Single....blockingGet() hangs in RxJava 2.1.3 when using Android/Firebase

When using the following pattern to synchronously get data from Firebase Realtime Database:

String s = Single.create(new SingleOnSubscribe<String>() {
	@Override
	public void subscribe(SingleEmitter<String> e) throws Exception {
		FirebaseDatabase.getInstance().getReference("path").orderByChild("child")
                     .equalTo("xyz").addListenerForSingleValueEvent(new ValueEventListener() {
    		@Override
    		public void onDataChange(DataSnapshot dataSnapshot) {
        		e.onSuccess("Got it");
    		}
    		@Override
    		public void onCancelled(DatabaseError databaseError) {
        		e.onError(databaseError.toException());
    		}
		});
	}
}).blockingGet();

It will hang and create an ANR error. If I use the same Firebase “innards” outside of the Single, it fires just fine. The Single without the Firebase code inside also will fire, so it seems there is some incompatibility between the two.

[Apologies on the formatting, I can’t for the life of me get it to look nice]

Any ideas?

[akarnokd]

If blockingGet doesn’t return it means the ValueEventListener was not called. We had some issues when dealing with Android features that use weak references for their listeners: If you don’t have a strong reference to them, they may disappear and events are never signalled. Try this:

ValueEventListener vel = new ValueEventListener() {
    		@Override
    		public void onDataChange(DataSnapshot dataSnapshot) {
        		e.onSuccess("Got it");
    		}
    		@Override
    		public void onCancelled(DatabaseError databaseError) {
        		e.onError(databaseError.toException());
    		}
};

WhatEverThisTypeIs fb = FirebaseDatabase.getInstance()
                     .getReference("path")
                     .orderByChild("child")
                     .equalTo("xyz");
fb.addListenerForSingleValueEvent(vel);

e.setCancellation(() -> fb.removeListenerForSingleValueEvent(vel));

Also blocking on the main thread is discouraged so not sure on which you blocked originally and in your Rx. Assuming from the ANR that you block the main thread and the Firebase listener signals on the main thread but can’t since you are blocking it, hence a deadlock.

[akarnokd]

Please try replacing blockingGet with subscribe? If it works, then please consider my comment again:

you block the main thread and the Firebase listener signals on the main thread but can’t since you are blocking it, hence a deadlock.