Service通信详解

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ID
private LocalBinder mLocalBinder = new LocalBinder();//用于返回的localBinder对象
private Callback mCallback;


@Override
public IBinder onBind(Intent intent) {
// TODO Auto-generated method stub
return 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
@Override
protected 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
@Override
protected 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
@Override
protected void onPostExecute(String result) {
if (mCallback != null) {
mCallback.onOperationCompleted(result);
}
mCallback.onOperationProgress(10);
stopForeground(true);
}

//进度条更新
@Override
protected 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
@Override
protected void onCancelled(String result) {
// TODO Auto-generated method stub
super.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;


@Override
protected 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
@Override
public 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();
}
}


@Override
public 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
@Override
public void onServiceConnected(ComponentName name, IBinder service) {
this.mService = ((MyLocalService.LocalBinder) service).getService();
this.mService.setCallback(this);
this.start.setOnClickListener(new OnClickListener() {


@Override
public 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
@Override
public void onServiceDisconnected(ComponentName name) {
this.mService = null;
}


@Override
protected void onResume() {
super.onResume();
Intent bindIntent = new Intent(this, MyLocalService.class);
bindService(bindIntent, this, BIND_AUTO_CREATE);//绑定Service
}


@Override
protected 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.

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