComponent` should fetch and display heroes without a fuss.
Where do we call the `getHeroes` method? In a constructor? We do *not*!
Years of experience and bitter tears have taught us to keep complex logic out of the constructor,
especially anything that might call a server as a data access method is sure to do.
The constructor is for simple initializations like wiring constructor parameters to properties.
It's not for heavy lifting. We should be able to create a component in a test and not worry that it
might do real work &mdash; like calling a server! &mdash; before we tell it to do so.
If not the constructor, something has to call `getHeroes`.
Angular will call it if we implement the Angular **ngOnInit** *Lifecycle Hook*.
Angular offers a number of interfaces for tapping into critical moments in the component lifecycle:
at creation, after each change, and at its eventual destruction.
Each interface has a single method. When the component implements that method, Angular calls it at the appropriate time.
.l-sub-section
:marked
Learn more about lifecycle hooks in the [Lifecycle Hooks](../guide/lifecycle-hooks.html) chapter.
:marked
Here's the essential outline for the `OnInit` interface:
+makeExample('toh-4/dart/lib/app_component_1.dart', 'on-init', 'app_component.dart (OnInit protocol)')(format=".")
:marked
We write an `ngOnInit` method with our initialization logic inside and leave it to Angular to call it
at the right time. In our case, we initialize by calling `getHeroes`.
+makeExample('toh-4/dart/lib/app_component_1.dart', 'ng-on-init', 'app_component.dart (OnInit protocol)')(format=".")
:marked
Our application should be running as expected, showing a list of heroes and a hero detail view
when we click on a hero name.
We're getting closer. But something isn't quite right.
## Async Services and Futures
Our `HeroService` returns a list of mock heroes immediately.
Its `getHeroes` signature is synchronous
+makeExample('toh-4/dart/lib/app_component_1.dart', 'get-heroes')(format=".")
:marked
Ask for heroes and they are there in the returned result.
Someday we're going to get heroes from a remote server. We don’t call http yet, but we aspire to in later chapters.
When we do, we'll have to wait for the server to respond and we won't be able to block the UI while we wait,
even if we want to (which we shouldn't) because the browser won't block.
We'll have to use some kind of asynchronous technique and that will change the signature of our `getHeroes` method.
We'll use *futures*.
### The Hero Service returns a future
We ask an asynchronous service to do some work and give us the result in the future.
The service does that work (somewhere) and eventually it updates the future with the results of the work or an error.
.l-sub-section
:marked
We are simplifying. Learn about Futures in the tutorial
[Asynchronous Programming: Futures](https://www.dartlang.org/docs/tutorials/futures/).
:marked
Update the `HeroService` with this future-returning `getHeroes` method:
+makeExample('toh-4/dart/lib/hero_service.dart', 'get-heroes', 'hero_service.dart (getHeroes)')(format=".")
:marked
We're still mocking the data. We're simulating the behavior of an ultra-fast, zero-latency server,
by returning a future that will quickly resolve with our mock heroes as the result.
.l-sub-section
:marked
Marking the method's body with `async` makes the method immediately return a `Future` object.
That future later completes with the method's return value.
For more information on async functions, see
[Declaring async functions](https://www.dartlang.org/docs/dart-up-and-running/ch02.html#async) in the Dart language tour.
:marked
### Act on the Futures
Returning to the `AppComponent` and its `getHeroes` method, we see that it still looks like this:
+makeExample('toh-4/dart/lib/app_component_1.dart', 'getHeroes', 'app_component.dart (getHeroes - old)')(format=".")
:marked
As a result of our change to `HeroService`, we're now setting `heroes` to a future rather than a list of heroes.
We have to change our implementation to *act on the future when it resolves*.
We can *await* the future to resolve, and then display the heroes:
+makeExample('toh-4/dart/lib/app_component.dart', 'get-heroes', 'app_component.dart (getHeroes - revised)')(format=".")
:marked
Our code waits until the future completes, and then
sets the component's `heroes` property to the list of heroes returned by the service. That's all there is to it!
Our app should still be running, still showing a list of heroes, and still
responding to a name selection with a detail view.
.l-sub-section
:marked
Check out the "[Take it slow](#slow)" appendix to see what the app might be like with a poor connection.
:marked
### Review the App Structure
Let’s verify that we have the following structure after all of our good refactoring in this chapter:
.filetree
.file angular2_tour_of_heroes
.children
.file lib
.children
.file app_component.dart
.file hero.dart
.file hero_detail_component.dart
.file hero_service.dart
.file mock_heroes.dart
.file web
.children
.file index.html
.file main.dart
.file pubspec.yaml
:marked
Here are the code files we discussed in this chapter.
+makeTabs(`
toh-4/dart/lib/hero_service.dart,
toh-4/dart/lib/app_component.dart,
toh-4/dart/lib/mock_heroes.dart
`,'',`
lib/hero_service.dart,
lib/app_component.dart,
lib/mock_heroes.dart
`)
:marked
## The Road We’ve Travelled
Let’s take stock of what we’ve built.
* We created a service class that can be shared by many components
* We used the `ngOnInit` Lifecycle Hook to get our heroes when our `AppComponent` activates
* We defined our `HeroService` as a provider for our `AppComponent`
* We created mock hero data and imported them into our service
* We designed our service to return a future and our component to get our data from the future
### The Road Ahead
Our Tour of Heroes has become more reusable using shared components and services.
We want to create a dashboard, add menu links that route between the views, and format data in a template.
As our app evolves, we’ll learn how to design it to make it easier to grow and maintain.
We learn about Angular Component Router and navigation among the views in the [next tutorial](toh-pt5.html) chapter.
.l-main-section
<a id="slow"></a>
:marked
### Appendix: Take it slow
We can simulate a slow connection.
Add the following `getHeroesSlowly` method to the `HeroService`:
+makeExample('toh-4/dart/lib/hero_service.dart', 'get-heroes-slowly', 'hero_service.dart (getHeroesSlowly)')(format=".")
:marked
Like `getHeroes`, it also returns a future.
But this future waits 2 seconds before resolving the future with mock heroes.
Back in the `AppComponent`, replace
`_heroService.getHeroes` with `_heroService.getHeroesSlowly`
and see how the app behaves.
.l-main-section
<a id="shadow-provider"></a>
:marked
### Appendix: Shadowing the parent's service
We stated [earlier](#child-component) that if we injected the parent `AppComponent` `HeroService`
into the `HeroDetailComponent`, *we must not add a providers list* to the `HeroDetailComponent` metadata.
Why? Because that tells Angular to create a new instance of the `HeroService` at the `HeroDetailComponent` level.
The `HeroDetailComponent` doesn't want its *own* service instance; it wants its *parent's* service instance.
Adding the `providers` list creates a new service instance that shadows the parent instance.
Think carefully about where and when to register a provider.
Understand the scope of that registration. Be careful not to create a new service instance at the wrong level.