You could achieve it using defer to track subscriptions and finalize to track completions, e.g. as an operator:
// a custom operator that will count number of subscribers
function customOperator(onCountUpdate = noop) {
return function refCountOperatorFunction(source$) {
let counter = 0;
return defer(()=>{
counter++;
onCountUpdate(counter);
return source$;
})
.pipe(
finalize(()=>{
counter--;
onCountUpdate(counter);
})
);
};
}
// just a stub for `onCountUpdate`
function noop(){}
And then use it like:
const source$ = new Subject();
const result$ = source$.pipe(
customOperator( n => console.log('Count updated: ', n) )
);
Heres a code snippet illustrating this:
const { Subject, of, timer, pipe, defer } = rxjs;
const { finalize, takeUntil } = rxjs.operators;
const source$ = new Subject();
const result$ = source$.pipe(
customOperator( n => console.log('Count updated: ', n) )
);
// emit events
setTimeout(()=>{
source$.next('one');
}, 250);
setTimeout(()=>{
source$.next('two');
}, 1000);
setTimeout(()=>{
source$.next('three');
}, 1250);
setTimeout(()=>{
source$.next('four');
}, 1750);
// subscribe and unsubscribe
const subscriptionA = result$
.subscribe(value => console.log('A', value));
setTimeout(()=>{
result$.subscribe(value => console.log('B', value));
}, 500);
setTimeout(()=>{
result$.subscribe(value => console.log('C', value));
}, 1000);
setTimeout(()=>{
subscriptionA.unsubscribe();
}, 1500);
// complete source
setTimeout(()=>{
source$.complete();
}, 2000);
function customOperator(onCountUpdate = noop) {
return function refCountOperatorFunction(source$) {
let counter = 0;
return defer(()=>{
counter++;
onCountUpdate(counter);
return source$;
})
.pipe(
finalize(()=>{
counter--;
onCountUpdate(counter);
})
);
};
}
function noop(){}<script src="https://unpkg.com/[email protected]/bundles/rxjs.umd.min.js"></script>* NOTE: if your source$ is cold — you might need to share it.
Hope it helps