Angular 2: A guide to simulating ChangeDetectorRef in unit tests

Question

Angular 2: A guide to simulating ChangeDetectorRef in unit tests

Recently, I embarked on my journey with Unit-Testing and successfully managed to mock my own services along with some Angular and Ionic elements. However, no matter what I try, ChangeDetectorRef remains unaltered.

It almost feels like magic, doesn't it?

beforeEach(async(() => 
    TestBed.configureTestingModule({
      declarations: [MyComponent],
      providers: [
        Form, DomController, ToastController, AlertController,
        PopoverController,

        {provide: Platform, useClass: PlatformMock},
        {
          provide: NavParams,
          useValue: new NavParams({data: new PageData().Data})
        },
        {provide: ChangeDetectorRef, useClass: ChangeDetectorRefMock}

      ],
      imports: [
        FormsModule,
        ReactiveFormsModule,
        IonicModule
      ],
    })
    .overrideComponent(MyComponent, {
      set: {
        providers: [
          {provide: ChangeDetectorRef, useClass: ChangeDetectorRefMock},
        ],
        viewProviders: [
          {provide: ChangeDetectorRef, useClass: ChangeDetectorRefMock},
        ]
      }
    })
    .compileComponents()
    .then(() => {
      let fixture = TestBed.createComponent(MyComponent);
      let cmp = fixture.debugElement.componentInstance;

      let cdRef = fixture.debugElement.injector.get(ChangeDetectorRef);

      console.log(cdRef); // logs ChangeDetectorRefMock
      console.log(cmp.cdRef); // logs ChangeDetectorRef , why ??
    })
  ));

 it('fails no matter what', async(() => {
    spyOn(cdRef, 'markForCheck');
    spyOn(cmp.cdRef, 'markForCheck');

    cmp.ngOnInit();

    expect(cdRef.markForCheck).toHaveBeenCalled();  // fail, why ??
    expect(cmp.cdRef.markForCheck).toHaveBeenCalled(); // success

    console.log(cdRef); // logs ChangeDetectorRefMock
    console.log(cmp.cdRef); // logs ChangeDetectorRef , why ??
  }));

@Component({
  ...
})
export class MyComponent {
 constructor(private cdRef: ChangeDetectorRef){}

 ngOnInit() {
   // do something
   this.cdRef.markForCheck();
 }
}

I have tried everything , async, fakeAsync,

injector([ChangeDetectorRef], () => {})

.

But alas, nothing seems to do the trick.

unit-testing angular typescript jasmine testbed

Answer 1

Answer №1

Update 2020:

This was originally written in May 2017 and continues to be an effective solution even now.

When unable to configure the injection of a changeDetectorRef mock through the test bed, I have found success with the following approach:

 it('detects changes', () => {
      // Creating a unique instance for testing
      const changeDetectorRef = fixture.debugElement.injector.get(ChangeDetectorRef); 
     
      // Spying directly on the prototype.
      const detectChangesSpy = spyOn(changeDetectorRef.constructor.prototype, 'detectChanges');

      component.someMethod(); // Calls the detectChanges method internally

      expect(detectChangesSpy).toHaveBeenCalled();
    });

By utilizing this method, private attributes are not of concern.

If encountered, here is a technique that has proven effective for me:

Since you are injecting the ChangeDetectorRef instance in your constructor:

 constructor(private cdRef: ChangeDetectorRef) { }

You can spy on the component, stub the attribute cdRef, and control its behavior as needed. Additionally, assertions can be made on its calls and parameters.

In your spec file, do not provide the ChangeDetectorRef when calling your TestBed. Set up the component in the beforeEach block so it resets between specs, similar to the process outlined in the Angular docs here:

component = fixture.componentInstance;

Then, in your tests, spy directly on the attribute:

describe('someMethod()', () => {
  it('calls detect changes', () => {
    const spy = spyOn((component as any).cdRef, 'detectChanges');
    component.someMethod();

    expect(spy).toHaveBeenCalled();
  });
});

The spy allows you to use .and.returnValue() to customize its return value.

Note that (component as any) is used because cdRef is a private attribute. However, since "private" does not exist in compiled Javascript, it is accessible in this manner during testing.

It is at your discretion whether accessing private attributes in this way is suitable for your testing purposes.

Answer 2

Update 2020:

This was originally written in May 2017 and continues to be an effective solution even now.

When unable to configure the injection of a changeDetectorRef mock through the test bed, I have found success with the following approach:

 it('detects changes', () => {
      // Creating a unique instance for testing
      const changeDetectorRef = fixture.debugElement.injector.get(ChangeDetectorRef); 
     
      // Spying directly on the prototype.
      const detectChangesSpy = spyOn(changeDetectorRef.constructor.prototype, 'detectChanges');

      component.someMethod(); // Calls the detectChanges method internally

      expect(detectChangesSpy).toHaveBeenCalled();
    });

By utilizing this method, private attributes are not of concern.

If encountered, here is a technique that has proven effective for me:

Since you are injecting the ChangeDetectorRef instance in your constructor:

 constructor(private cdRef: ChangeDetectorRef) { }

You can spy on the component, stub the attribute cdRef, and control its behavior as needed. Additionally, assertions can be made on its calls and parameters.

In your spec file, do not provide the ChangeDetectorRef when calling your TestBed. Set up the component in the beforeEach block so it resets between specs, similar to the process outlined in the Angular docs here:

component = fixture.componentInstance;

Then, in your tests, spy directly on the attribute:

describe('someMethod()', () => {
  it('calls detect changes', () => {
    const spy = spyOn((component as any).cdRef, 'detectChanges');
    component.someMethod();

    expect(spy).toHaveBeenCalled();
  });
});

The spy allows you to use .and.returnValue() to customize its return value.

Note that (component as any) is used because cdRef is a private attribute. However, since "private" does not exist in compiled Javascript, it is accessible in this manner during testing.

It is at your discretion whether accessing private attributes in this way is suitable for your testing purposes.

Answer 3

Answer №2

Is it possible that changeDetectorRef can now be accessed through the fixture? This could be a recent development worth noting.

To learn more, refer to the documentation provided here: https://angular.io/guide/testing#componentfixture-properties

We encountered a similar challenge with mocking the change detector, and found this method to be the solution for us.

Answer 4

Is it possible that changeDetectorRef can now be accessed through the fixture? This could be a recent development worth noting.

To learn more, refer to the documentation provided here: https://angular.io/guide/testing#componentfixture-properties

We encountered a similar challenge with mocking the change detector, and found this method to be the solution for us.

Answer 5

Answer №3

One important aspect to note is that the focus should be on testing your own code rather than unit testing the change detector itself, which has already been validated by the Angular team. To achieve this, consider extracting the call to the change detector into a local private method (marked as private since it's not meant for unit testing), like so:

private runChangeDetection(): void {
    this.cdRef.detectChanges();
}

In your unit test, ensure that your code actually invokes this function and thereby triggers the ChangeDetectorRef method. For instance:

it('should trigger the change detector',
    () => {
        const cdrSpy = spyOn((component as any).cdRef, 'detectChanges' as any);
        component.ngOnInit();
        expect(cdrSpy).toHaveBeenCalled();
    }
);

I encountered a similar scenario where a senior developer advised me to follow this approach for unit testing, emphasizing that it can lead to better code structuring. By restructuring in this manner, you establish flexibility in your codebase, allowing for smoother adjustments if there are changes in Angular's change detection mechanisms.

Answer 6

One important aspect to note is that the focus should be on testing your own code rather than unit testing the change detector itself, which has already been validated by the Angular team. To achieve this, consider extracting the call to the change detector into a local private method (marked as private since it's not meant for unit testing), like so:

private runChangeDetection(): void {
    this.cdRef.detectChanges();
}

In your unit test, ensure that your code actually invokes this function and thereby triggers the ChangeDetectorRef method. For instance:

it('should trigger the change detector',
    () => {
        const cdrSpy = spyOn((component as any).cdRef, 'detectChanges' as any);
        component.ngOnInit();
        expect(cdrSpy).toHaveBeenCalled();
    }
);

I encountered a similar scenario where a senior developer advised me to follow this approach for unit testing, emphasizing that it can lead to better code structuring. By restructuring in this manner, you establish flexibility in your codebase, allowing for smoother adjustments if there are changes in Angular's change detection mechanisms.

Answer 7

Answer №4

When it comes to unit testing, if you find yourself in a situation where you need to mock ChangeDetectorRef just for the sake of satisfying dependency injection during component creation, you have the flexibility to pass in any value.

In a specific scenario I encountered, this approach worked effectively:

TestBed.configureTestingModule({
  providers: [
    FormBuilder,
    MyComponent,
    { provide: ChangeDetectorRef, useValue: {} }
  ]
}).compileComponents()
injector = getTestBed()
myComponent = injector.get(MyComponent)

This code snippet successfully creates an instance of MyComponent. It is crucial to ensure that the test execution flow does not rely on ChangeDetectorRef. If it does, then you should consider replacing useValue: {} with a suitable mock object.

In my particular case, the objective was to test aspects related to form creation using FormBuilder.

Answer 8

When it comes to unit testing, if you find yourself in a situation where you need to mock ChangeDetectorRef just for the sake of satisfying dependency injection during component creation, you have the flexibility to pass in any value.

In a specific scenario I encountered, this approach worked effectively:

TestBed.configureTestingModule({
  providers: [
    FormBuilder,
    MyComponent,
    { provide: ChangeDetectorRef, useValue: {} }
  ]
}).compileComponents()
injector = getTestBed()
myComponent = injector.get(MyComponent)

This code snippet successfully creates an instance of MyComponent. It is crucial to ensure that the test execution flow does not rely on ChangeDetectorRef. If it does, then you should consider replacing useValue: {} with a suitable mock object.

In my particular case, the objective was to test aspects related to form creation using FormBuilder.

Answer 9

Answer №5

// Initialize component
constructor(private changeDetectorRef: ChangeDetectorRef) {}

// Method to handle some action
public handleAction() {
  this.changeDetectorRef.detectChanges();
}     

// Unit test for the method
const changeDetectorRef = fixture.componentRef.changeDetectorRef;
jest.spyOn(changeDetectorRef, 'detectChanges');
fixture.detectChanges(); // <--- Don't forget this!!

component.handleAction();

expect(changeDetectorRef.detectChanges).toHaveBeenCalled();

Answer 10

// Initialize component
constructor(private changeDetectorRef: ChangeDetectorRef) {}

// Method to handle some action
public handleAction() {
  this.changeDetectorRef.detectChanges();
}     

// Unit test for the method
const changeDetectorRef = fixture.componentRef.changeDetectorRef;
jest.spyOn(changeDetectorRef, 'detectChanges');
fixture.detectChanges(); // <--- Don't forget this!!

component.handleAction();

expect(changeDetectorRef.detectChanges).toHaveBeenCalled();

Answer 11

Answer №6

I have come across numerous insightful responses.

For the year 2023, my preferred approach using jest would be:

it('verifies changes are detected', () => {
  const changeDetectorRef = fixture.changeDetectorRef; 
   
  // Setting up spying on your method.
  jest.spyOn(changeDetectorRef, 'detectChanges');

  component.someMethod(); // This triggers detectChanges internally.

  expect(changeDetectorRef.detectChanges).toHaveBeenCalled();
});

Answer 12

I have come across numerous insightful responses.

For the year 2023, my preferred approach using jest would be:

it('verifies changes are detected', () => {
  const changeDetectorRef = fixture.changeDetectorRef; 
   
  // Setting up spying on your method.
  jest.spyOn(changeDetectorRef, 'detectChanges');

  component.someMethod(); // This triggers detectChanges internally.

  expect(changeDetectorRef.detectChanges).toHaveBeenCalled();
});

Answer 13

Answer №7

My testing approach involved the following steps: I included ChangeDetectorRef in my constructor and invoked it in ngAfterViewChecked

constructor(private cdRef: ChangeDetectorRef)
  ngAfterViewChecked() {
this.cdRef.detectChanges();
}

const cdRef = fixture.debugElement.injector.get(ChangeDetectorRef); //Fake changeDetectorRef
    const detectChangesSpy = jest.spyOn(cdRef.constructor.prototype,'detectChanges');//using jest.spyon to call detectChances
fixture.detectChanges();
component.ngAfterViewInit(); //initiating ngAfterViewInit
expect(detectChangesSpy).toHaveBeenCalled();

Answer 14

My testing approach involved the following steps: I included ChangeDetectorRef in my constructor and invoked it in ngAfterViewChecked

constructor(private cdRef: ChangeDetectorRef)
  ngAfterViewChecked() {
this.cdRef.detectChanges();
}

const cdRef = fixture.debugElement.injector.get(ChangeDetectorRef); //Fake changeDetectorRef
    const detectChangesSpy = jest.spyOn(cdRef.constructor.prototype,'detectChanges');//using jest.spyon to call detectChances
fixture.detectChanges();
component.ngAfterViewInit(); //initiating ngAfterViewInit
expect(detectChangesSpy).toHaveBeenCalled();

Angular 2: A guide to simulating ChangeDetectorRef in unit tests

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

Answer №6

Answer №7

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