Dfyjkt

I have to following problem:

Many API calls go through an API interface (Google API) and have to be limited in request per seconds/concurrency because of the Google API limitation.

I use a subject (sink/call pool), which manages all API requests with mergeMap and returns a result to another, piped subject.

Because API requests can unsubscribe before they finish, they shouldn't block my sink. So I have to stop the API request (task) after unsubscription.

The issue:
I don't know how to capture this unsubscribed state correctly. What I currently do is overwriting subscribe and unsubscribe to catch this state. It works but it does not look to "rxjs"ish for me.

What could I improve it?

import Observable, Subject, Subscription, Subscribable, EMPTY from 'rxjs';
import mergeMap, tap from 'rxjs/operators';

function doHeavyRequest() 
 return new Observable(subscribe => 
 // Simulate delay.
 setTimeout(() => 
 subscribe.next(1);
 subscribe.complete();
 , 1000);
 );


const sink = new Subject<[Subject<any>, number]>();

sink.pipe(
 mergeMap(([subject, id]) => 
 // Stop request here if already unsubscribed.
 if (subject.closed) 
 console.log('Request cancelled:', id);
 return EMPTY;
 
 return doHeavyRequest()
 .pipe(
 tap(res => 
 if (!subject.closed) 
 subject.next(res);
 subject.complete();
 else 
 console.log('Request aborted:', id);
 
 )
 );
 , 2)
).subscribe();

// Insert request into sink.
// Overwrite subscribe and unsubscribe.
// Track unsubscribe over the flag alive.
function getSomething(id: number) 
 const task = new Subject();

 const ob = task.asObservable();

 ob.subscribe = (...args: any) => 
 const sub = Observable.prototype.subscribe.call(ob, ...args);
 sub.unsubscribe = () => 
 if (!task.isStopped)
 task.unsubscribe();
 Subscription.prototype.unsubscribe.call(sub);
 ;
 return sub;
 ;

 sink.next([task, id]);

 return ob;


// Make 3 requests and unsubscribe.
export function test() 
 const ob0 = getSomething(0);
 const ob1 = getSomething(1);
 const ob2 = getSomething(2);

 const sub0 = ob0.subscribe(e => 
 console.log('0:', e);
 );
 setTimeout(() => sub0.unsubscribe(), 1500);

 const sub1 = ob1.subscribe(e => 
 console.log('1:', e);
 );
 setTimeout(() => sub1.unsubscribe(), 900);

 const sub2 = ob2.subscribe(e => 
 console.log('2:', e);
 );
 setTimeout(() => sub2.unsubscribe(), 100);

See the test.ts at plunker and the console output:

https://next.plnkr.co/edit/KREjMprTrjHu2zMI?preview

I'm not sure I understood it properly, but it looks like you want to do some cleanup upon unsubscribing, correct?

You can add teardown logic to a single subscription like so:

const subscription = obs.subscribe(() => ...)
subscription.add(() => /* do cleanup here. This is executed upon unsubscribing. */)

Perhaps the finalize pipeable operator might be useful as well. This one adds logic to an observable when it completes, which most of the time is upon complete OR unsubscription. Varies a bit for hot observables, so be aware.

When creating an observable, you can also add teardown logic into it by returning a function from its inner logic function, much like pipe'ing a finalize:

const obs = new Observable(subject => /* subject.next/error/complete somewhere */
 return () => /* cleanup resources upon unsubscribe OR complete */ 
)

Thanks to @Badashi, using finalize worked and looks much better:

import Observable, Subject, Subscription, Subscribable, EMPTY from 'rxjs';
import mergeMap, tap, finalize from 'rxjs/operators';

function doHeavyRequest() 
 return new Observable(subscribe => 
 // Simulate delay.
 setTimeout(() => 
 subscribe.next(1);
 subscribe.complete();
 , 1000);
 );


const sink = new Subject<[Subject<any>, number]>();

sink.pipe(
 mergeMap(([subject, id]) => 
 // Stop request here if already unsubscribed.
 if (subject.closed) 
 console.log('Request cancelled:', id);
 return EMPTY;
 
 return doHeavyRequest()
 .pipe(
 tap(res => 
 if (!subject.closed) 
 subject.next(res);
 subject.complete();
 else 
 console.log('Request aborted:', id);
 
 )
 );
 , 2)
).subscribe();

// Insert request into sink.
// Overwrite subscribe and unsubscribe.
// Track unsubscribe.
function getSomething(id: number) 
 const task = new Subject();
 const ob = task.pipe(finalize(() => 
 if (!task.isStopped) 
 task.unsubscribe();
 
 ));

 sink.next([task, id]);

 return ob;


// Make 3 requests and unsubscribe.
export function test() 
 const ob0 = getSomething(0);
 const ob1 = getSomething(1);
 const ob2 = getSomething(2);

 const sub0 = ob0.subscribe(e => 
 console.log('0:', e);
 );
 setTimeout(() => sub0.unsubscribe(), 1500);

 const sub1 = ob1.subscribe(e => 
 console.log('1:', e);
 );
 setTimeout(() => sub1.unsubscribe(), 900);

 const sub2 = ob2.subscribe(e => 
 console.log('2:', e);
 );
 setTimeout(() => sub2.unsubscribe(), 100);

Output:

0: 1
Request cancelled: 2
Request aborted: 1