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:
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.
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>
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<meta name="viewport" content="width=device-width">
<title>JS Bin</title>
</head>
<body>
</body>
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