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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,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

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