Injecting Dependencies with Angular 2 and the Ability to Include Optional Parameters

One issue I'm facing is that I have multiple components in my Angular 2 application that require the same dependency. This specific dependency needs a string for the constructor. How can I instruct angular2 to use a specific instance of this type for Dependency Injection?

For example:

In ChatUsers.ts:

@Component({
    selector: "chat-users"
})
@View({
    directives: [],
    templateUrl: '/js/components/ChatUsers.html'
})
export class ChatUsers {

    constructor(public currentUser : User) {
    }
}

And in app.ts:

/// <reference path="../libs/typings/tsd.d.ts" />

import {Component, View, bootstrap} from 'angular2/angular2';

import {User} from "User";

// How do I instantiate a user (e.g. new User('John')) and use it for DI?

@Component({
    selector: 'chat-app'
})
@View({
    directives: [ ],
    template: `
      <div> Some text
      </div>`
})
class ChatApp {
    constructor(public user: User) {
        // Perform actions using the user object
    }

}
bootstrap(ChatApp, [ User ]);

In User.ts:

export class User {
    name: string;
    constructor(name: string) {
        this.name = name;
    }
}

When running this code, an error occurs:

Cannot resolve all parameters for User(?). Make sure they all have valid type or annotations.

I am currently using the latest version of Angular 2: 2.0.0-alpha.44

Answer №1

If you want to make a dependency optional, simply use the parameter decorator @Optional (check out this plunker):

class User {
  name: string;
  constructor(@Optional() name: string) {
    this.name = name;
  }
}

There are two ways to inject `name` into `User`:

  1. Add a provider for `'userName'` in the app providers and use @Inject('userName') parameter decorator to inject it into `User` (see this plunker).
class User {
  name: string;
  constructor(@Inject('userName') name: string) {
      this.name = name;
  }
}
// ...
bootstrap(ChatApp, [
  User, 
  provide('userName', { useValue: 'Bob'})
]);
  1. Alternatively, you can use useFactory to instantiate your user specifically (see this plunker):
bootstrap(ChatApp, [
  provide(User, { useFactory: () => new User('John') })
]);

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