UIWindow-WindowLevel

WindowLevel是UIWindow的一个属性。系统定义的一共有3种。

UIKIT_EXTERN const UIWindowLevel UIWindowLevelNormal;
UIKIT_EXTERN const UIWindowLevel UIWindowLevelAlert;
UIKIT_EXTERN const UIWindowLevel UIWindowLevelStatusBar;

代码：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.

    NSLog(@"UIWindowLevelNormal=%f UIWindowLeveStatusBar=%f UIWindowLevelAlert=%f",UIWindowLevelNormal,UIWindowLevelStatusBar,UIWindowLevelAlert);

    return YES;
}

输出结果：

2015-07-19 21:02:45.448 WindowLevelDemo[775:24283] UIWindowLevelNormal=0.000000 UIWindowLeveStatusBar=1000.000000 UIWindowLevelAlert=2000.000000

参考资料：《iOS开发进阶》-唐巧