Android runOnUiThread not async

Question

I've got this piece of code:

public void updateOptionLists() {
    Log.d("UI", "Called update");
    if (updating){
        return;
    }
    updating = true;
    runOnUiThread(
            new Runnable() {
                @Override
                public void run() {
                    updating = false;
                    updateOptionList();
                    scrollToLastTapped();
                    Log.d("UI","Updating");
                }
            });
    Log.d("UI", "Posted update");
}

What I'd expect from logcat would be something like this:

• Called update
• Posted update
• Updating

As far as I know runOnUi should be asynchronous, right? And considering that the functions called alter the views, which takes a while, this should be running asynchronous. Right?

So I look at my logcat:

• Called update
• Updating
• Posted update

Why does this happen? And how do I make sure this runs asynchronous?

Answer 1

runOnUiThread will execute your code on the main thread, which (in this example) also appears to be where it's called from. This explains the ordering of the log statements you see - all code is executing on a single thread, so is synchronous per the documentation (my emphasis):

Runs the specified action on the UI thread. If the current thread is the UI thread, then the action is executed immediately. If the current thread is not the UI thread, the action is posted to the event queue of the UI thread.

runOnUiThread is typically used to execute code on the main thread from a different (i.e. background thread). The use of this separate background thread is what will make a task asynchronous. Calling back to the UI thread at the end of that task is required if you want to modify UI with the results of your background thread calculations.

Android provides several mechanisms for doing work on a background thread and then posting back to the main thread (not all use runOnUiThread explicitly for the latter operation). Good things to read up on include Thread, Handler, AsyncTask, Service, etc.

Answer 2

As far as I know runOnUi should be asynchronous, right?

runOnUiThread, as the name states, runs on UI thread, which is a main thread of an application. It runs synchronously with other code running within that thread and asynchronously with code in other threads. The word 'asynchronous' has no meaning without a context: some code can run asynchronously with other parts of the code, which means these parts of the code run in different threads. Saying that something 'should be asynchronous' makes no sense without this kind of context.