設定>顯示：這裡可以設定自動休眠超時時間。

當設定為30s時，到24s左右螢幕會先變暗，告知使用者螢幕快熄滅了，6s以後才會真正熄屏。

現在需要去掉這個功能，在PowerManagerService裡可以看到：

    private int getScreenDimDurationLocked(int screenOffTimeout) {
        return Math.min(SCREEN_DIM_DURATION,
                (int)(screenOffTimeout * MAXIMUM_SCREEN_DIM_RATIO));
    }

可以看到SCREEN_DIM_DURATION是7s：

    // The screen dim duration, in milliseconds.
    // This is subtracted from the end of the screen off timeout so the
    // minimum screen off timeout should be longer than this.
    private static final int SCREEN_DIM_DURATION = 7 * 1000;

    // The maximum screen dim time expressed as a ratio relative to the screen
    // off timeout.  If the screen off timeout is very short then we want the
    // dim timeout to also be quite short so that most of the time is spent on.
    // Otherwise the user won't get much screen on time before dimming occurs.
    private static final float MAXIMUM_SCREEN_DIM_RATIO = 0.2f;
 

最終getScreenDimDurationLocked()的返回值是6和7之間的最小值6s。

30S是系統裡最短的超時時間，也就是說其他screenOffTimeout乘以0.2是比6要大的，所以就不用考慮了。

要想去掉DIM這個Feature，直接把SCREEN_DIM_DURATION設為0，這樣getScreenDimDurationLocked()的返回值也就是0，這樣就不再有螢幕先變暗這個步驟了。

實測可行。