您是否尝试过将使用布局锚定创建的相关约束保存到属性中，然后只更改常量？

var ticketTop : NSLayoutConstraint?

func setup() {
    ticketTop = ticketContainer.topAnchor.constraintEqualToAnchor(self.topAnchor, constant:100)
    ticketTop.active = true
}

func update() {
    ticketTop?.constant = 25
}

可能更优雅

根据您编写扩展的喜好，这里有一种可能更优雅的方法，它不使用属性，而是在NSLayoutAnchor和UIView上创建扩展方法，以帮助更简洁地使用。
首先，你需要在NSLayoutAnchor上写一个扩展，如下所示：

extension NSLayoutAnchor {
    func constraintEqualToAnchor(anchor: NSLayoutAnchor!, constant:CGFloat, identifier:String) -> NSLayoutConstraint! {
        let constraint = self.constraintEqualToAnchor(anchor, constant:constant)
        constraint.identifier = identifier
        return constraint
    }
}

此扩展允许您在从锚创建约束的同一方法调用中设置约束的标识符。注意，Apple文档暗示XAxis锚点（左、右、行距等）不允许您使用Y轴锚点创建约束（顶部、底部等），但我没有观察到这是真的。如果你确实想要这种类型的编译器检查，您需要为NSLayoutXAxisAnchor、NSLayoutYAxisAnchor和NSLayoutDimension（用于宽度和高度约束）编写单独的扩展，以强制执行相同轴锚类型的要求。
接下来，您将在UIView上编写一个扩展，以通过标识符获得约束：

extension UIView {
    func constraint(withIdentifier:String) -> NSLayoutConstraint? {
        return self.constraints.filter{ $0.identifier == withIdentifier }.first
    }
}

有了这些扩展，您的代码就变成了：

func setup() {
    ticketContainer.topAnchor.constraintEqualToAnchor(anchor: self.topAnchor, constant:100, identifier:"ticketTop").active = true
}

func update() {
    self.constraint(withIdentifier:"ticketTop")?.constant = 25
}

请注意，使用常量或枚举而不是魔术字符串名称作为标识符将是对上述方法的改进，但我的回答要简明扼要。

你可以遍历视图的约束并检查匹配的项和锚点。记住，除非是维度约束，否则约束将位于视图的超级视图上。我写了一些帮助代码，可以让你找到视图的一个锚点上的所有约束。

import UIKit

class ViewController: UIViewController {

    let label = UILabel()
    let imageView = UIImageView()

    override func viewDidLoad() {
        super.viewDidLoad()

        label.text = "Constraint finder"

        label.translatesAutoresizingMaskIntoConstraints = false
        imageView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(label)
        view.addSubview(imageView)

        label.topAnchor.constraint(equalTo: view.topAnchor, constant: 30).isActive = true
        label.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 20).isActive = true
        label.widthAnchor.constraint(greaterThanOrEqualToConstant: 50).isActive = true

        imageView.topAnchor.constraint(equalTo: label.bottomAnchor).isActive = true
        imageView.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 60).isActive = true
        imageView.widthAnchor.constraint(equalTo: label.widthAnchor).isActive = true
        imageView.heightAnchor.constraint(equalToConstant: 70).isActive = true

        print("Label's top achor constraints: \(label.constraints(on: label.topAnchor))")
        print("Label's width achor constraints: \(label.constraints(on: label.widthAnchor))")
        print("ImageView's width achor constraints: \(imageView.constraints(on: imageView.widthAnchor))")
    }

}

public extension UIView {

    public func constraints(on anchor: NSLayoutYAxisAnchor) -> [NSLayoutConstraint] {
        guard let superview = superview else { return [] }
        return superview.constraints.filtered(view: self, anchor: anchor)
    }

    public func constraints(on anchor: NSLayoutXAxisAnchor) -> [NSLayoutConstraint] {
        guard let superview = superview else { return [] }
        return superview.constraints.filtered(view: self, anchor: anchor)
    }

    public func constraints(on anchor: NSLayoutDimension) -> [NSLayoutConstraint] {
        guard let superview = superview else { return [] }
        return constraints.filtered(view: self, anchor: anchor) + superview.constraints.filtered(view: self, anchor: anchor)
    }

}

extension NSLayoutConstraint {

    func matches(view: UIView, anchor: NSLayoutYAxisAnchor) -> Bool {
        if let firstView = firstItem as? UIView, firstView == view && firstAnchor == anchor {
            return true
        }
        if let secondView = secondItem as? UIView, secondView == view && secondAnchor == anchor {
            return true
        }
        return false
    }

    func matches(view: UIView, anchor: NSLayoutXAxisAnchor) -> Bool {
        if let firstView = firstItem as? UIView, firstView == view && firstAnchor == anchor {
            return true
        }
        if let secondView = secondItem as? UIView, secondView == view && secondAnchor == anchor {
            return true
        }
        return false
    }

    func matches(view: UIView, anchor: NSLayoutDimension) -> Bool {
        if let firstView = firstItem as? UIView, firstView == view && firstAnchor == anchor {
            return true
        }
        if let secondView = secondItem as? UIView, secondView == view && secondAnchor == anchor {
            return true
        }
        return false
    }
}

extension Array where Element == NSLayoutConstraint {

    func filtered(view: UIView, anchor: NSLayoutYAxisAnchor) -> [NSLayoutConstraint] {
        return filter { constraint in
            constraint.matches(view: view, anchor: anchor)
        }
    }
    func filtered(view: UIView, anchor: NSLayoutXAxisAnchor) -> [NSLayoutConstraint] {
        return filter { constraint in
            constraint.matches(view: view, anchor: anchor)
        }
    }
    func filtered(view: UIView, anchor: NSLayoutDimension) -> [NSLayoutConstraint] {
        return filter { constraint in
            constraint.matches(view: view, anchor: anchor)
        }
    }

}

定位单视图

extension UIView {
    func add(view: UIView, left: CGFloat, right: CGFloat, top: CGFloat, bottom: CGFloat) {
        
        view.translatesAutoresizingMaskIntoConstraints = false
        self.addSubview(view)

        view.leftAnchor.constraint(equalTo: self.leftAnchor, constant: left).isActive = true
        view.rightAnchor.constraint(equalTo: self.rightAnchor, constant: right).isActive = true
        
        view.topAnchor.constraint(equalTo: self.topAnchor, constant: top).isActive = true
        view.bottomAnchor.constraint(equalTo: self.bottomAnchor, constant: bottom).isActive = true
        
    }
}

用法

headerView.add(view: headerLabel, left: 20, right: 0, top: 0, bottom: 0)

我找到了另一个解决方案。如果你想改变一个通过界面生成器添加的现有约束，你实际上需要迭代超级视图的约束。这至少在你试图改变对齐约束的常量值时是正确的。
下面的例子显示了底部对齐，但我假设相同的代码将工作的前导/尾随/顶部对齐。

private func bottomConstraint(view: UIView) -> NSLayoutConstraint {
        guard let superview = view.superview else {
            return NSLayoutConstraint()
        }

        for constraint in superview.constraints {
            for bottom in [NSLayoutConstraint.Attribute.bottom, NSLayoutConstraint.Attribute.bottomMargin] {
                if constraint.firstAttribute == bottom && constraint.isActive && view == constraint.secondItem as? UIView {
                    return constraint
                }

                if constraint.secondAttribute == bottom && constraint.isActive && view == constraint.firstItem as? UIView {
                    return constraint
                }
            }
        }

        return NSLayoutConstraint()
    }

有用的扩展

extension UIView {

    //Note: Use `leadingAnchor and trailingAnchor` instead of `leftAnchor and rightAnchor`, it is going to help in RTL
    func addConstraint(top: NSLayoutYAxisAnchor?, leading: NSLayoutXAxisAnchor?, bottom: NSLayoutYAxisAnchor?, trailing: NSLayoutXAxisAnchor?, paddingTop: CGFloat? = nil, paddingLeft: CGFloat? = nil, paddingBottom: CGFloat? = nil, paddingRight: CGFloat? = nil, width: CGFloat? = nil, height: CGFloat? = nil) {
        translatesAutoresizingMaskIntoConstraints = false
        //top constraint
        if let top = top {
            self.topAnchor.constraint(equalTo: top, constant: paddingTop ?? 0).isActive = true
        }
    
        // leading/left constraint
        if let leading = leading {
            self.leadingAnchor.constraint(equalTo: leading, constant: paddingLeft ?? 0).isActive = true
        }

        //  bottom constraint
        if let bottom = bottom {
            self.bottomAnchor.constraint(equalTo: bottom, constant: -(paddingBottom ?? 0) ).isActive = true
        }

        // trailing/right constraint
        if let trailing = trailing {
            self.trailingAnchor.constraint(equalTo: trailing, constant: -(paddingRight ?? 0)).isActive = true
        }

        //width constraint
        if let width = width {
            widthAnchor.constraint(equalToConstant: width).isActive = true
        }

        //height constraint
        if let height = height {
            heightAnchor.constraint(equalToConstant: height).isActive = true
        }
    }
}

如何使用

let myView = UIView()
 self.view.addSubview(myView)
 myView.backgroundColor = .yellow
 myView.addConstraint(top: self.view.safeAreaLayoutGuide.topAnchor, leading: self.view.safeAreaLayoutGuide.leadingAnchor, bottom: self.view.safeAreaLayoutGuide.bottomAnchor, trailing: self.view.safeAreaLayoutGuide.trailingAnchor, paddingTop: 100)