java – RxJava single background thread scheduler-ThrowExceptions

Exception or error:

I’m fairly new to RxJava so this is probably a dumb question. I am going to describe my scenario.

I have some code running on the UI thread which will update some images but those images are not very important and they consume a bit of resources while generating them so I want to generate them on a single thread (not the UI thread of course) and generate them one by one. I’m guessing the trampoline scheduler is what I want but my problem is that if I use it then it does the work on the UI thread and I want it to do it on another thread.

Obviously I can write my own thread in which I can queue items and then it processes those one by one but I thought maybe RxJava would have a simple solution for me?

My current code looks like this:

Observable<Bitmap> getImage = Observable.create(new Observable.OnSubscribe<Bitmap>() {
    @Override public void call(Subscriber<? super Bitmap> subscriber) {
        Log.w(TAG,"On ui thread? "+ UIUtils.isRunningOnUIThread());

getImage.subscribeOn(Schedulers.trampoline()).subscribe(new Action1<Bitmap>() {
    @Override public void call(Bitmap bitmap) {

My log that says “On ui thread?” always has true. So how do I make that code and all subsequent attempts to do the same thing run on a single thread (not the ui thread) in order without writing a bunch of code to queue that work?


I believe that this can now be accomplished using Schedulers.single() or if you want your own you can use new SingleScheduler(). I’m still testing but I think it does what I wanted back when I posted this.

How to solve:

You can create a single reusable thread to create a Scheduler for the Observable in one of the following ways:

  • Create a ThreadPoolExecuter with a pool size of 1 (Executors.newSingleThreadExecutor() is a convenient static factory method for doing that), then use it to generate the schedulers via the Schedulers.from() method.
  • RxAndroid provides a custom Scheduler implementation that uses a Handler to schedule the actions, and thus can be used with any Thread that has a Looper running by passing it’s Handler to the AndroidSchedulers.handlerThread() factory method.

Note that you will need to observe on a main thread Scheduler if you’re interacting with the UI at the conclusion of these tasks.


In RxJava 2 you can use Schedulers.single() which:

Returns a default, shared, single-thread-backed Scheduler instance for
work requiring strongly-sequential execution on the same background

Please see the documentation for more details.

I don’t see it available in RxJava 1 Schedulers documentation.


You are using trampoline scheduler, so it means your source observable will be run on the current thread (here is main thread).

And the subscribeOn will work for both upstream and downstream. That why your log shows your are running on the main thread

To fix this, you can use Schedulers.single() in the subscribeOn, and then observeOn the main thread.

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