android的PowerManager和PowerManager.WakeLock

PowerManager.WakeLock

　　PowerManager.WakerLock是我分析Standup Timer源代碼時發現的一個小知識點，Standup Timer 用WakeLock保證程序運行時保持手機屏幕的恆亮（程序雖小但也作得至關的細心，考慮的很周到）。PowerManager 和PowerManager.WakerLock7用於對Android設備的電源進行管理。

　　 PowerManager:This class gives you control of the power state of the device.

　　 PowerManager.WakeLock: lets you say that you need to have the device on.

　　Android中經過各類Lock鎖對電源進行控制，須要注意的是加鎖和解鎖必須成對出現。先上一段Standup Timer裏的代碼而後進行說明。

代碼

private void acquireWakeLock() {
if (wakeLock == null ) {
    Logger.d( " Acquiring wake lock " );
    PowerManager pm = (PowerManager) getSystemService(Context.POWER_SERVICE);
    wakeLock = pm.newWakeLock(PowerManager.SCREEN_DIM_WAKE_LOCK, this .getClass().getCanonicalName());
    wakeLock.acquire();
    }
}


private void releaseWakeLock() {
if (wakeLock != null && wakeLock.isHeld()) {
    wakeLock.release();
    wakeLock = null ;
    }
}

 

 

acquireWakeLock()方法中獲取了 SCREEN_DIM_WAKE_LOCK鎖，該鎖使 CPU 保持運轉，屏幕保持亮度（能夠變灰）。這個函數在Activity的 onResume中被調用。releaseWakeLock()方法則是釋放該鎖。它在Activity的 onPause中被調用。利用Activiy的生命週期，巧妙的讓 acquire()和release()成對出現。

 

@Override
protected void onResume()
{
super .onResume();
// 獲取鎖，保持屏幕亮度
acquireWakeLock();
startTimer();
}

 

代碼

protected void onPause()
{
super .onPause();
synchronized ( this ) {
cancelTimer();
releaseWakeLock();

if (finished) {
clearState();
} else {
saveState();
}
}
}

 

PowerManager和WakeLock的操做步驟

1. 　　PowerManager pm = (PowerManager) getSystemService(Context.POWER_SERVICE);經過 Context.getSystemService().方法獲取PowerManager實例。
2. 　　而後經過PowerManager的newWakeLock((int flags, String tag)來生成WakeLock實例。int Flags指示要獲取哪一種WakeLock，不一樣的Lock對cpu 、屏幕、鍵盤燈有不一樣影響。
3. 　　獲取WakeLock實例後經過acquire()獲取相應的鎖，而後進行其餘業務邏輯的操做，最後使用release()釋放（釋放是必須的）。

關於int flags

　　各類鎖的類型對CPU 、屏幕、鍵盤的影響：

PARTIAL_WAKE_LOCK:保持CPU 運轉，屏幕和鍵盤燈有多是關閉的。

SCREEN_DIM_WAKE_LOCK：保持CPU 運轉，容許保持屏幕顯示但有多是灰的，容許關閉鍵盤燈

SCREEN_BRIGHT_WAKE_LOCK：保持CPU 運轉，容許保持屏幕高亮顯示，容許關閉鍵盤燈

FULL_WAKE_LOCK：保持CPU 運轉，保持屏幕高亮顯示，鍵盤燈也保持亮度

ACQUIRE_CAUSES_WAKEUP：Normal wake locks don't actually turn on the illumination. Instead, they cause the illumination to remain on once it turns on (e.g. from user activity). This flag will force the screen and/or keyboard to turn on immediately, when the WakeLock is acquired. A typical use would be for notifications which are important for the user to see immediately.

ON_AFTER_RELEASE：f this flag is set, the user activity timer will be reset when the WakeLock is released, causing the illumination to remain on a bit longer. This can be used to reduce flicker if you are cycling between wake lock conditions.

權限獲取

要進行電源的操做須要在AndroidManifest.xml中聲明該應用有設置電源管理的權限。

< uses-permission android:name ="android.permission.WAKE_LOCK" />
你可能還須要
< uses-permission android:name ="android.permission.DEVICE_POWER" />

另外WakeLock的設置是 Activiy 級別的，不是針對整個Application應用的。