RxAndroid: UI changes on Schedulers.io() thread

Question

I have simple job on IO thread which is changing home screen wallpaper, after that I'm trying to run some animation on UI thread:

     AppObservable.bindFragment(this, Observable.just(0))
       .observeOn(Schedulers.io())
       .subscribe(v -> setWallpaperOnSeparateThread());

private void setWallpaperOnSeparateThread() {
     WallpaperHelper.setBitmapAsWallpaper(photoViewAttacher.getVisibleRectangleBitmap(), getBaseActivity());

     AppObservable.bindFragment(this, Observable.just(0))
       .delay(500, TimeUnit.MILLISECONDS)
       .observeOn(AndroidSchedulers.mainThread())
       .subscribe(integer -> loadFinishAnimationAfterSetWallpaper());
}

but this approach results in error: java.lang.IllegalStateException: Observers must subscribe from the main UI thread, but was Thread[RxCachedThreadScheduler-1,5,main]

I've tried to change second Observable to:

  AppObservable.bindFragment(this, Observable.just(0))
    .delay(2000, TimeUnit.MILLISECONDS)
    .observeOn(Schedulers.io())
    .subscribeOn(AndroidSchedulers.mainThread())
    .subscribe(integer -> loadFinishAnimationAfterSetWallpaper());

But it didn't help.

Answer 1

AppObservable.bindFragment(this, Observable.just(0)) throw an exception as it's not called from the Main Thread

This code is not called in the main thread because you observe on Schedulers.io in this code (see bellow), than latter call AppObservable.bindFragment(this, Observable.just(0))

AppObservable.bindFragment(this, Observable.just(0))
   .observeOn(Schedulers.io())
   .subscribe(v -> setWallpaperOnSeparateThread());

You want to perform a task in io thread, then perform a task in main thread. To do so, you can chain you call using one Observable .

AppObservable.bindFragment(this, Observable.just(0))
   .observeOn(Schedulers.io())
   .flatMap(v -> Observable.defer(() -> WallpaperHelper.setBitmapAsWallpaper(photoViewAttacher.getVisibleRectangleBitmap(), getBaseActivity())))
   .delay(500, TimeUnit.MILLISECONDS)
   .observeOn(AndroidSchedulers.mainThread())
   .subscribe(v -> loadFinishAnimationAfterSetWallpaper());

Please note I use defer to represente you async task as an Observable but you can replace the flatMap call with doOnNext call.

AppObservable.bindFragment(this, Observable.just(0))
   .observeOn(Schedulers.io())
   .doOnNext(v -> WallpaperHelper.setBitmapAsWallpaper(photoViewAttacher.getVisibleRectangleBitmap(), getBaseActivity()))
   .delay(500, TimeUnit.MILLISECONDS)
   .observeOn(AndroidSchedulers.mainThread())
   .subscribe(v -> loadFinishAnimationAfterSetWallpaper());

Answer 2

Actually observeOn is for the subcscriber thread while subscribeOn is for observable thread. So you should reverse them

.observeOn(AndroidSchedulers.mainThread())
.subscribeOn(Schedulers.io())