Angular CanDeactivate Router Guard具有可观察的订阅值

我的服务就是这样

import { Injectable } from '@angular/core';
import { ReplaySubject } from 'rxjs/ReplaySubject';

import { Observable } from 'rxjs/Observable';

@Injectable()
export class EditModelService {

  private savingModelDataSource = new ReplaySubject<boolean>();

  savingModelData$= this.savingModelDataSource.asObservable();

  public setSavingModelData(state: boolean) {
    this.savingModelDataSource.next(state);
  }

}

我的后卫就像

import { Injectable } from '@angular/core';
import { CanDeactivate,ActivatedRouteSnapshot,RouterStateSnapshot } from '@angular/router';

import { Observable } from 'rxjs/Observable';

import { EditModelService } from './edit-model.service';

@Injectable()
export class EditModelCanDeactivateGuardGuard {
  private savingState: boolean;

  constructor(
    private editModelService: EditModelService
  ) { }

  CanDeactivate(
    currentRoute: ActivatedRouteSnapshot,currentState: RouterStateSnapshot
  ): Observable<boolean> | Promise<boolean> | boolean {

    this.editModelService.savingModelData$.subscribe(saving => {
      this.savingState = saving;
    });


    if (this.savingState) {
      return this.editModelService.savingModelData$.first();
    } else {
      return this.editModelService.savingModelData$.first();
    }

  }
}

基本上,它从服务获取savingState的值并根据状态返回,因此路由器的canDeactivate属性接收一个布尔值.
由于守护中的this.savingState值始终被订阅,因此我认为条件不存在问题.

通过一些搜索,我遇到了这个answer,并尝试将守卫代码作为

return this.editModelService.savingModelData$.first();

当然,我在这里导入了first()作为import’rxjs / add / operator / first’;.

它返回一个来自守卫的观察者只是为了看它是否正常工作或者是否与我返回的方式有关.但是,在这两种情况下,我都会收到错误,如下图所示.

我在导入路由器配置的模块中提供了路由器防护.这也是提供服务的地方.

我在这做错了什么？我是新手使用路由器防护(尤其是canDeactivate with observable),任何帮助表示赞赏.