What is the relationship between Looper, Handler and MessageQueue in Android?

Question

I have checked the official Android documentation/guide for Looper, Handler and MessageQueue . But I couldn\'t get it. I am new to and

Answer 1

Extending the answer, by @K_Anas, with an example, As it stated

It's widely known that it's illegal to update UI components directly from threads other than main thread in android.

for instance if you try to update the UI using Thread.

    int count = 0;
    new Thread(new Runnable(){
        @Override
        public void run() {
            try {
                while(true) {
                    sleep(1000);
                    count++;
                    textView.setText(String.valueOf(count));
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }


   ).start();

your app will crash with exception.

android.view.ViewRoot$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.

in other words you need to use Handler which keeps reference to the MainLooper i.e. Main Thread or UI Thread and pass task as Runnable.

  Handler handler = new Handler(getApplicationContext().getMainLooper);
        int count = 0;
        new Thread(new Runnable(){
            @Override
            public void run() {
                try {
                    while(true) {
                        sleep(1000);
                        count++;
                        handler.post(new Runnable() {
                           @Override
                           public void run() {
                                 textView.setText(String.valueOf(count));
                           }
                         });

                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }

    ).start() ;