Angular 5, rxjs- wait for observable to finish only if it is in a middle of a run before running another process

Question

I have a timer in my main component of 3 seconds. inside the timer I perform http call-

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

     }

In another component I have a button that suppose to perform a different http call, pressing on the button invoke the sumbit function-

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

When a user clicks on the button, if the timer is in process (the http inside of it was called and not resolved yet) I want to wait before I perform the http call on the button until it resolved, but if the timer is not in process (the time from the previous call did not passed yet) I want to execute it immediately.

I think that forkJoin and concat is not relevant here (this is a timer and not a 'regular' subscription that I want to wait to its execution either way) and I could not find a pretty way to do it, any idea?

Answer 1

You need to share some information between your two components, i.e. when the polling request is in progress and when it isn't. You should use a Service for this. It's also always good practice to move your http request logic to a Service instead of using the HttpClient directly in the component. This allows you to do your general error handling in one place. Let's call this Service ApiService.

ApiService

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
    );
  }
}

MainComponent

This is the Component where you want to start your polling from.

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();
}

FeatureComponent

This is the Component where you want to perform a http request when you click a button.

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
}

Check out a simple example of the code logic above here: https://stackblitz.com/edit/rxjs-t4hjcr

Answer 2

You can use Angular Subject

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();
    }
}

The app component uses the call service to subscribe to timer complete and make them available to the app component template.

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();
    }
}

add below code in your timer

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

For more reference you can check http://jasonwatmore.com/post/2018/06/25/angular-6-communicating-between-components-with-observable-subject