在呈现视图控制器后，我的自动布局会发生什么？

Question

在我解雇一个呈现的视图控制器之后，我试图理解我的视图布局会发生什么。任何人都能解释为什么我的布局在以下场景中断开：1个包含1个子视图控制器的主视图控制器。主视图控制器成功呈现第三视图控制器，但在解除显示的vc后，子视图控制器已更改其布局。以下是演示我的困境的示例RubyMotion代码：

class AppDelegate
    def application(application, didFinishLaunchingWithOptions:launchOptions)
    @presenting = false

    window.rootViewController = main_vc

    main_vc.addChildViewController(content_vc)
    main_vc.view.addSubview(content_vc.view)

    constraints = NSLayoutConstraint.constraintsWithVisualFormat("H:|[content]|",
                                                             options: NSLayoutAttributeLeft,
                                                             metrics: nil,
                                                             views: { 'content' => content_vc.view })
    window.addConstraints(constraints)

    constraints = NSLayoutConstraint.constraintsWithVisualFormat("V:|[content]|",
                                                             options: NSLayoutAttributeTop,
                                                             metrics: nil,
                                                             views: { 'content' => content_vc.view })
    window.addConstraints(constraints)

    [ content_vc.view.subviews.first ].each do |item|
      main_vc.view.addConstraint(NSLayoutConstraint.constraintWithItem(item,
                                                              attribute: NSLayoutAttributeCenterX,
                                                              relatedBy: NSLayoutRelationEqual,
                                                              toItem: main_vc.view,
                                                              attribute: NSLayoutAttributeCenterX,
                                                              multiplier: 1, constant: 0))
      main_vc.view.addConstraint(NSLayoutConstraint.constraintWithItem(item,
                                                              attribute: NSLayoutAttributeCenterY,
                                                              relatedBy: NSLayoutRelationEqual,
                                                              toItem: main_vc.view,
                                                              attribute: NSLayoutAttributeCenterY,
                                                              multiplier: 1, constant: -40))
    end

    brand_vc.view.when_tapped do
      puts "tapped brand_vc"
      main_vc.dismissViewControllerAnimated(true, completion: lambda { })
    end

    content_vc.view.when_tapped do
      puts "tapped content_vc"
      main_vc.presentViewController(brand_vc, animated: true, completion: lambda { })
    end

    true
  end

  def window
    @window ||= UIWindow.alloc.initWithFrame(UIScreen.mainScreen.bounds).tap do |w|
      w.backgroundColor = UIColor.greenColor
      w.makeKeyAndVisible
    end
  end

  def main_vc
    @main_vc ||= UIViewController.alloc.initWithNibName(nil, bundle: nil).tap do |vc|
      vc.view.translatesAutoresizingMaskIntoConstraints = false
      vc.view.backgroundColor = UIColor.redColor
      vc.view.addSubview(MyLabel.alloc.init_with_text("Main VC - (red color)"))
    end
  end

  def content_vc
    @content_vc ||= UIViewController.alloc.initWithNibName(nil, bundle: nil).tap do |vc|
      vc.view.translatesAutoresizingMaskIntoConstraints = false
      vc.view.backgroundColor = UIColor.blueColor
      vc.view.addSubview(MyLabel.alloc.init_with_text("Content VC (blue color)"))
    end
  end

  def brand_vc
     @brand_vc ||= UIViewController.alloc.initWithNibName(nil, bundle: nil).tap do |vc|
       vc.view.translatesAutoresizingMaskIntoConstraints = false
       vc.view.backgroundColor = UIColor.yellowColor
     end
  end

end

class MyLabel < UILabel

  def init_with_text(str)
    self.init

    self.translatesAutoresizingMaskIntoConstraints = false
    self.text = str
    self
  end

end