android - BlockingGet block UI thread RxJava 2

Question

I am dealing with the problem.

I am trying to call RxJava in the sync manner, however doing that results in blocking the Main thread.

Here is my code

   @Override
    public Single<SettingsBundle> getSettings() {
        SettingsBundle settingsModel = mSettingsManager.getSettings();
        return Single.just(settingsModel).map(mSettingsMapper);
    }

And here is my sync call

   @Override
    public SettingsBundle getSettingsSync() {
        return getSettings().blockingGet();
    }

When calling the getSettingsSync the Main thread is blocked, however sometimes it works fine, what is more problematic.

I have tried something like that

@Override
public SettingsBundle getSettingsSync() {
    return getSettings()
            .subscribeOn(Schedulers.newThread())
            .observeOn(AndroidSchedulers.mainThread())
            .blockingGet();
}

But it stills remains blocked.

What I am doing wrong, I would be grateful for any help.

Thanks.

Answer 1

TL;TR

never use observeOn(AndroidSchedulers.mainThread()) with blockingGet()

---

Long version

The output for:

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val result =
                Single.just("Hello")
                .subscribeOn(Schedulers.io())
               // .observeOn(AndroidSchedulers.mainThread())
                .map {
                    println("1. blockingGet `$it` thread: ${Thread.currentThread()}")
                    return@map it
                }
                .blockingGet()
        println("2. blockingGet `$result` thread: ${Thread.currentThread()}")
    }
}

is

 1. blockingGet `Hello` thread: Thread[RxCachedThreadScheduler-1,5,main]
 2. blockingGet `Hello` thread: Thread[main,5,main]

As you can see result was generated on main thread (line 2), the map function was execute in the RxCachedThreadScheduler thread.

With the line .observeOn(AndroidSchedulers.mainThread()) decommented the blockingGet() never return and all is stucked.