- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {

    // Override point for customization after application launch.

    //程序启动之后执行，只在第一次启动后执行，以后不再执行。

    //删除storyboard之后，需要在这里new出一个新的window，将该window初始化后，置为该应用程序的keywindow并展现出来。

    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];

    self.window.backgroundColor = [UIColor whiteColor];

    ViewController *root = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];  //加载nib文件，生成View实例

    [self.window setRootViewController:root];  //将new出来的view实例设置为window的root view。

    [self.window makeKeyAndVisible];   //将当前winsow置为当前状态下的key window，并呈现出来。Makes the receiver the key window and visible.

    return YES;

}