NSLayoutAnchor 作为 iOS9 的新特性是对AutoLayout创建约束的补充，核心还是 NSLayoutConstraint,可以避免过长创建约束代码

早期一条条创建 NSLayoutConstraint 约束的方式的确太啰嗦，因此绝大部分iOS 开发者应该都是使用 Masonry/SnapKit 或其他成熟的库来实现代码布局

简单的布局，也有使用 NSLayoutAnchor 实现过，不过一直没有研究过更新布局的方式，今天有空，换成原生的API，试一下最常见的两种情形下更新布局的实现

• 示例代码，每一次点击按钮的时候，更改红色View的宽度

  import UIKit
  
  class ViewController: UIViewController {
      
      private lazy var testButton = {
          let btn = UIButton()
          btn.setTitle("test", for: .normal)
          btn.setTitleColor(.black, for: .normal)
          btn.backgroundColor = UIColor.systemGreen
          btn.layer.cornerRadius = 10
          btn.layer.maskedCorners = [.layerMinXMinYCorner, .layerMinXMaxYCorner, .layerMaxXMinYCorner, .layerMaxXMaxYCorner]
          btn.layer.masksToBounds = true
          btn.translatesAutoresizingMaskIntoConstraints = false
          btn.addTarget(self, action: #selector(_target(_:)), for: .touchUpInside)
          return btn
      }()
      
      private lazy var redView = {
          let v = UIView()
          v.backgroundColor = UIColor.systemRed
          v.layer.cornerRadius = 10
          v.layer.maskedCorners = [.layerMinXMinYCorner, .layerMinXMaxYCorner, .layerMaxXMinYCorner, .layerMaxXMaxYCorner]
          v.layer.masksToBounds = true
          v.translatesAutoresizingMaskIntoConstraints = false
          return v
      }()
      
      private var feedback: UIImpactFeedbackGenerator?
      
      private var widthAnchorConstrainst: NSLayoutConstraint?
  
      override func viewDidLoad() {
          super.viewDidLoad()
          
          // Do any additional setup after loading the view.
          view.backgroundColor = .systemBlue
          view.addSubview(testButton)
          view.addSubview(redView)
          
          NSLayoutConstraint.activate([
              redView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 40),
              redView.centerXAnchor.constraint(equalTo: view.centerXAnchor),
              redView.heightAnchor.constraint(equalToConstant: 200),
              testButton.topAnchor.constraint(equalTo: redView.bottomAnchor, constant: 40),
              testButton.widthAnchor.constraint(equalToConstant: 120),
              testButton.centerXAnchor.constraint(equalTo: view.centerXAnchor),
              testButton.heightAnchor.constraint(equalToConstant: 44)
          ])
          
          widthAnchorConstrainst = redView.widthAnchor.constraint(equalToConstant: 200)
          widthAnchorConstrainst?.isActive = true
      }
      
      
      @objc private func _target(_ sender: UIButton) {
          if widthAnchorConstrainst?.constant == 200 {
              widthAnchorConstrainst?.constant = 100
          }
          else {
              widthAnchorConstrainst?.constant = 200
          }
      }
      
  }

• 示例代码，每一次点击按钮的时候，更改按钮的相对位置

  import UIKit
  
  class ViewController: UIViewController {
      
      private lazy var testButton = {
          let btn = UIButton()
          btn.setTitle("test", for: .normal)
          btn.setTitleColor(.black, for: .normal)
          btn.backgroundColor = UIColor.systemGreen
          btn.layer.cornerRadius = 10
          btn.layer.maskedCorners = [.layerMinXMinYCorner, .layerMinXMaxYCorner, .layerMaxXMinYCorner, .layerMaxXMaxYCorner]
          btn.layer.masksToBounds = true
          btn.translatesAutoresizingMaskIntoConstraints = false
          btn.addTarget(self, action: #selector(_target(_:)), for: .touchUpInside)
          return btn
      }()
      
      private lazy var redView = {
          let v = UIView()
          v.backgroundColor = UIColor.systemRed
          v.layer.cornerRadius = 10
          v.layer.maskedCorners = [.layerMinXMinYCorner, .layerMinXMaxYCorner, .layerMaxXMinYCorner, .layerMaxXMaxYCorner]
          v.layer.masksToBounds = true
          v.translatesAutoresizingMaskIntoConstraints = false
          return v
      }()
      
      private var feedback: UIImpactFeedbackGenerator?
      
      private var buttonTopConstraint: NSLayoutConstraint?
  
      override func viewDidLoad() {
          super.viewDidLoad()
          
          // Do any additional setup after loading the view.
          view.backgroundColor = .systemBlue
          view.addSubview(testButton)
          view.addSubview(redView)
          
          NSLayoutConstraint.activate([
              redView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 40),
              redView.centerXAnchor.constraint(equalTo: view.centerXAnchor),
              redView.heightAnchor.constraint(equalToConstant: 200),
              redView.widthAnchor.constraint(equalToConstant: 200),
              testButton.widthAnchor.constraint(equalToConstant: 120),
              testButton.centerXAnchor.constraint(equalTo: view.centerXAnchor),
              testButton.heightAnchor.constraint(equalToConstant: 44)
          ])
          
          buttonTopConstraint = testButton.topAnchor.constraint(equalTo: redView.bottomAnchor, constant: 40)
          buttonTopConstraint?.isActive = true
      }
      
      
      @objc private func _target(_ sender: UIButton) {
          guard let buttonTopConstraint else { return }
          
          if let secondItem = buttonTopConstraint.secondItem as? UIView , secondItem == redView {
              NSLayoutConstraint.deactivate([buttonTopConstraint])
              self.buttonTopConstraint = testButton.topAnchor.constraint(equalTo: view.centerYAnchor, constant: 0)
              self.buttonTopConstraint?.isActive = true
          }
          else {
              NSLayoutConstraint.deactivate([buttonTopConstraint])
              self.buttonTopConstraint = testButton.topAnchor.constraint(equalTo: redView.bottomAnchor, constant: 40)
              self.buttonTopConstraint?.isActive = true
          }
      }
  }