程序的入口及AppDelegate窗口显示原理

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AppDelegate.m

- (void)dealloc{    [_window release];    [super dealloc];}- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{        self.window = [[UIWindow alloc] initWithFrame:CGRectMake(0, 0, 320, 480)];    self.window.backgroundColor = [UIColor redColor];        /*     makeKeyAndVisible的作用：     1.让window在屏幕上直接显示，这是window的属性，view没有这个属性，如果室view，需要添加到其他的视图上显示     2.将当前窗口设置为主窗口     */    [_window makeKeyAndVisible];        _window.windowLevel = UIWindowLevelStatusBar;        UIWindow *w2 = [[UIWindow alloc] initWithFrame:CGRectMake(0, 0, 90, 90)];    w2.backgroundColor = [UIColor greenColor];//    [w2 makeKeyAndVisible];   不能这么写        //hidden的作用：在主窗口上显示w2，但是w2不是主窗口    w2.hidden = NO;        /*     设置window的优先级：UIWindowLevelNormal < UIWindowLevelStatusBar < UIWindowLevelAlert;     默认是：UIWindowLevelNormal     */    w2.windowLevel = UIWindowLevelAlert;        //拿到当前程序的主window    UIWindow *w3 = application.keyWindow;    NSLog(@"self.window:%@",self.window);    NSLog(@"w3:%@",w3);            return YES;}

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