第三章--四大組件之一Service

一直是聽過其大名，現在終於是學到了。

1、什麼是Service

作爲android四大組件之一，它一直默默付出，不像Activity，可以被人所見。Service一般進行長時間的後臺操作，沒有界面，不是進程也不是線程，比avtivity有更高的優先級。使用Service，有兩步。

1. 定義一個繼承Service的子類
2. 在AndroidManifest中註冊該Service

Service有一些可以重寫的方法：

1. IBinder onBind(Intent intent)：這是必須重寫的方法，當有一個客戶端和Service綁定時，返回的對象可以作爲communication channel（我不知道怎麼翻譯）和Service通信，如果沒有綁定，返回null。IBinder是一個接口，但是一般不直接實現，而是繼承IBinder的實現類Binder。
2. void onCreate()：第一次創建Service後會被調用。
3. onStartCommand(Intent intent, int flags, int startId)：客戶端調用startService(Intent intent)時會被調用。取代了原本的方法onStart(Intent intent, int startId)
4. void onDestroy()：Service關閉之前會調用的方法

Service的生命週期

說完了service的一些基本知識，下面看下它的生命週期：

從圖中可以看出Service有兩個運行方式，一個是startService()，另一個是bindService()。所以下面我用兩種方式運行Service來看一下它的運行過程

2、啓動和停止service

寫好Service的子類之後，然後寫了兩個按鈕，爲其添加點擊事件如下

 @Override
    public void onClick(View v) {
        switch (v.getId()){
            case R.id.start:
                startService(new Intent(MusicButtonActivity.this, MusicService.class));
                break;
            case R.id.stop:
                stopService(new Intent(MusicButtonActivity.this, MusicService.class));
                break;

        }
    }

然後Service的代碼如下，使用MediaPlayer播放了一段放在res\raw下的音樂

public class MusicService extends Service{

    private static final String TAG = MusicService.class.getSimpleName();
    //private MyBinder mMyBinder = new MyBinder();
    @Override
    public void onCreate() {
        super.onCreate();
        Log.i(TAG, "onCreate");
        mMediaPlayer = MediaPlayer.create(this, R.raw.my);
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.i(TAG, "onStartCommand");
        mMediaPlayer.start();
        return START_NOT_STICKY;

    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.i(TAG, "onDestroy");
        mMediaPlayer.stop();
    }

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        Log.i(TAG, "onBind");
        return mMyBinder;
    }

    @Override
    //客戶端和Service解綁時回調的方法
    public boolean onUnbind(Intent intent) {
        Log.i(TAG, "onUnbind");
        return super.onUnbind(intent);
    }
}

按下start，然後stop，可以看到後臺打印

I/MusicService: onCreate
I/MusicService: onStartCommand
I/MusicService: onDestroy

按下start，然後再按start，後臺打印如下：

I/MusicService: onCreate
I/MusicService: onStartCommand
I/MusicService: onStartCommand

可見onCreate()只會調用一次，當Service已經啓動後，之後就只會調用onStartCommand()方法。
以上使用Service的方法，Activity和Service基本上沒有什麼關聯，兩者之間無法進行通信，交換數據什麼的。所以想實現通信應該用bindService()和unBindService()

3、綁定Service

bindService()裏面有三個參數boolean bindService(Intent service, ServiceConnection conn, int flags)

• 第一個就是通過Intent啓動指定的Service。
• 第二個是ServiceConnection對象，監聽訪問者(clients)和Service的連接情況，我們可以看一下它的代碼

private ServiceConnection mServiceConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            System.out.println("connected");
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {

            System.out.println("disconnected");
        }
    };

當訪問者和Servive連接成功時會回調onServiceConnected(ComponentName name, IBinder service)其中service就是onBinder(Intent intent)返回的IBinder對象，通過這個IBinder對象與Service就可以進行通信。另外onServiceDisconnected(ComponentName name)只會在異常斷開和Service連接時，纔會調用該方法，正常斷開不會回調

• 最後一個指定綁定的時候如果還沒有創建Service，是否自動創建Service，0爲不創建，BIND_AUTO_CREATE爲自動創建。
  在上個代碼上修改，在activity裏得到歌曲的總時長（毫秒）。

public class MusicService extends Service{

    private static final String TAG = MusicService.class.getSimpleName();
    private MyBinder mMyBinder = new MyBinder();

    private MediaPlayer mMediaPlayer;
    @Override
    public void onCreate() {
        super.onCreate();
        Log.i(TAG, "onCreate");
        mMediaPlayer = MediaPlayer.create(this, R.raw.my);
        mMediaPlayer.start();
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.i(TAG, "onStartCommand");
       // mMediaPlayer.start();
        return START_NOT_STICKY;

    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.i(TAG, "onDestroy");
        mMediaPlayer.stop();
    }

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        Log.i(TAG, "onBind");
        return mMyBinder;
    }

    @Override
    public boolean onUnbind(Intent intent) {
        Log.i(TAG, "onUnbind");
        return super.onUnbind(intent);
    }

    public class MyBinder extends Binder{
        public int getProgress(){

            return mMediaPlayer.getDuration();
        }
    }
}

上面onBind()方法返回了一個可以得到歌曲總時長的Binder對象，該對象將會傳給訪問該Service的訪問者。
在activity裏的代碼：

public class MusicButtonActivity extends AppCompatActivity implements View.OnClickListener{

    private Button mStop_button;
    private Button mStart_button;
    MusicService.MyBinder mBinder;

    private ServiceConnection mServiceConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {

            System.out.println("connected");
            //得到傳入的對象
            mBinder = (MusicService.MyBinder) service;
            //得到了歌曲的時長
            int a = mBinder.getProgress();
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {

            System.out.println("disconnected");
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_music_button);

        mStart_button = (Button) findViewById(R.id.start);
        mStop_button = (Button) findViewById(R.id.stop);

        mStart_button.setOnClickListener(this);
        mStop_button.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()){
            case R.id.start:
                //startService(new Intent(MusicButtonActivity.this, MusicService.class));
                bindService(new Intent(MusicButtonActivity.this, MusicService.class), mServiceConnection, Service.BIND_AUTO_CREATE);

                break;
            case R.id.stop:
                unbindService(mServiceConnection);
                //stopService(new Intent(MusicButtonActivity.this, MusicService.class));
                break;

        }
    }
}

點擊start，後臺打印

MusicService: onCreate
MusicService: onBind

點擊stop，後臺打印

08-07 15:59:01.430 14412-14412/com.example.myactionbardemo I/MusicService: onCreate
08-07 15:59:01.485 14412-14412/com.example.myactionbardemo I/MusicService: onBind
08-07 15:59:04.333 14412-14412/com.example.myactionbardemo I/MusicService: onUnbind
08-07 15:59:04.334 14412-14412/com.example.myactionbardemo I/MusicService: onDestroy

通過這種方法可以對Service的數據進行我們需要的操作。
除了以上的兩種情況，如果在Service已經啓動的情況下，再去綁定的話，這個時候如果只是先解綁，在stop才能把Service銷燬

08-07 16:01:29.551 17091-17091/com.example.myactionbardemo I/MusicService: onCreate
08-07 16:01:29.596 17091-17091/com.example.myactionbardemo I/MusicService: onStartCommand
08-07 16:01:29.599 17091-17091/com.example.myactionbardemo I/MusicService: onBind
08-07 16:01:40.265 17091-17091/com.example.myactionbardemo I/MusicService: onUnbind
08-07 16:01:40.275 17091-17091/com.example.myactionbardemo I/MusicService: onDestroy

4、IntentService

我們開篇說過Service不是一個線程，它其實運行在主線程裏，所以它裏面不能處理一個耗時操作，當Service處理耗時操作超過20秒時，將會出現ANR(application not responding)異常，這是不能允許的。如果確實想在Service處理耗時操作，建議另開一條線程處理。除此之外，還可以使用Service的子類IntentService。如下面的代碼

public class MyIntentService  extends IntentService{
    /**
     * Creates an IntentService.  Invoked by your subclass's constructor.
     *
     * @param name Used to name the worker thread, important only for debugging.
     */
    public MyIntentService(String name) {
        super(name);
    }

    @Override
    protected void onHandleIntent(Intent intent) {

    }
}

看了下IntentService的源碼

public abstract class IntentService extends Service {
    private volatile Looper mServiceLooper;
    private volatile ServiceHandler mServiceHandler;
    private String mName;
    private boolean mRedelivery;
    //處理Intent請求
    private final class ServiceHandler extends Handler {
        public ServiceHandler(Looper looper) {
            super(looper);
        }

        @Override
        public void handleMessage(Message msg) {
            onHandleIntent((Intent)msg.obj);
            stopSelf(msg.arg1);
        }
    }

    @Override
    //創建了一個單獨的線程
    public void onCreate() {
        // TODO: It would be nice to have an option to hold a partial wakelock
        // during processing, and to have a static startService(Context, Intent)
        // method that would launch the service & hand off a wakelock.

        super.onCreate();
        HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
        thread.start();

        mServiceLooper = thread.getLooper();
        mServiceHandler = new ServiceHandler(mServiceLooper);
    }

    //發送處理intent請求的消息
    @Override
    public void onStart(Intent intent, int startId) {
        Message msg = mServiceHandler.obtainMessage();
        msg.arg1 = startId;
        msg.obj = intent;
        mServiceHandler.sendMessage(msg);
    }

    /**
     * You should not override this method for your IntentService. Instead,
     * override {@link #onHandleIntent}, which the system calls when the IntentService
     * receives a start request.
     * @see android.app.Service#onStartCommand
     */
    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        onStart(intent, startId);
        return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
    }

    @Override
    public void onDestroy() {
        mServiceLooper.quit();
    }

    /**
     * Unless you provide binding for your service, you don't need to implement this
     * method, because the default implementation returns null. 
     * @see android.app.Service#onBind
     */
    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }

    /**
     * This method is invoked on the worker thread with a request to process.
     * Only one Intent is processed at a time, but the processing happens on a
     * worker thread that runs independently from other application logic.
     * So, if this code takes a long time, it will hold up other requests to
     * the same IntentService, but it will not hold up anything else.
     * When all requests have been handled, the IntentService stops itself,
     * so you should not call {@link #stopSelf}.
     *
     * @param intent The value passed to {@link
     *               android.content.Context#startService(Intent)}.
     */
    @WorkerThread
    protected abstract void onHandleIntent(Intent intent);
}

IntentService會創建一個HandlerThread這樣的線程來處理所有的Intent請求，而我們只用重寫void onHandleIntent(Intent intent)。IntentService就會幫我們處理。