在应用程序启动时，确定用户是否已经登录.

At the start of the app, determine whether or not the user is logged in already.

如果用户未登录:

• 将rootViewController设置为UINavigationController并添加您的 欢迎查看. (如果希望导航栏看起来像您发布的照片​​，则可以将其隐藏.)
• 当他们点击注册或登录按钮时， 将适当的视图控制器推到导航控制器上 堆栈.
• 成功登录后，用UITabBarController 替换根视图控制器.

• Set the rootViewController to a UINavigationController and add your Welcome view to it. (You can hide the navigation bar if you want it to look like the photo that you posted.)
• When they tap the register or sign in buttons, push the appropriate view controller onto the navigation controller stack.
• After a successful login, replace the root view controller with the UITabBarController.

如果用户已登录:

将rootViewController设置为UITabBarController.

Set the rootViewController to the UITabBarController.