[RxJS] AsyncSubject: representing a computation that yields a final value

This lesson will teach you about AsyncSubject, another type of Subject with some replaying logic inside. We will also look at some use cases for this peculiar RxJS subject variant.

AsyncSubject:

Emit last value only when sequence completed. 

BehaviorSubject:

Replay onces, only before compleation.

ReplaySubject:

Replay many, before of after compleation.

var subject = new Rx.AsyncSubject();

// Subject
// ReplaySubject: replays many, before or after completion
// BehaviorSubject: replays one, only before completion
// AsyncSubject: replays one, only if completed

var observerA = {
  next: function (x) { console.log('A next ' + x); },
  error: function (err) { console.log('A error ' + err); },
  complete: function () { console.log('A done'); },
};

subject.subscribe(observerA);
console.log('observerA subscribed');

var observerB = {
  next: function (x) { console.log('B next ' + x); },
  error: function (err) { console.log('B error ' + err); },
  complete: function () { console.log('B done'); },
};

setTimeout(() => subject.next(1), 100);
setTimeout(() => subject.next(2), 200);
setTimeout(() => subject.next(3), 300);
setTimeout(() => subject.complete(), 350);

/*
----1---2---3--|       
  .............3|
                   3|
*/

setTimeout(function () {
  subject.subscribe(observerB);
  console.log('observerB subscribed');
}, 400);

/*
"observerA subscribed"
"A next 3"
"A done"
"B next 3"
"B done"
"observerB subscribed"
*/