这通常是由于测试代码与被测试的代码之间存在异步问题所导致的。我们可以在测试代码中使用fakeAsync和tick()函数来解决此问题。以下是一个示例：

// HTTP拦截器 @Injectable() export class MyInterceptor implements HttpInterceptor { intercept(request: HttpRequest, next: HttpHandler): Observable> { request = request.clone({ headers: request.headers.set('X-Requested-With', 'XMLHttpRequest') });

return next.handle(request);

} }

// 单元测试 describe('MyInterceptor', () => { let httpMock: HttpTestingController; let httpClient: HttpClient;

beforeEach(() => { TestBed.configureTestingModule({ imports: [HttpClientTestingModule], providers: [ { provide: HTTP_INTERCEPTORS, useClass: MyInterceptor, multi: true } ] });

httpMock = TestBed.inject(HttpTestingController);
httpClient = TestBed.inject(HttpClient);

});

afterEach(() => { httpMock.verify(); });

it('should add X-Requested-With header', fakeAsync(() => { const url = 'https://example.com'; httpClient.get(url).subscribe();

const httpRequest = httpMock.expectOne(url);
expect(httpRequest.request.headers.get('X-Requested-With')).toBe('XMLHttpRequest');

httpRequest.flush({});
tick();

})); });

在上面的代码中，我们使用了fakeAsync()函数来避免异步问题。接下来，我们通过httpClient.get()方法来执行HTTP请求。然后，我们调用httpMock.expectOne()方法来断言我们的拦截器是否已经正确地添加了'X-Requested-With'头部。最后，我们使用httpRequest.flush()方法来模拟HTTP响应，并使用tick()方法来等待异步操作完成。

通过这种方式，我们可以在Angular 12中正确地测试HTTP拦截器中的expectOne()方法。