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

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.