Handler 和callback 机制

Handler主要用来在线程之间的通信的机制。如在Activity或Service中需要接收其他线程的消息，则在需要接收消息的Activity或Service中需要实现Callback接口。下面是PowerManagerService中用于接收其他线程消息的handleMessage()的例子：

private final class PowerManagerHandler extends Handler {
    public PowerManagerHandler(Looper looper) {
        super(looper, null, true /*async*/);
    }

    @Override
    public void handleMessage(Message msg) {
        switch (msg.what) {
            case MSG_USER_ACTIVITY_TIMEOUT:
                handleUserActivityTimeout();
                break;
            case MSG_SANDMAN:
                handleSandman();
                break;
            case MSG_SCREEN_ON_BLOCKER_RELEASED:
                handleScreenOnBlockerReleased();
                break;
            case MSG_CHECK_IF_BOOT_ANIMATION_FINISHED:
                checkIfBootAnimationFinished();
                break;
        }
    }
}

然后在创建Handler的地方将实现了Callback的类的实例传入:

public void init(Context context, LightsService ls,
        ActivityManagerService am, BatteryService bs, IBatteryStats bss,
        DisplayManagerService dm) {
    mContext = context;
    mLightsService = ls;
    mBatteryService = bs;
    mBatteryStats = bss;
    mDisplayManagerService = dm;
    mHandlerThread = new HandlerThread(TAG);
    mHandlerThread.start();
    mHandler = new PowerManagerHandler(mHandlerThread.getLooper());//将实现了Callback类的实例传入

    Watchdog.getInstance().addMonitor(this);

    // Forcibly turn the screen on at boot so that it is in a known power state.
    // We do this in init() rather than in the constructor because setting the
    // screen state requires a call into surface flinger which then needs to call back
    // into the activity manager to check permissions.  Unfortunately the
    // activity manager is not running when the constructor is called, so we
    // have to defer setting the screen state until this point.
    mDisplayBlanker.unblankAllDisplays();
}

然后当在线程中可使用如下代码向PowerManagerService发送消息：

private void shutdownOrRebootInternal(final boolean shutdown, final boolean confirm,
        final String reason, boolean wait) {
    if (mHandler == null || !mSystemReady) {
        throw new IllegalStateException("Too early to call shutdown() or reboot()");
    }

    Runnable runnable = new Runnable() {
        @Override
        public void run() {
            synchronized (this) {
                if (shutdown) {
                    ShutdownThread.shutdown(mContext, confirm);
                } else {
                    ShutdownThread.reboot(mContext, reason, confirm);
                }
            }
        }
    };
    // ShutdownThread must run on a looper capable of displaying the UI.
    Message msg = Message.obtain(mHandler, runnable);
    msg.setAsynchronous(true);
    mHandler.sendMessage(msg);//向PowerManagerService发送message

    // PowerManager.reboot() is documented not to return so just wait for the inevitable.
    if (wait) {
        synchronized (runnable) {
            while (true) {
                try {
                    runnable.wait();
                } catch (InterruptedException e) {
                }
            }
        }
    }

当执上面的代码之后，创建这个Handler（PowerManagerHandler）时使用Callback实例的handleMessage将会被调用。

 

可以使用如下代码将一个线程实例放入到Handler中使其执行：

if(mHandler != null) {
    mHandler.removeCallbacks(mStopDetectionTimeoutRunnable);
    mHandler.postDelayed(mStopDetectionTimeoutRunnable, WAIT_FOR_DETECTION_TIMEOUT);
}

可使用如下代码删除这个线程：

if(mHandler != null) {
    mHandler.removeMessages(MSG_OPEN_LENOVO_SMART_STANDBY_CAMERA_SUCCESS);
    mHandler.removeCallbacks(mStopDetectionTimeoutRunnable);
}

 Handler上还有许多类似的发送消息或添加线程的方法。