Initializing various unique instances in the Angular2 DI constructor

Question

Initializing various unique instances in the Angular2 DI constructor

I'm facing a dilemma with Angular2 DI. Let's say I have a TestService and I need to use 2 distinct instances of this service in one component. However, when I add the provider to the component and include both instances in the constructor, I end up getting the same service instance for both. Here is an 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 displayed in console

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA Id: 0.24242492129168425
BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB Id: 0.24242492129168425

I'm seeking advice on whether there's a way to leverage Angular2's DI to inject multiple unique instances of a service within the same component or if it's better to forego DI in this scenario and manually create the instances using a custom constructor.

Thank you in advance

dependency-injection typescript angular

Answer 1

Answer №1

When dealing with a finite amount of instances, there are different approaches that can be taken:

@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
    ) {}
    ...
}

Alternatively, using the OpaqueToken counterpart can prevent 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
) {}

This approach does not interfere with DI in the TestService constructor and maintains the testability of TestComponent. It allows for independent mocking of both service instances.

Answer 2

When dealing with a finite amount of instances, there are different approaches that can be taken:

@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
    ) {}
    ...
}

Alternatively, using the OpaqueToken counterpart can prevent 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
) {}

This approach does not interfere with DI in the TestService constructor and maintains the testability of TestComponent. It allows for independent mocking of both service instances.

Answer 3

Answer №2

To achieve a new instance every time the factory is called, you can implement a factory function that returns a fresh instance:

@NgModule({
   providers: [{
      provide: 'customService', 
      useFactory: (/* Dependencies for CustomService like `http`*/) => 
        (/* parameters */) => new CustomService(/* http */), 
      deps: [/* Dependencies for CustomService like `Http`*/ ]
    }]
})


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

    constructor(@Inject('customService') private _customServiceFactory) { };

    ngOnInit() {
        console.log("Creating a new instance:", this._customServiceFactory( /* parameters */).toString());
        console.log("Another new instance created:", this._customServiceFactory().toString());
    }
}

Custom Plunker Demo (view the browser console for the output when interacting with the demo)

Answer 4

To achieve a new instance every time the factory is called, you can implement a factory function that returns a fresh instance:

@NgModule({
   providers: [{
      provide: 'customService', 
      useFactory: (/* Dependencies for CustomService like `http`*/) => 
        (/* parameters */) => new CustomService(/* http */), 
      deps: [/* Dependencies for CustomService like `Http`*/ ]
    }]
})


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

    constructor(@Inject('customService') private _customServiceFactory) { };

    ngOnInit() {
        console.log("Creating a new instance:", this._customServiceFactory( /* parameters */).toString());
        console.log("Another new instance created:", this._customServiceFactory().toString());
    }
}

Custom Plunker Demo (view the browser console for the output when interacting with the demo)

Answer 5

Answer №3

To enhance efficiency, I propose implementing a static method within the service to generate a new instance that can be injected into the component using Dependency Injection (DI). Here's an example:

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

@Injectable()
export class CustomService {

    public id: number = Math.random();

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

    static createInstance() {
      return new CustomService();
    }
}

Next, in the component file:

import {Component, OnInit} from "@angular/core";
import {CustomService} from "../../services/custom.service";

@Component({
    providers: [CustomService]
})
export class TestComponent implements OnInit {
    _customService1: CustomService;
    _customService2: CustomService;

    constructor(private customServiceFactory: CustomService) { 
       this._customService1 = this.customServiceFactory.createInstance();
       this._customService2 = this.customServiceFactory.createInstance();
    };

    ngOnInit() {
        console.log("First Instance:", this._customService1.toString());
        console.log("Second Instance:", this._customService2.toString());
    }
}

Answer 6

To enhance efficiency, I propose implementing a static method within the service to generate a new instance that can be injected into the component using Dependency Injection (DI). Here's an example:

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

@Injectable()
export class CustomService {

    public id: number = Math.random();

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

    static createInstance() {
      return new CustomService();
    }
}

Next, in the component file:

import {Component, OnInit} from "@angular/core";
import {CustomService} from "../../services/custom.service";

@Component({
    providers: [CustomService]
})
export class TestComponent implements OnInit {
    _customService1: CustomService;
    _customService2: CustomService;

    constructor(private customServiceFactory: CustomService) { 
       this._customService1 = this.customServiceFactory.createInstance();
       this._customService2 = this.customServiceFactory.createInstance();
    };

    ngOnInit() {
        console.log("First Instance:", this._customService1.toString());
        console.log("Second Instance:", this._customService2.toString());
    }
}

Initializing various unique instances in the Angular2 DI constructor

Answer №1

Answer №2

Answer №3

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