android - 在 BroadcastReceiver 中获取 WakeLock 并在 Service 中释放它的正确模式

Question

即使经过大量研究，我仍然不能完全确定我是如何为由 BroadcastReceiver 启动的 Service 实现 WakeLock 的方式> 是正确的——尽管它似乎工作正常。广播接收器从警报中获取 Intent 发送给它，因此首先从 AlarmManager 的 API 文档开始:

If your alarm receiver called Context.startService(), it is possible that the phone will sleep before the requested service is launched. To prevent this, your BroadcastReceiver and Service will need to implement a separate wake lock policy to ensure that the phone continues running until the service becomes available.

所以，在 onReceive() 我这样做:

    Intent serviceIntent = new Intent(context, SomeService.class);
    context.startService(serviceIntent);

    if(SomeService.wakeLock == null) {
        PowerManager powerManager = (PowerManager)context.getSystemService(Context.POWER_SERVICE);
        SomeService.wakeLock = powerManager.newWakeLock(
                PowerManager.PARTIAL_WAKE_LOCK, 
                SomeService.WAKE_LOCK_TAG);
    }
    if(! SomeService.wakeLock.isHeld()) {
        SomeService.wakeLock.acquire();
    }

在我所做的服务中:

    try {
        // Do some work
    } finally {
        if(wakeLock != null) {
            if(wakeLock.isHeld()) {
                wakeLock.release();
            }
            wakeLock = null;
        }
    }

SomeService.wakeLock字段是包私有(private)、静态和易变的。

我不确定的是使用 isHeld() 进行的检查 - 它是否真的告诉我是否获得了 WakeLock，我是否需要这样做检查吗？

Answer 1

What I am unsure about is the check using isHeld() - does it really tell me if a WakeLock is acquired or not, and do I need to do this check at all?

实际上回答起来有点棘手。查看 PowerManager 的来源和 PowerManager.WakeLock here WakeLock.acquire()和 WakeLock.acquireLocked()方法如下...

public void acquire(long timeout) {
    synchronized (mToken) {
        acquireLocked();
        mHandler.postDelayed(mReleaser, timeout);
    }
}

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, mWorkSource);
        } catch (RemoteException e) {
        }
        mHeld = true;
    }
}

... mService是 IPowerManager接口(interface)及其来源不可用，因此在尝试调用 acquireWakeLock(...) 时很难判断可能会或可能不会出错的地方.

无论如何，唯一可以捕获的异常是RemoteException和 catch block 什么都不做。紧接在 try/catch 之后，mHeld设置true无论如何。

简而言之，如果您调用 isHeld()紧接在 acquire() 之后结果将始终为 true .

进一步查找 PowerManager.WakeLock 的来源release() 的行为相似调用 release(int flags) mHeld成员始终设置为 false不管发生什么。

最后，我建议检查 isHeld() 总是一个好主意。作为一种最佳实践，以防更高版本的 Android 更改 WakeLock 的这种行为。方法。