Class ComponentContext<T>

Provides the foundation for an opinionated pattern for component tests.

• Includes all features from AngularContext
• Automatically creates your component at the beginning of run().
• Sets up Angular to call ngOnChanges() like it would in production. This is not the case if you use the standard TestBed.createComponent() directly.
• Wraps your component in a parent that you can easily style however you like.
• Lets you use component harnesses in the fakeAsync zone, which is normally a challenge.
• Causes async APP_INITIALIZERs to complete before instantiating the component. Two caveats:
  • this requires all work in your initializers to complete with a call to tick()
  • this requires delaying app initialization until inside the fakeAsync zone, i.e. with the callback to #run. If you have async initializers, you must be careful not to do things that finalize the app setup before then, such as #inject.

A very simple example:

@Component({ template: 'Hello, {{name()}}!' })
class GreeterComponent {
  readonly name = input.required<string>();
}

it('greets you by name', () => {
  const ctx = new ComponentContext(GreeterComponent);
  ctx.assignInputs({ name: 'World' });
  ctx.run(() => {
 expect(ctx.fixture.nativeElement.textContent).toBe('Hello, World!');
  });
});
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A full example, with routing and a component harness. This is the full code for a tiny Angular app:

 /////////////////
 // app-context.ts

 // To re-use your context setup, make a subclass of ComponentContext to import into any spec
 class AppContext extends ComponentContext<AppComponent> {
   constructor() {
     // Import `appConfig` from `app.config.ts`
     super(AppComponent, appConfig);
   }
 }

 ////////////////////////
 // app.component.spec.ts

 describe('AppComponent', () => {
   let ctx: AppContext;
   beforeEach(() => {
     ctx = new AppContext();
   });

   it('can navigate to the first page', () => {
     ctx.run(async () => {
       const app = await ctx.getHarness(AppComponentHarness);
       await app.navigateToFirstPage();
       expect(ctx.fixture.nativeElement.textContent).toContain(
         'First works!',
       );
     });
   });
 });

 ///////////////////////////
 // app.component.harness.ts

 // A simple component harness to demonstrate its integration with component contexts
 class AppComponentHarness extends ComponentHarness {
   static hostSelector = 'app-root';

   #getFirstPageLink = this.locatorFor('a');

   async navigateToFirstPage(): Promise<void> {
     const link = await this.#getFirstPageLink();
     await link.click();
   }
 }

 /////////////////////
 // first.component.ts

 // A minimal component for demonstration purposes
 @Component({ template: '<p>First works!</p>' })
 class FirstComponent {}

 ///////////////////
 // app.component.ts

 // A minimal app component with routing for demonstration purposes
 @Component({
   selector: 'app-root',
   imports: [RouterOutlet, RouterLink],
   template: `
     <a routerLink="/first-page">First Page</a>
     <router-outlet />
   `,
 })
 class AppComponent {}

 ////////////////////////
 // app.routes.ts

 const routes: Routes = [{ path: 'first-page', component: FirstComponent }];

 ////////////////
 // app.config.ts

 const appConfig: ApplicationConfig = { providers: [provideRouter(routes)] };
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