Android sample showing how to use Handlers for efficient foreground/background operations.

MyActivity.java

package se.hellsoft.demo.codesnippetdemo;

import android.app.Activity;
import android.net.Uri;
import android.os.Bundle;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.Message;
import android.os.SystemClock;
import android.view.View;
import android.widget.EditText;
import android.widget.ProgressBar;

/**
 * Example to illustrate how to work with the Handler class in Android.
 * This example is very naive and is nothing you should do in real life (networking should be handled by a service).
 * However, it serves to show how you can use two Handlers to perform operations on background and foreground thread
 * without having to allocate new objects all the time. 
 */
public class MyActivity extends Activity {
	private static final int START_DOWNLOAD_MSG = 1001;
	private static final int UPDATE_PROGRESS_MSG = 2002;
	private static final int TOGGLE_START_BTN_MSG = 3003;

	private Handler mBackgroundHandler;
	private Handler mForegroundHandler;
	private boolean mPaused = false;

	class MyCallback implements Handler.Callback {

		@Override
		public boolean handleMessage(Message msg) {
			switch (msg.what) {
				case START_DOWNLOAD_MSG:
					startDownload((Uri) msg.obj);
					break;
				case UPDATE_PROGRESS_MSG:
					if (!mPaused) {
						ProgressBar progressBar = ((ProgressBar) findViewById(R.id.download_progress));
						if (progressBar != null) {
							progressBar.setProgress(msg.arg1);
							progressBar.invalidate();
						}
					}
					break;
				case TOGGLE_START_BTN_MSG:
					if (!mPaused) {
						View view = findViewById(R.id.start_download_btn);
						if(view != null) {
							view.setEnabled((Boolean) msg.obj);
						}
					}
					break;
			}
			return true;
		}
	}

	private void startDownload(Uri url) {
		// Disable the button while downloading...
		mForegroundHandler.obtainMessage(TOGGLE_START_BTN_MSG, false).sendToTarget();
		// Fake downloading of file...
		for(int i = 0; i < 1000; i++) {
			SystemClock.sleep(50);
			mForegroundHandler.obtainMessage(UPDATE_PROGRESS_MSG, i, 0).sendToTarget();
		}
		// Enable button again...
		mForegroundHandler.obtainMessage(TOGGLE_START_BTN_MSG, true).sendToTarget();
	}

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		HandlerThread handlerThread = new HandlerThread("BackgroundThread");
		handlerThread.start();
		MyCallback callback = new MyCallback();
		mBackgroundHandler = new Handler(handlerThread.getLooper(), callback);
		mForegroundHandler = new Handler(callback);
		setContentView(R.layout.activity_my);
	}

	@Override
	protected void onResume() {
		super.onResume();
		mPaused = false;
	}

	@Override
	protected void onPause() {
		super.onPause();
		mPaused = true;
	}

	@Override
	protected void onDestroy() {
		mBackgroundHandler.removeMessages(START_DOWNLOAD_MSG);
		mBackgroundHandler.getLooper().quit();
		super.onDestroy();
	}

	/**
	 * Callback for button click
	 *
	 * @param view
	 */
	public void doStartDownload(View view) {
		String fileUri = ((EditText) findViewById(R.id.file_url)).getText().toString();
		Uri uri = Uri.parse(fileUri);
		mBackgroundHandler.obtainMessage(START_DOWNLOAD_MSG, uri).sendToTarget();
	}
}