UIWindow的UIWindowLevel属性

@property(nonatomic) UIWindowLevel windowLevel;                   // default = 0.0

typedef CGFloat UIWindowLevel NS_TYPED_EXTENSIBLE_ENUM;    // CGFloat类型, 可主动赋值.
UIKIT_EXTERN const UIWindowLevel UIWindowLevelNormal;      // 0
UIKIT_EXTERN const UIWindowLevel UIWindowLevelAlert;      // 2000.00
UIKIT_EXTERN const UIWindowLevel UIWindowLevelStatusBar API_UNAVAILABLE(tvos);      // 1000.00

windowLevel越大, 显示就在越上层, 当windowLevel相等时, 后实例化的显示在最上层.

.End