UIWindow

1 window的创建（在AppDelegate。m里面）

//window的创建，跟屏幕尺寸一样大小
self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
//设置window的底色
self.window.backgroundColor = [UIColor whiteColor];
//设置根控制器
self.window.rootViewController = [[UIViewController alloc] init];
//设置self.window为keyWindows并显示
[self.window makeKeyAndVisible];

2 windowLevel优先级

UIWindowLevel UIWindowLevelNormal = 0;                      默认值
UIWindowLevel UIWindowLevelAlert = 2000;                    警告UIWindowLevel UIWindowLevelStatusBar = 1000                   状态栏值
[UIApplication sharedApplication].keyWindow.windowLevel     当前应用的主windowLevel

3 显示新建的window

//1 把arc关闭
//2 hidden设为no,默认为yes
//3 windowLevel>[UIApplication sharedApplication].keyWindow.windowLevel
//4 若要在新建的window上显示状态栏:
(1)[[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
(2)windowLevel<1000