Android的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。