UIWindow的使用

主要用途：

1、用作蒙版

2、用作手势View

UIWindow的创建和UIView不同，UIWindow一旦被创建他就会添加到整个界面上，但是默认是hidden的，创建时注意要把UIWindow设置成全局变量；谁的UIWindow的UIWindowLevel值越大，谁的UIWindow就在最上面；UIWindowLevel优先级相等的情况下，看谁后实例化了，谁后实例化谁先显示。

当前窗口只能有一个主窗口

@property(nonatomic) UIWindowLevel windowLevel;                   // default = 0.0
@property(nonatomic,readonly,getter=isKeyWindow) BOOL keyWindow;
- (void)becomeKeyWindow;                               // override point for subclass. Do not call directly
- (void)resignKeyWindow;                               // override point for subclass. Do not call directly

- (void)makeKeyWindow;
- (void)makeKeyAndVisible;                             // convenience. most apps call this to show the main window and also make it key. otherwise use view hidden property


资料：
http://blog.youkuaiyun.com/hello_hwc/article/details/50670079

http://blog.youkuaiyun.com/liangliang103377/article/details/45060209