iOS並沒有開放控制系統亮度的接口，所以app能做到的只是比系統設置的亮度更暗，具體做法就是在整個app的keywindow之上再放一個window，設置黑色背景，通過控制alpha值來達到明暗變化。

[cpp]
- (void)installBrightnessWindow 
{ 
    m_brightnessWindow = [[UIWindow alloc] initWithFrame:self.view.window.frame]; 
    m_brightnessWindow.windowLevel = UIWindowLevelStatusBar + 1; 
    m_brightnessWindow.userInteractionEnabled = NO; 
    m_brightnessWindow.backgroundColor = [UIColor blackColor]; 
    m_brightnessWindow.alpha = 0; 
    m_brightnessWindow.hidden = NO; 
} 

- (void)installBrightnessWindow
{
    m_brightnessWindow = [[UIWindow alloc] initWithFrame:self.view.window.frame];
    m_brightnessWindow.windowLevel = UIWindowLevelStatusBar + 1;
    m_brightnessWindow.userInteractionEnabled = NO;
    m_brightnessWindow.backgroundColor = [UIColor blackColor];
    m_brightnessWindow.alpha = 0;
    m_brightnessWindow.hidden = NO;
}