RxSwift — background task without freezing UI

Question

I would like to do some heavy lifting in the background thread of my iOS app and NOT freeze the UI while it is being performed. What I try is:

self.someDisposable = heavyLiftingFuncReturningObservable()
            .subscribeOn(ConcurrentDispatchQueueScheduler(qos: .background))
            .observeOn(MainScheduler.instance)
            .subscribe(
                onNext: { [weak self] image in
                    // update UI
                },
                onError: { ... }
            )

Why does the above not work as expected and how to make it work?

I think making that Observable `lazy` could help. It could help, that it will be actually called when needed (from `.background` queue) and not when defined. — Štefan Ľupták, Nov 09 '17 at 12:59

score 7 · Answer 1 · edited Mar 24 '19 at 17:50

I believe the problem lies in your implementation of .heavyLiftingFuncReturningObservable(), namely in the fact that apparently it starts working on current thread instead of waiting until being subscribed to and running on background scheduler. Solution for that is to use .deferred() inside of .heavyLiftingFuncReturningObservable() function.

See http://adamborek.com/top-7-rxswift-mistakes/

SoftDesigner · Answer 2 · 2021-01-26T10:06:23.043

5

You can do this:

self.someDisposable = Observable.just(0) // dummy
    .observeOn(ConcurrentDispatchQueueScheduler(qos: .background))
    .flatMap { [unowned self] _ in
        heavyLiftingFuncReturningObservable()
    }
    .observeOn(MainScheduler.instance)
    .subscribe(
        onNext: { [weak self] image in
            // update UI
        },
        onError: { ... }
    )

edited Jan 26 '21 at 10:06

answered Jun 05 '20 at 14:17

SoftDesigner

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I am unsure about the subscribeOn This applies only once, but shouldn't it be observeOn here to force the switch after the initial first observeOn(... .background)? – altagir Jan 26 '21 at 02:36

score 0 · Answer 3 · 2021-07-13T07:39:23.373

Don't do this... use the Observable.deferred { } method

Put the heavy task inside the block and it will run on subscription.

If the work has a single response, use a Single observable type.
If the work only has to complete, use a Completable observable type

Use the observeOn / subscribleOn operators to control the thread doing the work.

Best of all, don't use RX for this use case.

RxSwift — background task without freezing UI

3 Answers3