Why updating ProgressBar in background thread doesn't throw CalledFromWrongThreadException

Question

I am trying to review the concept of Handler. So I simply create a Thread to update a ProgressBar and see if it would throw an exception.

Here's my code

@Override
public void onResume() {
    super.onResume();
    new Thread(){
        @Override
        public void run() {
            super.run();
            while (progressBar.getProgress() <100){
                try {
                    sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                progressBar.setProgress(pb.getProgress()+10);
            }
        }
    }.start();
}

My question is simple. Why it doesn't throw any CalledFromWrongThreadException? If it meant to be this way, should we forget about Handler when we deal with progress updating?

Answer 1

https://cs.android.com/android/platform/superproject/+/android-10.0.0_r30:frameworks/base/core/java/android/widget/ProgressBar.java;l=1610

Bad luck--ProgressBar has special support for updating its progress from another thread. That's why it doesn't throw something.

But because it's so specifically this operation that has this extra support, no to the second question. You wouldn't, in general, stop using a handler. For cases where, for example, you need to do one other little UI thing while updating the progress.