1.使用Intent进行异步通讯
在Service任务一旦完成后，就发送广播。开发者只需要实现一个BroadcastReceiver来监听响应既可。
Activity.startService启动intentService，intentService完成任务后sendBroadcast()发送广播，BroadcastReceiver.startActivity（）通知Activity操作结果。
在任务结束后调用sendBroadcast(new Intent(Action));
然后在广播中通知Activity执行的结果。这种方法的优点是，Android提供了现成的机制，而不需要开发者自己构建复杂的组件间消息处理系统。
缺点是通知的结果受限于Intent，该方法也不适合在IntentService和Activity之间进行大规模快速更新操作，比如进度条，因为这会阻塞系统。
具体的代码就不多讲，和基本的Service一样使用，只是多实现了个广播.
2.本地绑定Service
本地绑定Service可以在Service中给Activity提供更复杂的回调，因为那些耗时的操作必须放到Service的后台线程中，所以Service大部分回调应该是异步的。实际的调用触发后台操作立即返回，一旦操作完成，Service使用回调接口来通知Activity相应的执行结果.
下面的例子添加了一个回调接口和一个实现AsyncTask的类，该类用来模拟后台操作。Service的onBind()方法返回一个localBinder对象，通过该对象客户端可以获取Service引用,并能执行doLongRunningOperation()方法。此方法创建了一个新的AsyncTask，并用客户端传递进来的参数执行execute()函数,在执行的过程中，回调函数会通知客户端新进度，当执行完毕会回调执行结果.
这个例子以下载图片讲解其详细的实现步骤
public class MyLocalService extends Service { private static final int NOTIFICATION_ID = 1001;//通知的IDprivate LocalBinder mLocalBinder = new LocalBinder();//用于返回的localBinder对象private Callback mCallback;@Overridepublic IBinder onBind(Intent intent) { // TODO Auto-generated method stubreturn this.mLocalBinder;}public class LocalBinder extends Binder { public MyLocalService getService() { return MyLocalService.this;}}public void setCallback(Callback callback) { this.mCallback = callback;}//回调接口，便于Activity与Service进行通信public interface Callback { void onOperationProgress(int progress);void onOperationCompleted(String result);}public void doLongRunningOperation(String urlString) { new MyAsyncTask().execute(urlString);}private class MyAsyncTask extends AsyncTask<String, Integer, String> { //处理任务之中@Overrideprotected String doInBackground(String... params) { publishProgress(0);//更新进度条String result = downloadPhoto(params[0]);publishProgress(10);return result;}//执行任务之前@Overrideprotected void onPreExecute() { super.onPreExecute();startForeground(NOTIFICATION_ID, buildNotification());//保持服务一直处于活跃状态。并创建通知}//返回任务执行结果@Overrideprotected void onPostExecute(String result) { if (mCallback != null) { mCallback.onOperationCompleted(result);}mCallback.onOperationProgress(10);stopForeground(true);}//进度条更新@Overrideprotected void onProgressUpdate(Integer... values) { if (mCallback != null && values.length > 0) { for (Integer value : values) { mCallback.onOperationProgress(value);}}}//任务(结束)调用取消后@Overrideprotected void onCancelled(String result) { // TODO Auto-generated method stubsuper.onCancelled(result);stopForeground(true);}}//创建通知private Notification buildNotification() { Builder builder = new Builder(this);builder.setSmallIcon(R.drawable.ic_launcher);builder.setContentTitle("正在下载");builder.setContentText("图片还在下载中");Notification notification = builder.getNotification();return notification;}//下载图片private String downloadPhoto(String uriString) { HttpURLConnection conn = null;try { Thread.sleep(8000);URL url = new URL(uriString);conn = (HttpURLConnection) url.openConnection();conn.setRequestMethod("GET");conn.setReadTimeout(8000);conn.setConnectTimeout(8000);InputStream is = conn.getInputStream();File file = new File(Environment.getExternalStorageDirectory().toString() + "/" + "book.jpg");FileOutputStream fos = new FileOutputStream(file);byte[] buf = new byte[8196];int line = 0;while ((line = is.read(buf)) != -1) { fos.write(buf, 0, line);}fos.close();return "下载完成";} catch (Exception e) { e.printStackTrace();} finally { if (conn != null) { conn.disconnect();}}return "下载失败";}}
下面的代码显示了更新后的Activity，值得注意的变化是Activity实现了MyLocalService.Callback接口，当在onServiceConnected()方法中获取到对Service的引用后，调用setCallback(this)方法，以便于Activity能在操作执行期间收到回调通知。还有一点非常重要，当用户离开Activity或者调用onPause()时，不要忘记移除回调监听，否则可能导致内存泄漏。
public class MainActivity extends Activity implements ServiceConnection,MyLocalService.Callback { private Button start;private TextView content;private MyLocalService mService;private ProgressDialog dialog;@Overrideprotected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);this.start = (Button) findViewById(R.id.start);this.content = (TextView) findViewById(R.id.content);
}
//启动Service里的AsyncTask线程
public void onTriggerLongRunningOperation(View view) { if (this.mService != null) { this.mService.doLongRunningOperation("http://image.baidu.com/i?tn=download&word=download&ie=utf8&fr=detail&url=http%3A%2F%2Fwenwen.soso.com%2Fp%2F20090901%2F20090901120123-329341688.jpg&thumburl=http%3A%2F%2Fimg5.imgtn.bdimg.com%2Fit%2Fu%3D390263343%2C2695552076%26fm%3D21%26gp%3D0.jpg");}}//进度条更新操作@Overridepublic void onOperationProgress(int progress) { if (dialog == null) { dialog = new ProgressDialog(this);dialog.setMessage("正在下载");dialog.setMax(10);dialog.setCanceledOnTouchOutside(false);dialog.show();} else { dialog.setProgress(progress);}if (progress == 10 && dialog != null) { dialog.dismiss();}}@Overridepublic void onOperationCompleted(String result) { this.content.setText(result);//设置下载图片的返回结果}//绑定服务后的初始化操作都写在该方法内@Overridepublic void onServiceConnected(ComponentName name, IBinder service) { this.mService = ((MyLocalService.LocalBinder) service).getService();this.mService.setCallback(this);this.start.setOnClickListener(new OnClickListener() { @Overridepublic void onClick(View v) { onTriggerLongRunningOperation(v);//执行Service下载线程start.setEnabled(false);//设置启动按钮为不可用}});}//注销之前结束后的收尾操作@Overridepublic void onServiceDisconnected(ComponentName name) { this.mService = null;}@Overrideprotected void onResume() { super.onResume();Intent bindIntent = new Intent(this, MyLocalService.class);bindService(bindIntent, this, BIND_AUTO_CREATE);//绑定Service}@Overrideprotected void onPause() { super.onPause();if (this.mService != null) { this.mService.setCallback(null);unbindService(this);//解绑Service}}}
如果用户在操作执行完前离开了Activity,Service还会继续执行，因为显式调用了StartForeground()方法。如果Activity在操作结束前又重新恢复，它会在成功绑定到Service后继续调用继续接受回调。这种行为很容易把耗时的操作和用户界面分开。并且允许Activity恢复后还能继续获取运行的状态。
如果Service内部维护了一些状态，允许客户端获取这些状态以及订阅这些状态的变化是个很好的做法。因为当Activity重新恢复并绑定到Service后，这些状态可能已经发生了变化.