一个特殊的UIView,现在一个app中会有1-3个UIWindow；如：状态栏的提醒自定义控件...
没有UIWindow也就不会看到任何UI界面，先创建UIWindow，再创建控制器，创建控制器的view，然后将控制器的view添加到UIWindow上；
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
// Override point for customization after application launch.
// 创建窗口
_window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
// 窗口背景色
_window.backgroundColor = [UIColor whiteColor];
// 实例控制器 a
AViewController *a = [[AViewController alloc] init];
BViewController *b = [[BViewController alloc] init];
// 实例导航控制器 anav
UINavigationController *anav = [[UINavigationController alloc] initWithRootViewController:a];
UINavigationController *bnav = [[UINavigationController alloc] initWithRootViewController:b];
// 实例tab
UITabBarController *tab = [[UITabBarController alloc] init];
// nav加到tab上
tab.viewControllers = @[anav, bnav];
// 设置根控制器为nav
self.window.rootViewController = tab;
// 设置为主窗口并显示出来
[_window makeKeyAndVisible];
return YES;
}
[_window makeKeyWindow]; //让UIWindow成为主窗口，但不显示；
