在另一个视图控制器中将视图控制器添加为子视图

一些观察：

1. 实例化第二个视图控制器时，您正在调用

   ViewControllerB()

   。如果该视图控制器以编程方式创建其视图（这是不寻常的），那就很好了。但是，IBOutlet建议的存在暗示了第二个视图控制器的场景是在

   Interface Builder1z

   中定义的，但是通过调用

   ViewControllerB()

   ，您并没有给情节提要板实例化该场景并连接所有出口的机会。因此，隐式解包的UILabel是nil，导致出现错误消息。
   相反，您想在

   Interface Builder

   中为目标视图控制器提供一个“ storyboard id” ，然后可以使用
   

   instantiateViewController(withIdentifier:

   )它实例化（并连接
   所有IB插座）。在Swift 3中

   let controller = storyboard!.instantiateViewController(withIdentifier: "scene storyboard id")

   
   
   

You can now access this

controller

‘s

view

.

1. But if you really want to do

   addSubview

   (i.e. you’re not transitioning to the next scene), then you are engaging in a practice called “view controller containment”. You do not just want to simply

   addSubview

   . You want to do some additional container view controller calls, e.g.:

   let controller = storyboard!.instantiateViewController(withIdentifier: "scene storyboard id")

   addChild(controller)
   controller.view.frame = … // or, better, turn off

   translatesAutoresizingMaskIntoConstraints

   and then define constraints for this subview
   view.addSubview(controller.view)
   controller.didMove(toParent: self)

For more information about why this

addChild

(previously called

addChildViewController

) and

didMove(toParent:)

(previously called

didMove(toParentViewController:)

) are necessary, see
WWDC 2011 video #102 - Implementing UIViewController
Containment. In short,
you need to ensure that your view controller hierarchy stays in sync with your
view hierarchy, and these calls to

addChild

and

didMove(toParent:)

ensure
this is the case.

另请参阅 《View Controller编程指南》中的“ 创建自定义容器View
Controller ”。

顺便说一句，以上说明了如何以编程方式执行此操作。它
实际上是容易得多，如果您使用界面生成器“容器视图”。

在此处输入图片说明

这样，您就不必担心与这些与收容相关的任何呼叫，
Interface Builder会为您解决。

For Swift 2 implementation, see previous revision of this
answer.