创建window

 1 - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
 2     // Override point for customization after application launch.
 3     
 4     //创建window分为四个步骤
 5     //1.创建一个window
 6     self.window = [[UIWindow alloc]initWithFrame:[UIScreen mainScreen].bounds];
 7     //2.创建一个视图控制器
 8     self.window.rootViewController = [[UIViewController alloc]init];
 9     //3.创建背景色
10     self.window.backgroundColor = [UIColor orangeColor];
11     //4.可以显示
12     [self.window makeKeyAndVisible];
13     return YES;
14 }