ios - 为什么 UIButton 不返回正确的约束？

Question

在我下面的代码中:我在垂直 ScrollView 中添加了 5 个按钮。每个按钮都被限制在 scrollViews 的顶部 + 20、前导、尾随边缘及其高度。我创建了一个 b1HeightConstraint 变量。它用于保存 b1 按钮的 heightConstraint。

在单击按钮时，我试图删除此约束。但是我遇到了一个奇怪的问题:

当我记录约束时，我只看到 2 个约束，即使我已经向它添加了 4 个约束。我的 View 调试层次结构如下所示:

import UIKit
import Foundation

class ViewController: UIViewController {
    var filterView: UIView!
    var scrollView: UIScrollView!
    var containerView: UIView!

    override func loadView() {
        filterView = UIView()
        view = filterView
        view.backgroundColor = #colorLiteral(red: 0.909803926944733, green: 0.47843137383461, blue: 0.643137276172638, alpha: 1.0)

        scrollView = UIScrollView()
        scrollView.backgroundColor = #colorLiteral(red: 0.474509805440903, green: 0.839215695858002, blue: 0.976470589637756, alpha: 1.0)
        view.addSubview(scrollView)
        scrollView.translatesAutoresizingMaskIntoConstraints = false
        scrollView.topAnchor.constraint(equalTo: view.topAnchor, constant: 0).isActive = true
        scrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        scrollView.widthAnchor.constraint(equalTo: view.widthAnchor).isActive = true
        scrollView.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 1).isActive = true
        scrollView.isScrollEnabled = true

        containerView = UIView()
        containerView.backgroundColor = #colorLiteral(red: 0.176470592617989, green: 0.498039215803146, blue: 0.756862759590149, alpha: 1.0)
        scrollView.addSubview(containerView)
        containerView.translatesAutoresizingMaskIntoConstraints = false

        // This is key:  connect all four edges of the containerView to
        // to the edges of the scrollView
        containerView.topAnchor.constraint(equalTo: scrollView.topAnchor).isActive = true
        containerView.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor).isActive = true
        containerView.trailingAnchor.constraint(equalTo: scrollView.trailingAnchor).isActive = true
        containerView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor).isActive = true

        // Making containerView and scrollView the same height means the
        // content will not scroll vertically
        containerView.widthAnchor.constraint(equalTo: scrollView.widthAnchor).isActive = true
    }



    let b1 = Buttons(titleText: "one")
    let b2 = Buttons(titleText: "two")
    let b3 = Buttons(titleText: "three")
    let b4 = Buttons(titleText: "four")
    let b5 = Buttons(titleText: "five")
    var b1HeightConstraint : NSLayoutConstraint?

    override func viewDidLoad() {
        super.viewDidLoad()


        let buttonArray = [b1, b2, b3, b4, b5]

        b1.button.addTarget(self, action: #selector(ViewController.shrink(_:)), for: .touchUpInside)

        var startPoint = containerView.topAnchor

        for btn in buttonArray {
            let theBtn = btn.button
            containerView.addSubview(theBtn)
            theBtn.translatesAutoresizingMaskIntoConstraints = false
            theBtn.topAnchor.constraint(equalTo: startPoint, constant: 20).isActive = true
            theBtn.leadingAnchor.constraint(equalTo: containerView.leadingAnchor).isActive = true
            theBtn.trailingAnchor.constraint(equalTo: containerView.trailingAnchor).isActive = true
            theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor).isActive = true

            startPoint = theBtn.bottomAnchor
            let btnHeight = theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor)
            if btn == b1{
                b1HeightConstraint = btnHeight
            }
        }

        containerView.bottomAnchor.constraint(equalTo: startPoint, constant: 20).isActive = true

    }

    @objc func shrink(_ sender: Any){
        guard let btn = sender as? UIButton else{
            return
        }
        print("count is: \(btn.constraints.count)")

        btn.removeConstraint(b1HeightConstraint!)
        containerView.removeConstraint(b1HeightConstraint!)
        print("count is: \(btn.constraints.count)")
        containerView.updateConstraintsIfNeeded()
        containerView.updateConstraints()
        scrollView.updateConstraintsIfNeeded()
        scrollView.updateConstraints()
    }
}

class Buttons : NSObject {
    let button = UIButton()
    init(titleText: String) {
        button.backgroundColor = #colorLiteral(red: 0.976470589637756, green: 0.850980401039124, blue: 0.549019634723663, alpha: 1.0)
        button.setTitle(titleText, for: .normal)
    }
}

_{代码已准备就绪，可以转储到 ViewController 类中。开箱即用。我的代码是编写的代码的副产品 here}

Answer 1

以下是关于您的代码的几条评论:

1. 您从未向任何 View 添加任何约束，因此您不应该删除它们。 iOS (CocoaTouch) 向这些 View 添加了这些约束，所以请不要碰它们。 (换句话说:不要在未调用 addConstraint 时调用 removeConstraint)。您对约束的控制是激活和停用它们。将添加和删除留给 iOS。
2. 当您激活一个约束时，一个约束被添加(由 iOS)到约束中提到的两个项目的最常见的祖先。因此，如果这两个 View 是兄弟 View ，它将被添加到父 View 中。如果两个 View 是父 View 和 subview ，则约束将添加到父 View 。如果两个 View 是祖 parent 和孙子女，则将其添加到祖 parent 。如果这两个 View 是堂兄弟，则约束将添加到它们的共同祖 parent 。

3. 这些代码行:

   let btnHeight = theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor)
   if btn == b1{
       b1HeightConstraint = btnHeight
   }
   

   正在创建一个新约束并将其分配给 b1HeightConstraint，但您从未激活此约束，因此它根本没有被添加到任何 View 中。因此，尝试删除它永远不会奏效，因为该约束仅存在于您的 b1HeightConstraint 属性中。由于它从未被激活，它实际上并没有约束任何东西。

4. 如果您想缩小按钮，您需要执行以下操作之一:a) 修改其高度约束的 constant 属性或 b) 设置其高度约束的 isActive 属性为 false 然后给它一个新的高度约束 OR c) 修改事件约束的优先级以具有 自动布局选择使用不同的约束。

5. 在您的 View 调试层次结构中，显示的所有约束都是事件约束(意味着它们可供自动布局使用)。灰色的是自动布局选择不使用的那些，因为更高优先级的约束优先于它。这不会引起冲突。 self.height = 34(内容大小) 约束由系统添加以解决内容压缩 和内容压缩。 UIButton 以 750 的优先级抵抗压缩，以 250 的优先级抵抗扩展。 self.height = 34 (content size) 约束变灰了，因为 content hugging 的优先级为 250 并且使用了另一个更高优先级的约束(设置按钮的高度等于 scrollView 的高度具有优先级 1000)。

---

更新代码:

这是您修改后的代码。我改变了两件事:

1. 我确定 b1HeightConstraint 是一个激活的约束。
2. 我更改了 shr​​ink 方法以停用旧的高度约束，然后创建并激活一个新的。

更新代码

import UIKit
import Foundation

class ViewController: UIViewController {
    var filterView: UIView!
    var scrollView: UIScrollView!
    var containerView: UIView!

    override func loadView() {
        filterView = UIView()
        view = filterView
        view.backgroundColor = #colorLiteral(red: 0.909803926944733, green: 0.47843137383461, blue: 0.643137276172638, alpha: 1.0)

        scrollView = UIScrollView()
        scrollView.backgroundColor = #colorLiteral(red: 0.474509805440903, green: 0.839215695858002, blue: 0.976470589637756, alpha: 1.0)
        view.addSubview(scrollView)
        scrollView.translatesAutoresizingMaskIntoConstraints = false
        scrollView.topAnchor.constraint(equalTo: view.topAnchor, constant: 0).isActive = true
        scrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        scrollView.widthAnchor.constraint(equalTo: view.widthAnchor).isActive = true
        scrollView.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 1).isActive = true
        scrollView.isScrollEnabled = true

        containerView = UIView()
        containerView.backgroundColor = #colorLiteral(red: 0.176470592617989, green: 0.498039215803146, blue: 0.756862759590149, alpha: 1.0)
        scrollView.addSubview(containerView)
        containerView.translatesAutoresizingMaskIntoConstraints = false

        // This is key:  connect all four edges of the containerView to
        // to the edges of the scrollView
        containerView.topAnchor.constraint(equalTo: scrollView.topAnchor).isActive = true
        containerView.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor).isActive = true
        containerView.trailingAnchor.constraint(equalTo: scrollView.trailingAnchor).isActive = true
        containerView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor).isActive = true

        // Making containerView and scrollView the same height means the
        // content will not scroll vertically
        containerView.widthAnchor.constraint(equalTo: scrollView.widthAnchor).isActive = true
    }

    let b1 = Buttons(titleText: "one")
    let b2 = Buttons(titleText: "two")
    let b3 = Buttons(titleText: "three")
    let b4 = Buttons(titleText: "four")
    let b5 = Buttons(titleText: "five")
    var b1HeightConstraint : NSLayoutConstraint?

    override func viewDidLoad() {
        super.viewDidLoad()

        let buttonArray = [b1, b2, b3, b4, b5]

        b1.button.addTarget(self, action: #selector(ViewController.shrink(_:)), for: .touchUpInside)

        var startPoint = containerView.topAnchor

        for btn in buttonArray {
            let theBtn = btn.button
            containerView.addSubview(theBtn)
            theBtn.translatesAutoresizingMaskIntoConstraints = false
            theBtn.topAnchor.constraint(equalTo: startPoint, constant: 20).isActive = true
            theBtn.leadingAnchor.constraint(equalTo: containerView.leadingAnchor).isActive = true
            theBtn.trailingAnchor.constraint(equalTo: containerView.trailingAnchor).isActive = true
            //theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor).isActive = true

            startPoint = theBtn.bottomAnchor
            let btnHeight = theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor)
            btnHeight.isActive = true
            if btn == b1{
                b1HeightConstraint = btnHeight
            }
        }

        containerView.bottomAnchor.constraint(equalTo: startPoint, constant: 20).isActive = true

    }

    @objc func shrink(_ sender: UIButton) {
        b1HeightConstraint?.isActive = false
        b1HeightConstraint = sender.heightAnchor.constraint(equalToConstant: 20)
        b1HeightConstraint?.isActive = true
    }
}

class Buttons : NSObject {
    let button = UIButton()
    init(titleText: String) {
        button.backgroundColor = #colorLiteral(red: 0.976470589637756, green: 0.850980401039124, blue: 0.549019634723663, alpha: 1.0)
        button.setTitle(titleText, for: .normal)
    }
}

---

缩小按钮高度的选项

1. 设置高度约束的常量属性

   // shrink button's height by 200 points
   b1HeightConstraint?.constant -= 200
   

2. 停用旧约束并创建并激活新约束

   // make button height 20 points
   b1HeightConstraint?.isActive = false
   b1HeightConstraint = sender.heightAnchor.constraint(equalToConstant: 20)
   b1HeightConstraint?.isActive = true
   

3. 更改高度约束的优先级

   // Set b1HeightConstraint's priority to less than 250, and the
   // *content hugging* with priority 250 will take over and resize
   // the button to its intrinsic height
   b1HeightConstraint?.priority = UILayoutPriority(rawValue: 100)
   

   注意:要使其正常工作，您必须为高度限制指定一个小于 1000 的初始优先级(999 效果很好)因为 Auto Layout 不允许您更改事件约束的优先级(如果需要)(优先级 1000)。

   或

   // Just deactivate the buttonHeight constraint and the *content
   // hugging* will take over and it will set the button's height
   // to its intrinsic height
   b1HeightConstraint?.isActive = false