ios - 为什么 UIButton 不返回正确的约束？

Question

在下面的代码中：我在垂直滚动视图中添加了 5 个按钮。每个按钮都被限制在滚动视图的 top + 20 、前缘、后缘及其高度。我创建了一个b1HeightConstraint变量。它用于保存按钮的 heightConstraint b1。

在单击按钮时，我正在尝试删除此约束。然而我面临一个奇怪的问题：

当我记录约束时，我只看到 2 个约束，即使我已经添加了 4 个约束。我的视图调试层次结构如下：

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)
    }
}

_{代码已准备好转储到 ViewController 类中。开箱即用。我的代码是这里编写的代码的衍生}

Answer 1

以下是关于您的代码的几条评论：

1. 您从未向任何视图添加任何约束，因此您不应该删除它们。iOS (CocoaTouch) 将这些约束添加到这些视图中，所以请不要触摸它们。（换句话说：当你没有打电话removeConstraintaddConstraint时不要打电话）。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（由 iOS）添加到约束中提到的两个项目的最常见的祖先。所以如果两个视图是兄弟，它会被添加到父视图中。如果两个视图是父视图和子视图，则将约束添加到父视图。如果这两个视图是祖父母和孙子，它将被添加到祖父母。如果这两个视图是表亲，则约束将被添加到它们的共同祖父母。

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，因此它根本没有添加到任何视图中。因此，试图删除它永远不会起作用，因为该约束仅存在于您的b1HeightConstraint财产中。由于它从未被激活，它实际上并没有限制任何东西。

4. 如果要缩小按钮，您需要执行以下操作之一：a) 修改constant其高度约束的属性或b) 将其高度约束的isActive属性设置为false然后给它一个新的高度约束或c) 修改优先级主动约束让自动布局选择使用不同的约束。

5. 在您的视图调试层次结构中，显示的所有约束都是活动约束（意味着它们可供Auto Layout使用）。灰色的那些是 Auto Layout 选择不使用的那些，因为更高优先级的约束优先于它。这不会导致冲突。该self.height = 34 (content size)约束由系统添加以说明内容压缩和内容合并。UIButton抗压优先750，抗膨胀优先250。该self.height = 34 (content size)约束是灰色的，因为内容拥抱的优先级为250，而使用了另一个更高优先级的约束（将按钮的高度设置为等于滚动视图的高度的约束具有优先级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shrink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constant高度约束的属性

   // 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：为此，您必须为高度约束赋予一个小于1000(999工作得很好) 的初始优先级，因为如果需要，自动布局将不允许您更改活动约束的优先级 (priority 1000)。

   或者

   // 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
   

Answer 2

这是因为视图和它的 superView 之间的约束被添加到 superView ，如果它们是静态添加到的，你只会看到高度/宽度约束UIButton，从 Vandad IOS Book 看这个图

看这个演示