[RxJS] Reusable multicasting with Subject factories

Posted 2020-08-16 Answer1215

The way we use publish() (or multicast with an RxJS Subject) makes the shared Observable not reusable if the shared execution happens to complete or emit an error. In this lesson we will see how to use a simple Subject factory function in order to create a new Subject, one for each shared execution, whenever connect() is called.

 

var shared = Rx.Observable.interval(1000).take(3)
  .do(x => console.log(‘source ‘ + x))
  .multicast(new Rx.Subject())
  .refCount();

The code above, after subject emit 0,1,2, three values, then it completes. It means if you want to subscribe the subject again, it won‘t emit anything because it is completed. 

 

If you want to reuse the ‘shared‘ subject even after subject complete, you need to use subject factories, which simply just a function return new Subject():

function subjectFactory() {
  return new Rx.Subject(); 
}

var shared = Rx.Observable.interval(1000).take(3)
  .do(x => console.log(‘source ‘ + x))
  .multicast(subjectFactory)
  .refCount();

 

So now even you resubscribe after subject complete, it will emit you new value.

function subjectFactory() {
  return new Rx.Subject(); 
}

var shared = Rx.Observable.interval(1000).take(3)
  .do(x => console.log(‘source ‘ + x))
  .multicast(subjectFactory)
  .refCount();

// subject: --0--1--2--3--4--5|
//                               A
// subject2:                     --0--1--2--3--4--5|

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

var subA = shared.subscribe(observerA); // 0 => 1
console.log(‘subscribed A‘);

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

var subB;
setTimeout(function () {
  subB = shared.subscribe(observerB);
  console.log(‘subscribed B‘);
}, 2000);

setTimeout(function () {
  subA.unsubscribe();
  console.log(‘unsubscribed A‘);
}, 3000);

setTimeout(function () {
  subB.unsubscribe();
  console.log(‘unsubscribed B‘);
}, 5000);

setTimeout(function () {
  subA = shared.subscribe(observerA); // 0 => 1 (connect)
  console.log(‘subscribed A‘);
}, 6000);

 

/**
"subscribed A"
"source 0"
"A next 0"
"source 1"
"A next 1"
"subscribed B"
"source 2"
"A next 2"
"B next 2"
"A done"
"B done"
"unsubscribed A"
"unsubscribed B"
"subscribed A"
"source 0"
"A next 0"
"source 1"
"A next 1"
"source 2"
"A next 2"
"A done"

*/