设置父控件的alpha 不影响子控件

UIView* superView=[[UiView alloc] init];
UIView* subView=[[UIView alloc] init];
[superView addSubView:subView];

//设置父控件的Alpha 而不影响子控件
[superView setBackgroundColor:[[UIColor blackColor] colorWithAlphaComponent:0.5f];