Tips for testing an Angular component with a dependency on a service that includes a BehaviorSubject

Question

Tips for testing an Angular component with a dependency on a service that includes a BehaviorSubject

I'm relatively new to Angular and I'm currently experimenting with testing the implementation of a component that relies on a RecipesServices service containing a BehaviorSubject named selectedRecipe:

@Component({
  selector: 'app-recipe',
  templateUrl: './recipe.page.html',
  styleUrls: ['./recipe.page.scss'],
})
export class RecipePage implements OnInit {
  selectedRecipe: Recipe;
  constructor(
    private recipesService: RecipesService
  ) {
    this.recipesService.selectedRecipe.subscribe(newRecipe => this.selectedRecipe = newRecipe);
  }
}

Below is the structure of the service:

@Injectable({
  providedIn: 'root'
})
export class RecipesService {

  /**
   * Representing the recipe selected by the user
   */
  readonly selectedRecipe : BehaviorSubject<Recipe> = new BehaviorSubject(null);

  constructor(
    private httpClient: HttpClient
  ) {}
...
}

I've been attempting various methods to mock this service and include it as a provider in the component's test, but I seem to be running out of ideas. The current test I'm working on yields an error stating "Failed: this.recipesService.selectedRecipe.subscribe is not a function":

import { HttpClient } from '@angular/common/http';
import { ComponentFixture, fakeAsync, TestBed, waitForAsync } from '@angular/core/testing';
import { Router, UrlSerializer } from '@angular/router';
import { IonicModule } from '@ionic/angular';
import { BehaviorSubject, defer, Observable, of, Subject } from 'rxjs';
import { Recipe } from '../recipes-list/recipe';
import { RecipesService } from '../recipes-list/services/recipes.service';

import { RecipePage } from './recipe.page';

let mockrecipesService = {
  selectedRecipe: BehaviorSubject
}

describe('RecipePage', () => {
  let component: RecipePage;
  let fixture: ComponentFixture<RecipePage>;
  var httpClientStub: HttpClient;
  let urlSerializerStub = {};
  let routerStub = {};

  beforeEach(waitForAsync(() => {

    TestBed.configureTestingModule({
      declarations: [ RecipePage ],
      imports: [IonicModule.forRoot()],
      providers: [
        { provide: HttpClient, useValue: httpClientStub },
        { provide: UrlSerializer, useValue: urlSerializerStub },
        { provide: Router, useValue: routerStub },
        { provide: RecipesService, useValue: mockrecipesService}
      ]
    }).compileComponents();
    spyOn(mockrecipesService, 'selectedRecipe').and.returnValue(new BehaviorSubject<Recipe>(null));

    fixture = TestBed.createComponent(RecipePage);
    component = fixture.componentInstance;
    fixture.detectChanges();
  }));

  it('should create', () => {
    expect(component).toBeTruthy();
  });
});

Your assistance is greatly appreciated!

angular typescript karma-jasmine

Answer 1

Answer №1

Impressive question with an abundance of code to analyze!

To address the issue, I recommend avoiding public access to your subject from the RecipesService to prevent potential loss of control when certain components start using the .next method. Instead, create a public observable that you can subscribe to within the RecipePage component.

Another concern lies in the constructor – it's best practice to avoid placing logic in constructors and utilize Angular life cycle hooks like ngOnInit/ngOnChanges. These hooks are called by Angular once the component setup is complete.

For testing purposes, you only need to mock the RecipesService. If your component does not depend on HttpClient, there's no need for a stub there.

In my approach, I developed a specialized mock class to manage this service during tests. This mock class mimics the functionality of the real service by offering a public observable ($selectedRecipeObs) and a utility method to update values in our tests.

I have also created a StackBlitz project demonstrating the running tests. You can find all relevant information related to your issue within the app/pagerecipe/ directory. Refer to the Angular tutorial on testing, explore their various test examples, and check out the corresponding GitHub repository for more insights on testing strategies.

RecipesService:

@Injectable({
    providedIn: 'root'
})
export class RecipesService {
  
    /**
     * The recipe selected by the user
     */
    private readonly selectedRecipe : BehaviorSubject<Recipe> = new BehaviorSubject(null);
    // It's recommended to restrict public access to subjects. 
    // Hence, we provide a public observable for component subscription.
    public $selectedRecipeObs = this.selectedRecipe.asObservable();

    constructor(
      private httpClient: HttpClient
    ) {}
}

Component:

@Component({
  selector: "app-recipe-page",
  templateUrl: "./recipe-page.component.html",
  styleUrls: ["./recipe-page.component.css"],
  providers: []
})
export class RecipePageComponent implements OnInit {
  selectedRecipe: Recipe;
  constructor(private recipesService: RecipesService) {
    // Keep the constructor simple; move most code to ngOnInit or other life cycle hooks.
  }

  ngOnInit(): void {
    // Subscribe to the new $selectedRecipeObs instead of the subject to maintain control.
    // Use service methods for set/get operations, etc.
    this.recipesService.$selectedRecipeObs.subscribe(
      newRecipe => (this.selectedRecipe = newRecipe)
    );
  }
}

Our RecipesService mock:

export class RecipesServiceMock {
    private selectedRecipe = new BehaviorSubject<Recipe>(null);
    // Must share the same name as the original service for consistency.
    public $selectedRecipeObs = this.selectedRecipe.asObservable();

    constructor() {
    } 

    /** Utility method for setting test values; naming can vary. */
    public setSelectedRecipeForTest(value: Recipe): void {
        this.selectedRecipe.next(value);
    }
}

Test file:

import {
  ComponentFixture,
  fakeAsync,
  TestBed,
  tick,
  waitForAsync
} from "@angular/core/testing";
import { Recipe } from "../recipe";

import { RecipesService } from "../recipes.service";
import { RecipesServiceMock } from "../test-recipes.service";
import { RecipePageComponent } from "./recipe-page.component";

////// Tests //////
describe("RecipePageComponent", () => {
  let component: RecipePageComponent;
  let fixture: ComponentFixture<RecipePageComponent>;
  let recipesServiceMock: RecipesServiceMock;

  beforeEach(
    waitForAsync(() => {
      recipesServiceMock = new RecipesServiceMock();
      TestBed.configureTestingModule({
        imports: [],
        providers: [{ provide: RecipesService, useValue: recipesServiceMock }]
      }).compileComponents();

      fixture = TestBed.createComponent(RecipePageComponent);
      component = fixture.componentInstance;
    })
  );

  it("should create", () => {
    fixture.detectChanges();
    expect(component).toBeTruthy();
  });

  it("should update component with new value", fakeAsync(() => {
    // Provide a new value instead of null
    const myNewRecipe = new Recipe("tasty");

    recipesServiceMock.setSelectedRecipeForTest(myNewRecipe);

    fixture.detectChanges(); //
    tick(); // )

    expect(component.selectedRecipe).toEqual(myNewRecipe);
  }));
});

Answer 2

Impressive question with an abundance of code to analyze!

To address the issue, I recommend avoiding public access to your subject from the RecipesService to prevent potential loss of control when certain components start using the .next method. Instead, create a public observable that you can subscribe to within the RecipePage component.

Another concern lies in the constructor – it's best practice to avoid placing logic in constructors and utilize Angular life cycle hooks like ngOnInit/ngOnChanges. These hooks are called by Angular once the component setup is complete.

For testing purposes, you only need to mock the RecipesService. If your component does not depend on HttpClient, there's no need for a stub there.

In my approach, I developed a specialized mock class to manage this service during tests. This mock class mimics the functionality of the real service by offering a public observable ($selectedRecipeObs) and a utility method to update values in our tests.

I have also created a StackBlitz project demonstrating the running tests. You can find all relevant information related to your issue within the app/pagerecipe/ directory. Refer to the Angular tutorial on testing, explore their various test examples, and check out the corresponding GitHub repository for more insights on testing strategies.

RecipesService:

@Injectable({
    providedIn: 'root'
})
export class RecipesService {
  
    /**
     * The recipe selected by the user
     */
    private readonly selectedRecipe : BehaviorSubject<Recipe> = new BehaviorSubject(null);
    // It's recommended to restrict public access to subjects. 
    // Hence, we provide a public observable for component subscription.
    public $selectedRecipeObs = this.selectedRecipe.asObservable();

    constructor(
      private httpClient: HttpClient
    ) {}
}

Component:

@Component({
  selector: "app-recipe-page",
  templateUrl: "./recipe-page.component.html",
  styleUrls: ["./recipe-page.component.css"],
  providers: []
})
export class RecipePageComponent implements OnInit {
  selectedRecipe: Recipe;
  constructor(private recipesService: RecipesService) {
    // Keep the constructor simple; move most code to ngOnInit or other life cycle hooks.
  }

  ngOnInit(): void {
    // Subscribe to the new $selectedRecipeObs instead of the subject to maintain control.
    // Use service methods for set/get operations, etc.
    this.recipesService.$selectedRecipeObs.subscribe(
      newRecipe => (this.selectedRecipe = newRecipe)
    );
  }
}

Our RecipesService mock:

export class RecipesServiceMock {
    private selectedRecipe = new BehaviorSubject<Recipe>(null);
    // Must share the same name as the original service for consistency.
    public $selectedRecipeObs = this.selectedRecipe.asObservable();

    constructor() {
    } 

    /** Utility method for setting test values; naming can vary. */
    public setSelectedRecipeForTest(value: Recipe): void {
        this.selectedRecipe.next(value);
    }
}

Test file:

import {
  ComponentFixture,
  fakeAsync,
  TestBed,
  tick,
  waitForAsync
} from "@angular/core/testing";
import { Recipe } from "../recipe";

import { RecipesService } from "../recipes.service";
import { RecipesServiceMock } from "../test-recipes.service";
import { RecipePageComponent } from "./recipe-page.component";

////// Tests //////
describe("RecipePageComponent", () => {
  let component: RecipePageComponent;
  let fixture: ComponentFixture<RecipePageComponent>;
  let recipesServiceMock: RecipesServiceMock;

  beforeEach(
    waitForAsync(() => {
      recipesServiceMock = new RecipesServiceMock();
      TestBed.configureTestingModule({
        imports: [],
        providers: [{ provide: RecipesService, useValue: recipesServiceMock }]
      }).compileComponents();

      fixture = TestBed.createComponent(RecipePageComponent);
      component = fixture.componentInstance;
    })
  );

  it("should create", () => {
    fixture.detectChanges();
    expect(component).toBeTruthy();
  });

  it("should update component with new value", fakeAsync(() => {
    // Provide a new value instead of null
    const myNewRecipe = new Recipe("tasty");

    recipesServiceMock.setSelectedRecipeForTest(myNewRecipe);

    fixture.detectChanges(); //
    tick(); // )

    expect(component.selectedRecipe).toEqual(myNewRecipe);
  }));
});

Tips for testing an Angular component with a dependency on a service that includes a BehaviorSubject

Answer №1

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