（三）IntentService

IntentService

前一篇介绍了service的混合调用，及AIDL的使用，使用混合调用的目的是调用service中的方法，同时在最后我们也看到了，service中的方法同样是运行在主线程的，那么就意味着这里不能执行耗时操作，如果有耗时操作的话，需要开线程。那么怎么办呢？其实Google已经为我们提供了一种优雅的方式IntentService，使用也非常方便，只要继承了IntentService，实现onHandleIntent方法（子线程，用来执行耗时操作）即可，执行完，IntentService会自行结束。

一、Demo

1、Activity

public class MainActivity extends AppCompatActivity implements View.OnClickListener{

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        findViewById(R.id.bt_start).setOnClickListener(this);
    }

    @Override
    public void onClick(View view) {
        Intent intent = new Intent(this, MyIntentService.class);
        Log.d("hjw","activity intent = "+intent.hashCode()+",threadId = "+Thread.currentThread().getId());
        startService(intent);
    }
}

2、IntentService

public class MyIntentService extends IntentService {
    private String TAG = "hjw";

    public MyIntentService() {
        super("MyIntentService");
    }

    @Override
    public void onCreate() {
        Log.d(TAG, "onCreate" + ",threadId = " + Thread.currentThread().getId());
        super.onCreate();
    }

    @Override
    public void onStart(@Nullable Intent intent, int startId) {
        Log.d(TAG, "onStart" + ",threadId = " + Thread.currentThread().getId());
        super.onStart(intent, startId);
    }

    @Override
    protected void onHandleIntent(@Nullable Intent intent) {
        Log.d(TAG, "onHandleIntent intent = " + intent.hashCode() + ",threadId = " + Thread.currentThread().getId());
        try {
            Thread.sleep(10000);
        } catch (Exception e) {
            e.printStackTrace();
        }

    }

    @Override
    public void onDestroy() {
        Log.d(TAG, "onDestroy" + ",threadId = " + Thread.currentThread().getId());
        super.onDestroy();
    }
}

操作就是在Activity开启IntentService，自动执行onHandleIntent方法，执行完自动结束IntentService。log如下

	Line 2351: 01-24 17:13:51.310 D/hjw     ( 8333): activity intent = 259321624,threadId = 2
	Line 2380: 01-24 17:13:51.364 D/hjw     ( 8333): onCreate,threadId = 2
	Line 2381: 01-24 17:13:51.371 D/hjw     ( 8333): onStart,threadId = 2
	Line 2382: 01-24 17:13:51.373 D/hjw     ( 8333): onHandleIntent intent = 146592482,threadId = 211
	Line 2989: 01-24 17:14:01.376 D/hjw     ( 8333): onDestroy,threadId = 2

2、源码

由于源码十分少，实在没什么可分析的，情不自禁就贴出来了

public abstract class IntentService extends Service {
    private volatile Looper mServiceLooper;
    private volatile ServiceHandler mServiceHandler;
    private String mName;
    private boolean mRedelivery;

    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);
        }
    }

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

    /**
     * Sets intent redelivery preferences.  Usually called from the constructor
     * with your preferred semantics.
     *
     * 
If enabled is true,
     * {@link #onStartCommand(Intent, int, int)} will return
     * {@link Service#START_REDELIVER_INTENT}, so if this process dies before
     * {@link #onHandleIntent(Intent)} returns, the process will be restarted
     * and the intent redelivered.  If multiple Intents have been sent, only
     * the most recent one is guaranteed to be redelivered.
     *
     * 
If enabled is false (the default),
     * {@link #onStartCommand(Intent, int, int)} will return
     * {@link Service#START_NOT_STICKY}, and if the process dies, the Intent
     * dies along with it.
     */
    public void setIntentRedelivery(boolean enabled) {
        mRedelivery = enabled;
    }

    @Override
    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);
    }

    @Override
    public void onStart(@Nullable 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(@Nullable 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
    @Nullable
    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)}.
     *               This may be null if the service is being restarted after
     *               its process has gone away; see
     *               {@link android.app.Service#onStartCommand}
     *               for details.
     */
    @WorkerThread
    protected abstract void onHandleIntent(@Nullable Intent intent);
}

注意：构造方法，必须要实现。

方法解析：（以下顺序也是调用顺）

1、onCreate

构造了一个线程，并且为线程绑定了handler和looper。

2、onStartCommand

调用了onStart，拓展下返回值：

1):START_STICKY：如果service进程被kill掉，保留service的状态为开始状态，但不保留递送的intent对象。随后系统会尝试重新创建service，由于服务状态为开始状态，所以创建服务后一定会调用onStartCommand(Intent,int,int)方法。如果在此期间没有任何启动命令被传递到service，那么参数Intent将为null。  
2):START_NOT_STICKY：“非粘性的”。使用这个返回值时，如果在执行完onStartCommand后，服务被异常kill掉，系统不会自动重启该服务  
3):START_REDELIVER_INTENT：重传Intent。使用这个返回值时，如果在执行完onStartCommand后，服务被异常kill掉，系统会自动重启该服务，并将Intent的值传入。   
4):START_STICKY_COMPATIBILITY：START_STICKY的兼容版本，但不保证服务被kill后一定能重启

3、onStart

构造Message，并通过handler发送

4、Handler处理

调用onHandleIntent，intent对象被封装在message中，执行完会执行stopSelf

    /**
     * Stop the service, if it was previously started.  This is the same as
     * calling {@link android.content.Context#stopService} for this particular service.
     *  
     * @see #stopSelfResult(int)
     */
    public final void stopSelf() {
        stopSelf(-1);
    }

    /**
     * Old version of {@link #stopSelfResult} that doesn't return a result.
     *  
     * @see #stopSelfResult
     */
    public final void stopSelf(int startId) {
        if (mActivityManager == null) {
            return;
        }
        try {
            mActivityManager.stopServiceToken(
                    new ComponentName(this, mClassName), mToken, startId);
        } catch (RemoteException ex) {
        }
    }

这里跟调用stopService效果是一样的。

通过看源码可以知道，任务一旦开启就不能中断了（系统没有提供给我们可以操作的api），执行完毕后会自动结束。