以编程方式添加约束不起作用答案

【问题标题】：Adding Constraint Programmatically Does Not Work以编程方式添加约束不起作用
【发布时间】：2020-05-01 21:33:28
【问题描述】：

我正在像这样以编程方式设置图像视图的约束：

        imgScrollView = UIScrollView(frame: CGRect(x: 0, y: 0, width: 100, height: 100))
        imgScrollView.showsHorizontalScrollIndicator = false
        imgScrollView.showsVerticalScrollIndicator = false
        imgScrollView.bouncesZoom = false
        imgScrollView.bounces = false

        view.addSubview(imgScrollView)

        imgScrollView.translatesAutoresizingMaskIntoConstraints = false
        imgScrollView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 0).isActive = true
        imgScrollView.bottomAnchor.constraint(equalTo: toolBar.topAnchor, constant: -100).isActive = true
        imgScrollView.leftAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leftAnchor, constant: 0).isActive = true
        imgScrollView.rightAnchor.constraint(equalTo: view.safeAreaLayoutGuide.rightAnchor, constant: 0).isActive = true



        // add image view to scrollview
        imgView = UIImageView(frame: CGRect(x: 0, y: 0,width: 100, height: 100))

        imgScrollView.addSubview(imgView)
        imgView.translatesAutoresizingMaskIntoConstraints = false

        imgView.topAnchor.constraint(equalTo: imgScrollView.contentLayoutGuide.topAnchor, constant: 0).isActive = true
        imgView.bottomAnchor.constraint(equalTo: imgScrollView.contentLayoutGuide.bottomAnchor, constant: 0).isActive = true
        imgView.leftAnchor.constraint(equalTo: imgScrollView.contentLayoutGuide.leftAnchor, constant: 0).isActive = true
        imgView.rightAnchor.constraint(equalTo: imgScrollView.contentLayoutGuide.rightAnchor, constant: 0).isActive = true


        imgView.widthAnchor.constraint(equalTo: imgScrollView.widthAnchor, multiplier: 1).isActive = true

现在稍后点击按钮，我将添加额外的约束

imgView.heightAnchor.constraint(equalTo: imgScrollView.heightAnchor, multiplier: 1).isActive = true

但是没有添加约束。为什么会这样？

【问题讨论】：

@mezzi 为什么不直接在开头添加高度约束，然后在点击按钮时更改其常量？
你想做什么？给予左右约束并给予宽度......它们会发生冲突......底部和顶部的情况也是如此......也给予高度......
当我在创建 imgView 的开头添加所有约束左、右和宽度、高度时没有问题。没有冲突。一切都按预期工作。当我尝试在按钮点击后添加高度约束时，没有添加约束
@badhanganesh 你是什么意思？我应该在开头输入 nil 吗？
我不知道你在点击按钮时做了什么，但不是在按钮点击上添加新的高度约束，你可以在开头添加高度约束，高度为零或其他东西（或任何默认值），然后点击按钮，您可以将该高度常数更改为您想要的任何新值。您必须存储要更改的（高度）约束。

标签： ios swift nslayoutconstraint

【解决方案1】：

您的实现逻辑对我有用。

左图显示没有高度限制的实现。
右图显示按钮按下高度限制。

在函数viewDidLoad中

override func viewDidLoad() {
  super.viewDidLoad()
  view.backgroundColor = .white
  scrollView.isScrollEnabled = true
  scrollView.bounces = false
  scrollView.alwaysBounceVertical = true
  imageView.image = UIImage(color: .lightGray)

  navigationItem.rightBarButtonItem = barButtonItem
  view.addSubview(scrollView)
  scrollView.addSubview(imageView)

  scrollView.translatesAutoresizingMaskIntoConstraints = false
  imageView.translatesAutoresizingMaskIntoConstraints = false
  NSLayoutConstraint.activate([
    scrollView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
    scrollView.leftAnchor.constraint(equalTo: view.leftAnchor),
    scrollView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor),
    scrollView.rightAnchor.constraint(equalTo: view.rightAnchor)
  ])
  NSLayoutConstraint.activate([
    imageView.topAnchor.constraint(equalTo: scrollView.contentLayoutGuide.topAnchor),
    imageView.leftAnchor.constraint(equalTo: scrollView.contentLayoutGuide.leftAnchor),
    imageView.bottomAnchor.constraint(equalTo: scrollView.contentLayoutGuide.bottomAnchor),
    imageView.rightAnchor.constraint(equalTo: scrollView.contentLayoutGuide.rightAnchor),
    imageView.widthAnchor.constraint(equalTo: scrollView.widthAnchor)
  ])
}

在功能按钮按下

@objc func buttonPressed() {
  navigationItem.rightBarButtonItem = nil
  UIView.animate(withDuration: 1) {
    NSLayoutConstraint.activate([
      self.imageView.heightAnchor.constraint(equalTo: self.scrollView.heightAnchor)
    ])
    self.view.setNeedsLayout()
    self.view.layoutIfNeeded()
  }
}

UIImage 工具Create UIImage with solid color in Swift

fileprivate extension UIImage {
  convenience init?(color: UIColor, size: CGSize = CGSize(width: 1, height: 1)) {
    let rect = CGRect(origin: .zero, size: size)
    UIGraphicsBeginImageContextWithOptions(rect.size, false, 0.0)
    color.setFill()
    UIRectFill(rect)
    let image = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    guard let cgImage = image?.cgImage else { return nil }
    self.init(cgImage: cgImage)
  }
}

【讨论】：

使用 NSLayoutConstraint.activate() 设置约束就可以了。不是它按预期工作。谢谢。