Android Programming: Pushing the Limits -- Chapter 6: Services and Background Tasks

什么时候使用Service

服务类型

开启服务

后台运行

服务通信

附加资源

什么时候使用Service：

@、任何与用户界面无关的操作，可移到后台线程，然后由一个Service来控制这个线程。

服务类型：

@、First is the one that performs work for the application independent of the user’s input.

如：后台执行的音乐播放器。

@、The other type of Service is one that’s directly triggered by an action from the user.

如：照片分享应用的拍照上传服务。 完成后系统自动终止服务。

@、Service的生命周期：

1、  对于Service来说，总是会被执行的两个回调方法：onCreate和onDestroy。

2、  在onCreate中完成大部分初始化工作，在onDestroy中完成大部分清理工作。

3、  由于onCreate，onDestroy都是在主线程执行，应此要把长时间运行的操作移到后台线程中。

开启服务：

@、Context.startService：

1、  如果需要提供给外部调用，则需要配置intent-filter。

2、  调用Context.startService会触发onStartCommand，系统根据onStartCommand的返回值决定服务被关闭后是否重启。

3、  START_STICKY：系统因某些原因，如内存不足，关闭服务，如果服务的onStartCommand返回START_STICKY，则系统会重启服务，不过参数Intent传入的是null。这种情况，可能需要在onDestroy中对数据进行保存。（音乐播放器）

4、  START_NOT_STICKY：服务被系统关闭后，不会被重启。（上传服务）

5、  START_REDELIVER_INTENT：类似START_STICKY，但是参数Intent传入的是初始的Intent。

6、  可通过BroadcastReceiver方式将结果回馈给Activity。

7、  此方式需要通过调用Service.stopSelf()或者Context.stopService()来停止服务。

8、  实例，演示Service.stopSelf()

public class MyMusicPlayer extends Service implements MediaPlayer.OnCompletionListener{
    public static final String ACTION_ADD_TO_QUEUE =
            "com.example.lsp.myactivity.ADD_TO_QUEUE";
    private ConcurrentLinkedQueue<Uri> mTrackQueue;
    private MediaPlayer mMediaPlayer;

    public IBinder onBind(Intent intent){
        return null;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        mTrackQueue = new ConcurrentLinkedQueue<Uri>();
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        String action = intent.getAction();
        if(ACTION_ADD_TO_QUEUE.equals(action)){
            Uri trackUri = intent.getData();
            addTrackToQueue(trackUri);
        }
        return START_NOT_STICKY;
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        if(mMediaPlayer != null){
            mMediaPlayer.release();
            mMediaPlayer = null;
        }
    }

    /**
     *  Add track to end of queue if already palying,
     *  otherwise create a new MediaPlayer and start playing.
     */
    private synchronized void addTrackToQueue(Uri trackUri){
        if(mMediaPlayer == null){
            try{
                mMediaPlayer = MediaPlayer.create(this, trackUri);
                mMediaPlayer.setOnCompletionListener(this);
                mMediaPlayer.prepare();
                mMediaPlayer.start();
            }catch (IOException e){
                stopSelf();
            }
        }else{
            mTrackQueue.offer(trackUri);
        }
    }

    // Track completed, start playing next or stop service...
    @Override
    public void onCompletion(MediaPlayer mp) {
        mMediaPlayer.reset();
        Uri nextTrackUri = mTrackQueue.poll();
        if(nextTrackUri != null){
            try{
                mMediaPlayer.setDataSource(this, nextTrackUri);
                mMediaPlayer.prepare();
                mMediaPlayer.start();
            }catch (IOException e){
                stopSelf();
            }
        }else{
            stopSelf();
        }
    }
}

MyMusicPlayer.java

@、Context.bindService：

1、  此方法用于同一应用内组件拥有Service对象，然后通过Service对象直接访问Service方法。这种方式叫local binder。

2、  如果是跨应用调用Service，则需要AIDL。

3、  任何不属于当前前台运行的应用的Service都有可能被系统终止掉(为了free RAM)。想要在用户退出应用后，Service能够进行运行，可调用Service.startForeground()方法。

4、  当最后一个client断开连接(Context.unbindService())，服务将自动停止，除非在对后一个client断开连接后调用Service.startForeground()来保持Service alive。这也是为什么正确地调用Service.stopForeground()如此重要。

后台运行：

@、IntentService：

1、  IntentService通过Handler的方式来实现后台运行。

2、  自定义Service继承IntentService，实现onHandleIntent方法：根据入参Intent的action(intent.getAction())，实现多action的切换。

3、  多次调用时，以队列的方式处理，即onHandleIntent一次只处理一个intent。

4、  实例：

public class MyIntentService extends IntentService {
    private static final String NAME = MyIntentService.class.getSimpleName();
    public static final String ACTION_UPLOAD_PHOTO =
            "com.example.lsp.myactivity.UPLOAD_PHOTO";
    public static final String EXTRA_PHOTO = "bitmapPhoto";
    public static final String ACTION_SEND_MASSAGE =
            "com.example.lsp.myactivity.SEND_MESSAGE";
    public static final String EXTRA_MESSAGE = "messageText";
    public static final String EXTRA_RECIPIENT = "messageRecipient";

    public MyIntentService(){
        super(NAME);
        // We don't want intents redelivered in case we're shut down unexpectedly
        setIntentRedelivery(false);
    }

    /**
     * This methos is executed on its own thread, one intent at a time...
     */
    @Override
    protected void onHandleIntent(Intent intent) {
        String action = intent.getAction();

        if(ACTION_SEND_MASSAGE.equals(action)){
            String messageText = intent.getStringExtra(EXTRA_MESSAGE);
            String messageRecipient = intent.getStringExtra(EXTRA_RECIPIENT);
            sendMessage(messageRecipient, messageText);
        }else if(ACTION_UPLOAD_PHOTO.equals(action)){
            Bitmap photo = intent.getParcelableExtra(EXTRA_PHOTO);
            uploadPhoto(photo);
        }
    }

    private void sendMessage(String messageRecipient, String messageText){
        // TODO Make network call...

        // TODO Send a broadcast that operation is completed
    }

    private void uploadPhoto(Bitmap photo){
        // TODO Make network call...

        // TODO Send a broadcast that operation is completed
        //sendBroadcast(new Intent(BROADCAST_UPLOAD_COMPLETED));
    }
}

MyIntentService.java

@、ExecutorService：

1、  实现操作并行执行。

2、  自定义服务类，继承Service类。

3、  添加ExecutorService对象作为成员变量。

4、  在自定义服务类中创建私有类，并实现Runnable接口，由此类来完成具体的功能操作。

5、  在onStartCommand()方法中通过ExecutorSerive的execute方法执行操作。

6、  实例：

public class MediaTranscoder extends Service {
    private static final int NOTIFICATION_ID = 1001;
    public static final String ACTION_TRANSCODE_MEDIA =
            "com.example.lsp.myactivity.TRANSCODE_MEDIA";
    public static final String EXTRA_OUTPUT_TYPE = "outputType";
    private ExecutorService mExecutorService;
    private int mRunningJobs = 0;
    private final Object mLock = new Object();
    private boolean mIsForeground = false;

    public IBinder onBind(Intent intent){
        return null;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        mExecutorService = Executors.newCachedThreadPool();
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        String action = intent.getAction();
        if(ACTION_TRANSCODE_MEDIA.equals(action)){
            String outputType = intent.getStringExtra(EXTRA_OUTPUT_TYPE);

            // Start new job and increase the running job counter
            synchronized(mLock){
                TranscodeRunnable transcodeRunnable =
                        new TranscodeRunnable(intent.getData(), outputType);
                mExecutorService.execute(transcodeRunnable);
                mRunningJobs++;
                startForegroundIfNeeded();
            }
        }
        return START_NOT_STICKY;
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        mExecutorService.shutdownNow();
        synchronized (mLock){
            mRunningJobs = 0;
            stopForegroundIfAllDone();
        }
    }

    public void startForegroundIfNeeded(){
        if(!mIsForeground){
            Notification notification = buildNotificatin();
            startForeground(NOTIFICATION_ID, notification);
            mIsForeground = true;
        }
    }

    private Notification buildNotificatin(){
        Notification notification = null;
        // TODO Build the notification here
        return notification;
    }

    private void stopForegroundIfAllDone(){
        if(mRunningJobs == 0 && mIsForeground){
            stopForeground(true);
            mIsForeground = false;
        }
    }

    private class TranscodeRunnable implements Runnable{
        private Uri mInData;
        private String mOutputType;

        private TranscodeRunnable(Uri inData, String outputType){
            mInData = inData;
            mOutputType = outputType;
        }

        @Override
        public void run() {
            // TODO Perform transcoding here...

            //Decrease counter when we're done..
            synchronized (mLock){
                mRunningJobs--;
                stopForegroundIfAllDone();
            }
        }
    }
}

MediaTranscoder.java

服务通信：

@、使用BroadcastReceiver的方式

@、在bindService()模式的基础上，增加回调接口来实现通信

1、实例：

public class MyLocalService extends Service {
    private static final int NOTIFICATION_ID = 1001;
    private LocalBinder mLocalBinder = new LocalBinder();
    private Callback mCallback;

    public IBinder onBind(Intent intent) {
        return mLocalBinder;
    }

    public void performLongRunningOperation(MyComplexDataObject dataObject) {
        new MyAsyncTask().execute(dataObject);
    }

    public void setCallback(Callback callback) {
        mCallback = callback;
    }

    public class LocalBinder extends Binder {
        public MyLocalService getService() {
            return MyLocalService.this;
        }
    }

    public interface Callback {
        void onOperationProgress(int progress);
        void onOperationCompleted(MyComplexResult complexResult);
    }

    private final class MyAsyncTask extends AsyncTask<MyComplexDataObject, Integer, MyComplexResult> {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            startForeground(NOTIFICATION_ID, buildNotification());
        }

        @Override
        protected void onProgressUpdate(Integer... values) {
            if(mCallback != null && values.length > 0) {
                for (Integer value : values) {
                    mCallback.onOperationProgress(value);
                }
            }
        }

        @Override
        protected MyComplexResult doInBackground(MyComplexDataObject... myComplexDataObjects) {
            MyComplexResult complexResult = new MyComplexResult();
            // Actual operation left out for brevity...
            return complexResult;
        }

        @Override
        protected void onPostExecute(MyComplexResult myComplexResult) {
            if(mCallback != null ) {
                mCallback.onOperationCompleted(myComplexResult);
            }
            stopForeground(true);
        }

        @Override
        protected void onCancelled(MyComplexResult complexResult) {
            super.onCancelled(complexResult);
            stopForeground(true);
        }
    }

    private Notification buildNotification() {
        Notification notification = null;
        // Create a notification for the service..
        return notification;
    }
}

MyLocalService.java

public class MyActivity extends Activity
        implements ServiceConnection, MyLocalService.Callback {
    private MyLocalService mService;

    /**
     * Called when the activity is first created.
     */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main_activity);
    }

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

    @Override
    protected void onPause() {
        super.onPause();
        if (mService != null) {
            mService.setCallback(null); // Important to avoid memory leaks
            unbindService(this);
        }
    }

    public void onTriggerLongRunningOperation(View view) {
        if(mService != null) {
            mService.performLongRunningOperation(new MyComplexDataObject());
        }
    }

    @Override
    public void onOperationProgress(int progress) {
        // TODO Update user interface with progress..
    }

    @Override
    public void onOperationCompleted(MyComplexResult complexResult) {
        // TODO Show result to user...
    }

    @Override
    public void onServiceConnected(ComponentName componentName,
                                   IBinder iBinder) {
        mService = ((MyLocalService.LocalBinder) iBinder).getService();
        mService.setCallback(this);

        // Once we have a reference to the service, we can update the UI and
        // enable buttons that should otherwise be disabled.
        findViewById(R.id.trigger_operation_button).setEnabled(true);
    }

    @Override
    public void onServiceDisconnected(ComponentName componentName) {
        // Disable the button as we are loosing the reference to the service.
        findViewById(R.id.trigger_operation_button).setEnabled(false);
        mService = null;
    }
}

MyActivity.java

附加资源：

Google’s changes to the Service API at

http://android-developers.blogspot.se/2010/02/service-api-changes-starting-with.html

Dianne Hackborn at

http://android-developers.blogspot.se/2010/04/multitaskingandroid-way.html