Handlers in Android

Sun May 10, 2015 by Bill Farmer. Categories: Hacking .

Using a handler to return results from a worker thread

This has taken me quite a while to put together. There are throw-away remarks in the Android docs about this, but no useful examples. You want to get the results back from a worker thread to your main activity so you can do stuff with them. There are several parts to this.

The main activity implements Handler.Callback, and has a handleMessage function.
When the worker thread is created, include a reference to the main activity.
In the worker thread constructor, create a Handler, including the reference to the main activity.
When some results need passing back, send a Message back to the main activity using the handler.

    public class MyActivity extends Activity implements Handler.Callback
    {
        //...
        // On create
        @Override
        public void onCreate(Bundle savedInstanceState)
        {
            //...
            WorkerThread worker = new WorkerThread(this);
        //...
        public boolean handleMessage(Message msg)
        {
        //...
    public class WorkerThread extends Thread
    {
        Handler handler;
        //...
        // Constructor
        public WorkerThread(Handler.Callback callback)
        {
            handler = new Handler(callback);
            //...
        private void someFunction()
        {
            handler.sendEmptyMessage(WHATEVER);
            // or
            Message msg = handler.obtainMessage(WHATEVER, thingy);
            handler.sendMessage(msg);
            //...

The handler will send the message back to the activity in the UI thread, so you can do whatever with the contents.

Bill Farmer

Handlers in Android

Using a handler to return results from a worker thread

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