Angular2 DI - initializing multiple different inst

问题:

I have an Angular2 DI question. Say I have a TestService and I want to use 2 different instances of this service inside the same component. If I simply add a provider to the component and I add the 2 instances to the constructor I end up with the same service instance. For example:

TestService

import {Injectable} from "@angular/core";

@Injectable()
export class TestService {

    public id: number = Math.random();

    public toString(): string {
        return "Id: " + this.id;
    }
}

Test component

import {Component, Input, OnInit} from "@angular/core";
import {TestService} from "../../services/test.service";

@Component({
    providers: [TestService]
})
export class TestComponent implements OnInit {

    constructor(private _testService1: TestService, private _testService2: TestService) { };

    ngOnInit() {
        console.log("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA", this._testService1.toString());
        console.log("BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB", this._testService2.toString());
    }
}

Result in console

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA Id: 0.24242492129168425
BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB Id: 0.24242492129168425

Can someone pls tell me if there's a way to use Angular2's DI mechanism to inject multiple different instances of a service within the same component or should I just drop the DI for this particular case and create my instances manually with a manual constructor?

Thanks in advance

回答1:

You can inject a factory that returns a new instance every time you call it:

@NgModule({
   providers: [{
      provide: 'testService', 
      useFactory: (/* TestService deps here like `http`*/) => 
        (/* params */) => new TestService(/* http */), 
      deps: [/* TestService deps here like `Http`*/ ]
    }]
})


@Component(...)
export class TestComponent implements OnInit {

    constructor(@Inject('testService') private _testServiceFactory) { };

    ngOnInit() {
        console.log("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA", this._testServiceFactory( /* params */).toString());
        console.log("BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB", this._testServiceFactory().toString());
    }
}

Plunker example (check the output in the browser console when you click the button)

回答2:

Given that there is finite amount of instances, the straightforward way may be:

@Component({
    providers: [
        { provide: 'TestService1', useClass: TestService },
        { provide: 'TestService2', useClass: TestService }
    ]
})
export class TestComponent implements OnInit {
    constructor(
        @Inject('TestService1') private _testService1: TestService,
        @Inject('TestService2') private _testService2: TestService
    ) {}
    ...
}

Or OpaqueToken counterpart to avoid overriding services with the same string identifier:

export const TestService1 = new OpaqueToken;
export const TestService2 = new OpaqueToken;

...
providers: [
    { provide: TestService1, useClass: TestService },
    { provide: TestService2, useClass: TestService }
]
...
constructor(
    @Inject(TestService1) private _testService1: TestService,
    @Inject(TestService2) private _testService2: TestService
) {}

It doesn't hurt DI in TestService constructor. And keeps the testability of TestComponent on par, both service instances can be mocked independently.

回答3:

I would create a static method that returns new instance in the service, and inject it only one through DI in component. Something like:

import {Injectable} from "@angular/core";

@Injectable()
export class TestService {

    public id: number = Math.random();

    public toString(): string {
        return "Id: " + this.id;
    }
    static init() {
      return new TestService();
    }
}

then in component:

import {Component, Input, OnInit} from "@angular/core";
import {TestService} from "../../services/test.service";

@Component({
    providers: [TestService]
})
export class TestComponent implements OnInit {
    _testService1: TestService;
    _testService2: TestService;

    constructor(_testFactory: TestService) { 
       this._testService1 = _testFactory.init();
       this._testService2 = _testFactory.init();
    };

    ngOnInit() {
        console.log("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA", this._testService1.toString());
        console.log("BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB", this._testService2.toString());
    }
}