iOS 使用View 为屏幕增加一个全屏的蒙层

UIView *view = [[UIView alloc] initWithFrame:CGRectMake(0, 0, [UIScreen mainScreen].bounds.size.width, [UIScreen mainScreen].bounds.size.height)];
view.backgroundColor = [UIColor blackColor];
view.alpha = 0.5;
[[UIApplication sharedApplication].keyWindow addSubview:view];