注入的服务未定义

Question

我有以下HandleService：

@Injectable()
export class HandleService {
  constructor(private notificationService: NotificationService) {
  }
  success(message: string) {
    this.notificationService.printSuccessMessage(message);
  }
  error(error: HttpErrorResponse) {
    if (error.error instanceof ErrorEvent) {
      // A client-side or network error occurred. Handle it accordingly.
      this.notificationService.printErrorMessage(error.error.message);
      console.error('An error occurred:', error.error.message);
    } else {
      // The backend returned an unsuccessful response code.
      // The response body may contain clues as to what went wrong,
      this.notificationService.printErrorMessage(error.message);
      console.error(
        `Backend returned code ${error.status}, ` +
        `body was: ${error.error}`);
    }
    // return an observable with a user-facing error message
    return throwError(
      'Something bad happened; please try again later.');
  };
}

这就是用过的NotificationService：

@Injectable()
export class NotificationService {
  private _notification: any = alertify;
  constructor() {
  }
  openConfirmationDialog(message: string, okCallback: () => any) {
    this._notification.confirm(message, function (e) {
      if (e) {
        okCallback();
      } else {
      }
    });
  }
  printSuccessMessage(message: string) {
    this._notification.success(message);
  }
  printErrorMessage(message: string) {
    this._notification.error(message);
  }
  openAlertDialog(message: string) {
    this._notification.alert(message);
  }
}

我使用这样的HandleService：

@Injectable()
export class CustomerService {
constructor(public http: HttpClient, private authService: AuthService,
              private handleService: HandleService) {
  }
deleteCustomer(customer: Customer): Observable<any> {
    return this.http.delete(BASE_URL + customer.id, this.authService.setHeaders('application/json'))
      .pipe(
        tap(() => this.handleService.success(`Deleted customer: ${customer.name}`)),
        catchError(this.handleService.error)
      );
  }
}

因此，当一切正常时this.handleService.success('Deleted customer: ${customer.name}')执行按预期工作。如果发生错误，则执行this.handleService.error，但它不会打印错误消息，而是cannot read property 'printErrorMessage' of undefined收到以下错误。所以我handleService中的notificationService在这里没有定义，但为什么呢？它注入到构造函数中，并且在 succes -方法上正常工作。为什么它在error方法中不起作用？

Answer 1

我的猜测： 当作为参数传递且未执行(在deleteCustomer函数内部(时，this.handleService.error的作用域是不同的。

将其替换为this.handleService.error.bind(this)

Answer 2

你的 app.module.ts 中有你的服务吗？

providers: [HandleService,NotificationService],

Answer 3

在 App.module.ts 中的提供程序数组上添加服务，或者可以使用：

 @Injectable({
  providedIn: 'root'
})

Answer 4

不需要向 app.module.ts 添加任何提供程序

只需在您的服务类之前添加装饰器

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

与句柄服务相同