Question

I came across this code in a very basic Handler tutorial. The code is working fine but I do not understand why I have to use Handler for progressDialog.dismiss() ??? I removed the Handler part and placed progressDialog.dismiss() in the run() method and it worked fine. So why used Handler???

 import android.app.Activity;
    import android.app.ProgressDialog;
    import android.os.Bundle;
    import android.os.Handler;
    import android.os.Message;
    import android.view.View;
    import android.view.View.OnClickListener;
    import android.widget.Button;
    import android.widget.Toast;


    public class HandlerThread extends Activity{

    private Button start;
    private ProgressDialog progressDialog;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        start = (Button) findViewById(R.id.Button01);
        start.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // TODO Auto-generated method stub
                fetchData();
            }

        });
    }



    protected void fetchData() {
        // TODO Auto-generated method stub
        progressDialog = ProgressDialog.show(this, "", "Doing...");
        new Thread() {
            public void run() {
                try {

                    Thread.sleep(8000);

                    } catch (InterruptedException e) {

                    }
                      messageHandler.sendEmptyMessage(0);

                    }
        }.start();


    }



    private Handler messageHandler = new Handler() {

        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            progressDialog.dismiss();

        }
    };
}

Answer 1

From the documentation of View:

You must always be on the UI thread when calling any method on any
view. If you are doing work on other threads and want to update the
state of a view from that thread, you should use a Handler.

In your example, when you’ve to call the dismiss() method on the ProgressDialog, as per the above documentation, you must do so from the UI thread. The messageHandler is initialized to an instance of a Handler when the HandlerThread class is instantiated (presumably on the UI thread).

From the documentation of Handler:

Each Handler instance is associated with a single thread and that
thread’s message queue. When you create a new Handler, it is bound to
the thread / message queue of the thread that is creating it — from
that point on, it will deliver messages and runnables to that message
queue and execute them as they come out of the message queue.

So to communicate with the UI thread from your new thread, just post a message to the Handler created on the UI thread.

If you call methods on a View from outside the UI thread, it invokes undefined behaviour, which means, it may appear to work fine. But it’s not always guaranteed to work fine.