如何在新订阅中将所有值发布到观察者？

Question

I have an observer that get different values published to it over time. For example

    sub = new Subject<any>();
    sub.next(1); sub.next(2); sub.next(3);
    #hack 1
    sub.next(4); sub.next(5); sub.next(6);
    #hack 2

If there is a subscription in #hack 1 it should get all values 1,2,3 and if there is subscription in #hack 2 it should get all values 1,2,3,4,5,6. Also, ReplaySubject gets old values, sure, but I want all the values on every publish. So, on every sub.next(7), I need all the values 1,2,3,4,5,6 and 7. So basically the observable stores all published or 'next'-ed values. How do I achieve this?

Answer 1

Use BehaviorSubject with scan :) It's basically like Map.reduce, but emits on every update.

import { Component } from '@angular/core';

import { scan } from 'rxjs/operators';
import { BehaviorSubject } from 'rxjs/BehaviorSubject';

@Component({
  selector: 'my-app',
  template: `
  <input (keydown.enter)="onMessage(input.value); input.value =''" #input />

  <div *ngFor="let message of messages$ | async">
    {{ message }}
  </div>
  `,
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  private messagesSource = new BehaviorSubject<any>([]);
  messages$ = this.messagesSource.asObservable()
    .pipe(scan((acc, curr) => [...acc, curr], []));

  onMessage(val: string) {
    this.messagesSource.next(val);
  }
}

Live demo

This line is what does all the magic: scan((acc, curr) => [...acc, curr]) It concats every value emitted by the subject into an array.

Answer 2

I like @TomaszKula's answer, but I think the question is about successive subscriptions picking up previous (and future) emits.

As far as I can tell, ReplaySubject does what is asked for...

 console.clear() const sub = new Rx.ReplaySubject(); sub.next(1); console.log('1st subscribe') sub.subscribe(x => console.log('1st', x)) sub.next(2); console.log('2nd subscribe') sub.subscribe(x => console.log('2nd', x)) sub.next(3) 

 <script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/5.5.8/Rx.js"></script> 

Answer 3

The answer is: You can't with a simple Subject as you only have access to it's value inside a Subscription but never locally in your Service. Using a BehaviorSubject you can.

But first of all, make your subject private and accessible via methods only. Then you can do magic like this:

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

@Injectable()
export class DataService {

   private subject = new BehaviorSubject<string>();

   setData(data: string) {
     this.subject.next(
       this.subject.getValue() + ',' + data;
     );
   }

   clearData() {
     this.subject.next();
   }

   getData(): Observable<any> {
      return this.subject.asObservable();
   }

}

And your Subscribers access this BehaviorSubject this way

constructor(private dataService:DataService){}

ngOnInit(): void {
   this.dataService.getData().subscribe(data => {
      // do something with data
   });
}

private aMethod(): void {
   this.dataService.setData(aString);
}