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javascriptecmascript-6observablereactivex

RxJS detect when observable has been subscribed to

I have a need to detect when an observable (observedEvents) has been subscribed to, and then subscribe to another observable (triggerEvent). I don't want to subscribe to triggerEvent manually, but only once and when observedEvents has a subscription.

Here is some code explaining what I am looking for:

// This just emits events
let emitting = new EventEmitter();

// This is the main Observable which someone else might
// have access to
let observedEvents = Rx.Observable.merge(
  Rx.Observable.fromEvent(emitting, 'aba'),
  Rx.Observable.fromEvent(emitting, 'bob')
)

// This trigger should get a subscription if observedEvents
// has one, i.e. when I subscribe to observedEvents
// that subscription activates this trigger

// I have made an attempt at this by calling skipUntil
// this however skips one event, but I don't want that
let triggerEvent = Rx.Observable.merge(
  // these actions are things that can
  // happen when the trigger is active
  Rx.Observable.of('a').delay(200),
  Rx.Observable.of('b').delay(400),
  Rx.Observable.of('c').delay(600)
)
.skipUntil(observedEvents);

// Something else should be used to activate trigger
// I don't want to do this part manually
triggerEvent.subscribe(val => {
    console.log(`Do something fancy with ${val}`);
});

//----------------------------------------------------
// Somewhere else in the code...
//----------------------------------------------------
observedEvents.subscribe(evt => {
  console.log(`Some event: ${evt}`);
});
// At this point I want triggerEvent to become active
// because observedEvents has a subscription

setTimeout(() => {
  emitting.emit('bob', 'world');
  setTimeout(() => emitting.emit('aba', 'stackoverflow!'), 500);
}, 200);
<!DOCTYPE html>
<html>
<head>
  <script src="https://npmcdn.com/@reactivex/[email protected]/dist/global/Rx.umd.js"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/EventEmitter/5.1.0/EventEmitter.min.js"></script>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>
<body>

</body>
</html>

Is this possible?

I hope that explains what I'm looking for.

As I'm writing this, I'm thinking a solution with Subjects is probably what I need. I'm not sure, but I just need a nudge in the right direction or a solution if possible.

Solution

  • For rxjs > v7, see this answer

    Answer

    Sure enough I was right about using Subjects. The key was the observers list for Subject. Here is what I finally did:

    let emitting = new EventEmitter();
let sub = new Rx.Subject();

// return this to users
let myGlobalSub = sub.merge(Rx.Observable.of(1, 2, 3));

// For internal use
let myObservers = Rx.Observable.fromEvent(emitting, 'evt');

console.log(`The number of subscribers is ${sub.observers.length}`);

// Only do something if myGlobalSub has subscribers
myObservers.subscribe(l => {
  if (sub.observers.length) { // here we check observers
    console.log(l);
  }
});

// Somewhere in the code...
emitting.emit('evt', "I don't want to see this"); // No output because no subscribers

myGlobalSub.subscribe(l => console.log(l)); // One sub

emitting.emit('evt', 'I want to see this'); // Output because of one sub

console.log(`The number of subscribers is ${sub.observers.length}`);
    <!DOCTYPE html>
<html lang=en>

<head>
  <script src="https://unpkg.com/[email protected]/bundles/Rx.min.js"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/EventEmitter/5.2.5/EventEmitter.min.js"></script>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>

<body>
</body>

</html>