Angular 5，rxjs-仅当可观察性在运行过程中时才等待可观察性完成，然后再运行另一个进程

Question

我的主要组件中有一个 3 秒的计时器。 在计时器内，我执行 http 调用-

    constructor(){
          this.timer = timer(3000, 3000);
          this.timerObservable = this.timer.subscribe(x => {
              this.http.get(URL).subscribe(()=>{
              //DO SOMETHING
              });
          });
     }

在另一个组件中，我有一个按钮，假设要执行不同的http调用，按下按钮调用sumbit函数-

        submit(){
            this.http.get("/sumbitForm").subscribe(()=> {
              //DO SOMETHING
             })
         }

当用户单击按钮时，如果计时器正在进行中(其中的http被调用但尚未解析(，我想在对按钮执行http调用之前等待，直到它解决，但如果计时器不在处理中(上一次调用的时间尚未过去(我想立即执行它。

我认为 forkJoin 和 concat 在这里无关紧要(这是一个计时器，而不是我想等待其执行的"常规"订阅(，而且我找不到一种漂亮的方法来做到这一点，知道吗？

Answer 1

您需要在

两个组件之间共享一些信息，即轮询请求何时正在进行，何时不在进行中。为此，您应该使用服务。将 http 请求逻辑移动到服务而不是直接在组件中使用 HttpClient 也始终是一种很好的做法。这允许您在一个地方进行常规错误处理。我们称之为服务ApiService。

API 服务

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable, BehaviorSubject, interval } from 'rxjs';
import { switchMap, tap } from 'rxjs/operators';
@Injectable({ providedIn: 'root' })
export class ApiService {
  // Use a BehaviorSubject to notify listeners about ongoing polling requests.
  // A BahaviorSubject always has a current value that late subscribers will 
  // receive immediately upon subscribing
  private pollRequestInProgress = new BehaviorSubject<boolean>(false);
  // Share this BehaviorSubject as an Observable
  public pollRequestInProgress$ = pollRequestInProgress.asObservable();
  constructor(private http: HttpClient)
  doPoll(url: string): Observable<any> {
    return interval(3000).pipe( // interval(3000) is equivalent to timer(3000, 3000)
      tap(_ => pollRequestInProgress.next(true)), // notify that a poll request is about to happen
      switchMap(_ => this.http.get(url)), // do your poll request
      tap(_ => pollRequestInProgress.next(false)) // notify that your poll request ended
    );
  }
}

主组件

这是您要从中开始轮询的组件。

private destroy$: Subject<void> = new Subject<void>();
constructor(private apiService: ApiService) {}
// move logic to ngOnInit instead of constructor
ngOnInit() {
  // subscribe and thereby start the polling
  this.apiService.doPoll(URL).pipe(takeUntil(this.destroy$))
    .subscribe(pollResponse => {
      //DO SOMETHING
    });
}
ngOnDestroy() {
  // unsubscribe when the Component gets destroyed.
  this.destroy$.next();
  this.destroy$.complete();
}

功能组件

这是您在单击按钮时要执行 http 请求的组件。

constructor(private http: HttpClient, private apiService: apiService) {}
submit() {
  // Listen to whether a polling request is currently in progress.
  // We will immediately receive a value upon subscribing here, because we used 
  // a BehaviorSubject as the underlying source.
  this.apiService.pollRequestInProgress$.pipe(
    // Emit the first item that tells you that no polling request is in progress.
    // i.e. let the first 'requestInProgress === false' through but not any 
    // other items before or after.
    first(requestInProgress => !requestInProgress),
    // If no polling request is in progress, switch to the http request you want 
    // to perform
    switchMap(this.http.get("/sumbitForm")) // <-- consider moving this http.get to your ApiService aswell
  ).subscribe(httpResponse => {
    // you've got your http response here
  });
  // you don't have to unsubscribe here as first and http.get both complete 
  // and thus unsubscribe automatically
}

在此处查看上述代码逻辑的简单示例：https://stackblitz.com/edit/rxjs-t4hjcr

Answer 2

您可以使用角度主题

import { Injectable } from '@angular/core';
import { Observable, Subject } from 'rxjs';
@Injectable({ providedIn: 'root' })
export class CallService {
    private subject = new Subject<any>();
    timerCompleted() {
        this.subject.next();
    }
    checkTimer(): Observable<any> {
        return this.subject.asObservable();
    }
}

应用组件

使用调用服务订阅计时器完成，并使其可用于应用组件模板。

import { Component, OnDestroy } from '@angular/core';
import { Subscription } from 'rxjs';
import { CallService } from './_services/index';
@Component({
    selector: 'app',
    templateUrl: 'app.component.html'
})
export class AppComponent implements OnDestroy {
    subscription: Subscription;
    constructor(private callService: CallService) {
        this.subscription = this.callService.checkTimer().subscribe(() => { 
           // call your api after timer complete
        });
    }
    ngOnDestroy() {
        // unsubscribe to ensure no memory leaks
        this.subscription.unsubscribe();
    }
}

在计时器中添加以下代码

this.timer = timer(3000, 3000);
        this.timerObservable = this.timer.subscribe(x => {
           this.http.get(URL).subscribe(()=>{
               this.callService.timerCompleted();
           });
        });

有关更多参考，您可以查看 http://jasonwatmore.com/post/2018/06/25/angular-6-communicating-between-components-with-observable-subject