Description
Dependency Injection is a design pattern that passes an object as dependencies in different components across the application. It creates a new instance of class along with its required dependencies. The Dependency Injection is stimulated into the framework and can be used everywhere.You must remember the below points while using Dependency Injection:
- The injector mechanism maintains service instance and can be created using a provider.
- The provider is a way of creating a service.
- You can register the providers along with injectors.
Example
The below example describes use of dependency injection in the Angular 2:<!DOCTYPE html> <html> <head> <title>Angular 2 Dependency Injection</title> <script src="https://cdnjs.cloudflare.com/ajax/libs/es6-shim/0.33.3/es6-shim.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/systemjs/0.19.20/system-polyfills.js"></script> <script src="https://code.angularjs.org/2.0.0-beta.6/angular2-polyfills.js"></script> <script src="https://code.angularjs.org/tools/system.js"></script> <script src="https://code.angularjs.org/tools/typescript.js"></script> <script src="https://code.angularjs.org/2.0.0-beta.6/Rx.js"></script> <script src="https://code.angularjs.org/2.0.0-beta.6/angular2.dev.js"></script> <script> System.config({ transpiler: 'typescript', typescriptOptions: { emitDecoratorMetadata: true }, packages: {'app': {defaultExtension: 'ts'}}, map: { 'app': './angular2/src/app' } }); System.import('app/dependency_injection_main') .then(null, console.error.bind(console)); </script> </head> <body> <my-app>Loading...</my-app> </body> </html>The above code includes the following configuration options:
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You can configure the index.html file using typescript version. The SystemJS transpile the TypeScript to JavaScript before running the application by using the transpiler option.
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If you do not transpile to JavaScript before running the application, you could see the compiler warnings and errors which are hidden in the browser.
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The TypeScript generates metadata for each and every class of the code when the emitDecoratorMetadata option is set. If you don't specify this option, large amount of unused metadata will be generated which affects the file size and impact on the application runtime.
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Angular 2 includes the packages form the app folder where files will have the .ts extension.
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Next it will load the main component file from the app folder. If there is no main component file found, then it will display the error in the console.
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When Angular calls the bootstrap function in main.ts, it reads the Component metadata, finds the 'app' selector, locates an element tag named app, and loads the application between those tags.
dependency_injection_main.ts
import {bootstrap} from 'angular2/platform/browser'; //importing bootstrap function import {AppComponent} from "./fruit-component"; //importing component function bootstrap(AppComponent);fruit-component.ts
import {Component} from 'angular2/core'; import {MyListComponent} from './play-component'; //@Component is a decorator that uses configuration object to create the component and its view. @Component({ selector: 'my-app', //The selector creates an instance of the component where it finds 'my-app' tag in parent HTML template:`<my-list></my-list>`, directives:[MyListComponent] //'MyListComponent' is the root component of fruits that governs child components }) export class AppComponent { }play-component.ts
import {Component} from "angular2/core"; import {FruitService} from "./fruits.service"; import {Fruits} from "./fruits"; import {OnInit} from "angular2/core"; @Component({ selector: "my-list", template: ` List of Fruits<br> <ul> <li *ngFor="#list of fruits"> //The NgFor directive instantiates a template once per item from an iterable {{list.id}} - {{ list.name }} </li> </ul> `, providers: [FruitService] //providers are part of @Component metadata }) //The 'MyListComponent' should get list of fruits from the injected 'FruitService' export class MyListComponent implements OnInit { public fruits : Country[]; //Using constructor, call the _fruitService and populate the fruits list constructor(private _fruitService: FruitService) {} getContacts(){ this._fruitService.getContacts().then((fruits: Country[]) => this.fruits = fruits); } //The 'ngOnInit()' hook is called when done with creating the component and evaluated the inputs ngOnInit():any{ this.getContacts(); } }fruits.service.ts
import {Injectable} from "angular2/core"; import {COUNTRIES} from "./fruits.lists"; //It is used for meta data generation and specifies that class is available to an injector for instantiation @Injectable() //'FruitService' exposes 'getContacts()' method that returns list of data export class FruitService { getContacts() { return Promise.resolve(COUNTRIES); // takes values from fruits.lists.ts file } }fruits.lists.ts
import {Fruits} from "./fruits"; //storing array of data in Fruits export const COUNTRIES: Fruits[] =[ {"id": 1, name :"Apple"}, {"id": 2, name: "Grapes"}, {"id": 3, name: "Orange"}, {"id": 4, name: "Banana"} ];fruits.ts
export interface Fruits{ id: number; name: string; }
Output
Let's carry out the following steps to see how above code works:- Save above HTML code as index.html file as how we created in environment chapter and use the above app folder which contains .ts files.
- Open the terminal window and enter the below command:
npm start - After few moments, a browser tab should open and displays the output as shown below.
- Save above HTML code as angular2_dependency_injection.html file in your server root folder.
- Open this HTML file as http://localhost/angular2_dependency_injection.html and output as below gets displayed.
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