PowerManager之WakeLock源码解析

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首先看源码注释里对WakeLock类的注释：

    /*** A wake lock is a mechanism to indicate that your application needs* to have the device stay on.* <p>* Any application using a WakeLock must request the {@code android.permission.WAKE_LOCK}* permission in an {@code <uses-permission>} element of the application's manifest.* Obtain a wake lock by calling {@link PowerManager#newWakeLock(int, String)}.* </p><p>* Call {@link #acquire()} to acquire the wake lock and force the device to stay* on at the level that was requested when the wake lock was created.* </p><p>* Call {@link #release()} when you are done and don't need the lock anymore.* It is very important to do this as soon as possible to avoid running down the* device's battery excessively.* </p>*/

用来控制设备保持运行状态的，需要配置申请权限，获取实例通过newWakeLock方法、使用acquire获取唤醒锁保持设备运行、使用release进行释放

加锁方式有两种，一种为永久锁，需要用户手动释放，另一种是超时锁，到时间后自动释放，源码如下

        /*** Acquires the wake lock.* <p>* Ensures that the device is on at the level requested when* the wake lock was created.* </p>*/public void acquire() {synchronized (mToken) {acquireLocked();}}

      /*** Acquires the wake lock with a timeout.* <p>* Ensures that the device is on at the level requested when* the wake lock was created.  The lock will be released after the given timeout* expires.* </p>** @param timeout The timeout after which to release the wake lock, in milliseconds.*/public void acquire(long timeout) {synchronized (mToken) {acquireLocked();mHandler.postDelayed(mReleaser, timeout);}}

mReleaser是释放锁的Runnable

        private final Runnable mReleaser = new Runnable() {public void run() {release();}};

再回到前面看加锁的源码acquireLocked：

        private void acquireLocked() {if (!mRefCounted || mCount++ == 0) {// Do this even if the wake lock is already thought to be held (mHeld == true)// because non-reference counted wake locks are not always properly released.// For example, the keyguard's wake lock might be forcibly released by the// power manager without the keyguard knowing.  A subsequent call to acquire// should immediately acquire the wake lock once again despite never having// been explicitly released by the keyguard.mHandler.removeCallbacks(mReleaser);try {mService.acquireWakeLock(mToken, mFlags, mTag, mPackageName, mWorkSource);} catch (RemoteException e) {}mHeld = true;}}

mRefCounted是用来控制是否关联计数的变量，默认为true，如果设置为false则后面的计数变量mCount一直不会改变

  private boolean mRefCounted = true;

下面来看释放锁的源码release

        /*** Releases the wake lock.* <p>* This method releases your claim to the CPU or screen being on.* The screen may turn off shortly after you release the wake lock, or it may* not if there are other wake locks still held.* </p>*/public void release() {release(0);}

        /*** Releases the wake lock with flags to modify the release behavior.* <p>* This method releases your claim to the CPU or screen being on.* The screen may turn off shortly after you release the wake lock, or it may* not if there are other wake locks still held.* </p>** @param flags Combination of flag values to modify the release behavior.* Currently only {@link #WAIT_FOR_PROXIMITY_NEGATIVE} is supported.** {@hide}*/public void release(int flags) {synchronized (mToken) {if (!mRefCounted || --mCount == 0) {mHandler.removeCallbacks(mReleaser);if (mHeld) {try {mService.releaseWakeLock(mToken, flags);} catch (RemoteException e) {}mHeld = false;}}if (mCount < 0) {throw new RuntimeException("WakeLock under-locked " + mTag);}}}

所以不计数模式，无论acquire多少次，只需调一次release就可以进行释放；而计数模式每调一次acquire，只是把计数变量值加1，释放时也需要--mCount == 0时才能释放，调用次数过多，导致mCount<0会throw new RuntimeException("WakeLock under-locked " + mTag);而不计数模式不会出现，因为mCount一直为初始值0

那设置计数模式的地方在哪里呢，看下面

        /*** Sets whether this WakeLock is reference counted.* <p>* Wake locks are reference counted by default.  If a wake lock is* reference counted, then each call to {@link #acquire()} must be* balanced by an equal number of calls to {@link #release()}.  If a wake* lock is not reference counted, then one call to {@link #release()} is* sufficient to undo the effect of all previous calls to {@link #acquire()}.* </p>** @param value True to make the wake lock reference counted, false to* make the wake lock non-reference counted.*/public void setReferenceCounted(boolean value) {synchronized (mToken) {mRefCounted = value;}}

注释已经很详细了

在实际开发中，可以这样简单使用，设置为不计数模式，然后在onResume中acquire，在onPause中release，用来控制在某个页面保持屏幕恒亮。

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