title: How to update constraints of a view when other view is hidden using Snapkit?
body: <p>There are three views from top to bottom: redView, yellowView, blueView.
  I want to hide yellowView and change the constraint of blueView in order to let blueView below redView.
  The picture is below:
    <a href="https://i.stack.imgur.com/yRuS9.png" rel="nofollow noreferrer">this picture is original</a>
    <a href="https://i.stack.imgur.com/owYLN.png" rel="nofollow noreferrer">this picture is what I want</a>
  The code is below:</p>
  
  <pre><code>private lazy var redView: UIView = {
     let redView = UIView()
      redView.backgroundColor = UIColor.red
      return redView
  }()
  private lazy var yellowView: UIView = {
     let yellowView = UIView()
      yellowView.backgroundColor = UIColor.yellow
      return yellowView
  }()
  private lazy var blueView: UIView = {
     let blueView = UIView()
      blueView.backgroundColor = UIColor.blue
      return blueView
  }()
  override func viewDidLoad() {
      super.viewDidLoad()
      view.addSubview(redView)
      view.addSubview(yellowView)
      view.addSubview(blueView)
      redView.snp.makeConstraints { (make) in
          make.top.left.right.equalTo(view)
          make.height.equalTo(40)
      }
      yellowView.snp.makeConstraints { (make) in
          make.top.equalTo(redView.snp.bottom)
          make.left.right.height.equalTo(redView)
      }
      blueView.snp.makeConstraints { (make) in
          make.top.equalTo(yellowView.snp.bottom)
          make.left.right.height.equalTo(yellowView)
      }
  </code></pre>
  
  <p>}</p>
  
  <pre><code>    if yellowView.isHidden == true {
          //how is the code?
      } else {
          //how is the code?
      }
  </code></pre>