@interface RootViewController : UIViewController

{

    UIView        *view;

}

-(void)createBackgroundView

{

    view = [[UIView alloc]initWithFrame:self.view.frame];

    view.backgroundColor = [[UIColor blackColor] colorWithAlphaComponent:0.0];

    view.opaque = NO;

UIWindow *appWindow = [[UIApplicationsharedApplication] keyWindow];

[appWindow addSubview:view]; //覆盖全屏

    [self.view addSubview:view];

    [UIView animateWithDuration:0.2 animations:^{

        view.backgroundColor = [[UIColor blackColor] colorWithAlphaComponent:0.25];

    }];

}