以编程方式超出UIView约束

我有3个UIView层的蛋糕，具有编程约束 .

用于以编程方式设置约束的构造函数：

func setupViewConstraints(item:UIView, leadingTo:NSLayoutXAxisAnchor, leadingCon:CGFloat, 
trailingTo:NSLayoutXAxisAnchor, trailingCon:CGFloat, topTo:NSLayoutYAxisAnchor, 
topCon:CGFloat, bottomTo:NSLayoutYAxisAnchor, bottomCon:CGFloat) {
    item.translatesAutoresizingMaskIntoConstraints = false
    item.leadingAnchor.constraint(equalTo: leadingTo, constant: leadingCon).isActive = true
    item.trailingAnchor.constraint(equalTo: trailingTo, constant: trailingCon).isActive = true
    item.topAnchor.constraint(equalTo: topTo, constant:topCon).isActive = true
    item.bottomAnchor.constraint(equalTo: bottomTo, constant:bottomCon).isActive = true
}

最低的基础层是lightGray .

view = UIView()
view.backgroundColor = .lightGray

第二层包含带有约束的2 UIView （红色和蓝色） .

let red = UIView()
red.backgroundColor = .red
view.addSubview(red)
setupViewConstraints(item: red, leadingTo: view.leadingAnchor, leadingCon: 0, trailingTo: view.trailingAnchor, trailingCon: -(view.frame.width)*0.2), topTo: view.topAnchor, topCon: 0, bottomTo: view.bottomAnchor, bottomCon: -(view.frame.width)*0.8)

let blue = UIView()
blue.backgroundColor = .blue
view.addSubview(blue)
setupViewConstraints(item: blue, leadingTo: view.leadingAnchor, leadingCon: 0, trailingTo: view.trailingAnchor, trailingCon: -(view.frame.width)*0.2), topTo: red.bottomAnchor, topCon: 0, bottomTo: view.bottomAnchor, bottomCon: 0)

在顶部我有黄色 UIView 层，它与所有较低层重叠 .

let yellow = UIView()
yellow.backgroundColor = .yellow
view.addSubview(yellow)
setupViewConstraints(item: yellow, leadingTo: view.leadingAnchor, leadingCon: 0, trailingTo: view.trailingAnchor, trailingCon: 0, topTo: view.topAnchor, topCon: 0, bottomTo: view.bottomAnchor, bottomCon: 0)

另外，我在 UIView 内有 UINavigationBar ， UINavigationItem .

//Add navigation item and buttons
        naviItem = UINavigationItem()
        naviItem.setRightBarButton(UIBarButtonItem(barButtonSystemItem:.add, target:self, action:#selector(goToDestVC)), animated: true)
        naviItem.setLeftBarButton(UIBarButtonItem(image: UIImage(named: "hamburger_slim_30"), style: .plain, target: self, action: #selector(hamburgerBtnPressed)), animated: true)

        //Add navigation bar with transparent background
        naviBar = UINavigationBar()
        naviBar.setBackgroundImage(UIImage(), for: .default)
        naviBar.shadowImage = UIImage()
        naviBar.isTranslucent = true

// Assign the navigation item to the navigation bar
        naviBar.items = [naviItem]

        view.addSubview(naviBar)
        setupViewConstraints(item: naviBar, leadingTo: yellow.leadingAnchor, leadingCon: 0, trailingTo: yellow.trailingAnchor, trailingCon: 0, topTo: yellow.topAnchor, topCon: 0, bottomTo: yellow.bottomAnchor, bottomCon: -(view.frame.height)*0.9))

我有 hamburgerBtnPressed 函数，它应该将黄色图层向右移动80％（我将前导和尾随常量的值更改为80％）， but this does not work!!!

var hamburgerMenuIsVisible = false

    @objc func hamburgerBtnPressed(_ sender: Any) {

        if !hamburgerMenuIsVisible {

            let menuWidth = (self.view.frame.width)*0.8

              setupViewConstraints(item: layoutView, leadingTo: view.leadingAnchor, leadingCon: menuWidth, trailingTo: view.trailingAnchor, trailingCon: menuWidth, topTo: view.topAnchor, topCon: 0, bottomTo: view.bottomAnchor, bottomCon: 0)

            hamburgerMenuIsVisible = true

        } else {

            setupViewConstraints(item: layoutView, leadingTo: view.leadingAnchor, leadingCon: 0, trailingTo: view.trailingAnchor, trailingCon: 0, topTo: view.topAnchor, topCon: 0, bottomTo: view.bottomAnchor, bottomCon: 0)

            hamburgerMenuIsVisible = false
        }

        // layoutIfNeeded() lays out the subviews immediately and forces the layout before drawing
        UIView.animate(withDuration: 0.2, delay:0.0, options: .curveEaseIn, animations: {

            self.view.layoutIfNeeded()

        }) { (animationComplete) in
            print("Animation is complete!")
        }
    }

但是如果我将前导和尾随常量的值更改为负数，则一切都会起作用，菜单将向左移动而没有任何问题 .

let menuWidth = -(self.view.frame.width)*0.8

请解释..问题是什么？为什么黄色 UIView 向左移动了约束的负值，并且不能与约束的正值一起使用？并给出一个错误：

Probably at least one of the constraints in the following list is one you don't want. 
(
    "<NSLayoutConstraint:0x6040002853c0 UIView:0x7fa947c35850.trailing == UIView:0x7fa947e1d2d0.trailing   (active)>",
    "<NSLayoutConstraint:0x604000092750 UIView:0x7fa947c35850.trailing == UIView:0x7fa947e1d2d0.trailing + 331.2   (active)>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x604000092750 UIView:0x7fa947c35850.trailing == UIView:0x7fa947e1d2d0.trailing + 331.2   (active)>

Update: 我选择 Option 2 ：保留对要更改的约束的引用，并调整其常量 . 在layoutIfNeeded之前也需要调用setNeedsLayout .

更新的代码：

var leadingC: NSLayoutConstraint!
var trailingC: NSLayoutConstraint!
var yellow: UIView!

的loadView（）：

yellow = UIView()
        yellow.backgroundColor = .yellow
        view.addSubview(yellow)

        //Set up leading and trailing constraints for handling yellow view shift
        leadingC = yellow.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 0)
        trailingC = yellow.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: 0)

//Put leadingC.constant and trailingC.constant into the function
        setupViewConstraints(item: yellow, leadingTo: view.leadingAnchor, leadingCon: leadingC!.constant, trailingTo: view.trailingAnchor, trailingCon: trailingC.constant, topTo: view.topAnchor, topCon: 0, bottomTo: view.bottomAnchor, bottomCon: 0)

更新汉堡功能：

@objc func hamburgerBtnPressed(_ sender: Any) {

        if !hamburgerMenuIsVisible {

            let menuWidth = (self.view.frame.width)*0.8


            leadingC!.constant = menuWidth
            trailingC!.constant = menuWidth

            print(leadingC.constant, trailingC.constant)

            hamburgerMenuIsVisible = true

        } else {

            leadingC!.constant = 0
            trailingC!.constant = 0


            hamburgerMenuIsVisible = false
        }

        // layoutIfNeeded() lays out the subviews immediately and forces the layout before drawing
        UIView.animate(withDuration: 0.2, delay:0.0, options: .curveEaseIn, animations: {

            self.view.setNeedsLayout()
            self.view.layoutIfNeeded()

        }) { (animationComplete) in
            print("Animation is complete!")
        }
    }
    var hamburgerMenuIsVisible = false

我没有错误和“动画完成！”也印了，但屏幕上没有任何反应，没有动画 .