android Phone 距离感应器锁的实现

android手机在正常打电话的过程中，靠近面部会灭屏，这个主要是PhoneApp中的mProximityWakeLock的作用。这个锁是在PhoneApp中被初始化的。

if ((pm.getSupportedWakeLockFlags()
                 & PowerManager.PROXIMITY_SCREEN_OFF_WAKE_LOCK) != 0x0) {
                mProximityWakeLock =
                        pm.newWakeLock(PowerManager.PROXIMITY_SCREEN_OFF_WAKE_LOCK, LOG_TAG);
            }

初始化这个锁之后，就有acquire与release这把锁来亮灭屏了，主要是函数updateProximitySensorMode(Phone.State state) 发挥的作用，这个函数在电话状态发生改变的时候会调用（还有类似手机插拔，手机方向改变等等）。那我们来看下这个函数主要做了什么。

此函数在源码中的解释已经很清楚了，在此不做赘述，这个函数主要是acquire与release mProximityWakeLock这把锁。那么，我们来看下，申请这把锁的时候，究竟干了什么事情。PhoneApp里的mProximityWakeLock.acquire() 调用PowerManager里的acquire()，然后调用PowerManagerService里的acquireWakeLock()，接着调用acquireWakeLockLocked()。这个函数就是我们最终申请锁的地方。

我们看下比较重要的地方

if ((flags & LOCK_MASK) == PowerManager.PROXIMITY_SCREEN_OFF_WAKE_LOCK) {
    mProximityWakeLockCount++;
    if (mProximityWakeLockCount == 1) {
        enableProximityLockLocked();
    }
}

这个地方就是判断，如果我们申请的是PROXIMITY_SCREEN_OFF_WAKE_LOCK，那么就enableProximityLockLocked();mProximityWakeLockCount说明的是PROXIMITY_SCREEN_OFF_WAKE_LOCK是采用计数方式的锁。

继续看enableProximityLockLocked()做了什么

private void enableProximityLockLocked() {
    if (mDebugProximitySensor) {
        Slog.d(TAG, "enableProximityLockLocked");
    }
    if (!mProximitySensorEnabled) {
        // clear calling identity so sensor manager battery stats are accurate
        long identity = Binder.clearCallingIdentity();
        try {
            mSensorManager.registerListener(mProximityListener, mProximitySensor,
                    SensorManager.SENSOR_DELAY_NORMAL);
            mProximitySensorEnabled = true;
        } finally {
            Binder.restoreCallingIdentity(identity);
        }
    }
}

原来，这个函数主要是注册了mProximityListener，也就是说，我们监听距离感应器的操作是在加锁之后发生的。灭屏是在距离感应器监听到数值才激发的，那么我们看下mProximityListener。

SensorEventListener mProximityListener = new SensorEventListener() {
        public void onSensorChanged(SensorEvent event) {
            long milliseconds = SystemClock.elapsedRealtime();
            synchronized (mLocks) {
                float distance = event.values[0];
                long timeSinceLastEvent = milliseconds - mLastProximityEventTime;
                mLastProximityEventTime = milliseconds;
                mHandler.removeCallbacks(mProximityTask);
                boolean proximityTaskQueued = false;

                // compare against getMaximumRange to support sensors that only return 0 or 1
                boolean active = (distance >= 0.0 && distance < PROXIMITY_THRESHOLD &&
                        distance < mProximitySensor.getMaximumRange());

                if (mDebugProximitySensor) {
                    Slog.d(TAG, "mProximityListener.onSensorChanged active: " + active);
                }
                if (timeSinceLastEvent < PROXIMITY_SENSOR_DELAY) {
                    // enforce delaying atleast PROXIMITY_SENSOR_DELAY before processing
                    mProximityPendingValue = (active ? 1 : 0);
                    mHandler.postDelayed(mProximityTask, PROXIMITY_SENSOR_DELAY - timeSinceLastEvent);
                    proximityTaskQueued = true;
                } else {
                    // process the value immediately
                    mProximityPendingValue = -1;
                    proximityChangedLocked(active);
                }

                // update mProximityPartialLock state
                boolean held = mProximityPartialLock.isHeld();
                if (!held && proximityTaskQueued) {
                    // hold wakelock until mProximityTask runs
                    mProximityPartialLock.acquire();
                } else if (held && !proximityTaskQueued) {
                    mProximityPartialLock.release();
                }
            }
        }

复写了onSensorChanged()函数，当检测到actiove的时候，执行了proximityChangedLocked并申请了一个mProximityPartialLock。(if else里的代码都会执行proximityChangedLocked)。proximityChangedLocked()函数会在active的时候执行goToSleepLocked。