在最顶层窗口添加一个视图

BackgroundView * backView = [[BackgroundView
alloc] initWithFrame:CGRectMake(0,
0, WIDTH,
HEIGHT)];

backView.backgroundColor =
ColorBlack60;

// 当前顶层窗口

UIWindow *window = [[UIApplication
sharedApplication].windows
lastObject];

// 添加到窗口

[window addSubview:backView];