Swift 更新约束

Question

我在UIView 中创建的按钮中添加了约束

func CreateButtonWithIndex(index:Int) {

    newButton.setTranslatesAutoresizingMaskIntoConstraints(false)

    self.view.addSubview(newButton)

    let newButtonConstraintL = NSLayoutConstraint(item: newButton, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 50)
    let newButtonConstraintH = NSLayoutConstraint(item: newButton, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 50)
    var newButtonConstraintX = NSLayoutConstraint(item: newButton, attribute: .CenterX, relatedBy: .Equal, toItem: view, attribute: .CenterX, multiplier: 1, constant: CGFloat(riga))
    var newButtonConstraintY = NSLayoutConstraint(item: newButton, attribute: .CenterY, relatedBy: .Equal, toItem: view, attribute: .CenterY, multiplier: 1, constant: CGFloat(colonna))

    view.addConstraints([newButtonConstraintX,newButtonConstraintY,newButtonConstraintH,newButtonConstraintL])

    var pan = UIPanGestureRecognizer(target:self, action:"pan:")
    pan.maximumNumberOfTouches = 2
    pan.minimumNumberOfTouches = 1
    newButton.addGestureRecognizer(pan)
}

func pan(rec:UIPanGestureRecognizer) {
  case .Ended: 
        if let subview = selectedView {
            if let button = rec.view as? UIButton {
                if let title = button.titleForState(.Normal){
                    button.setTranslatesAutoresizingMaskIntoConstraints(false)
                    let line = Float(p.x % snapX)
                    let column = Float(p.x % snapY)
                    let constraintL = NSLayoutConstraint(item: button, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 50)
                    let constraintH = NSLayoutConstraint(item: button, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 50)
                    var constraint2 = NSLayoutConstraint(item: button, attribute: .CenterX, relatedBy: .Equal, toItem: view, attribute: .CenterX, multiplier: 1, constant: CGFloat(line))
                    var constraint3 = NSLayoutConstraint(item: button, attribute: .CenterY, relatedBy: .Equal, toItem: view, attribute: .CenterY, multiplier: 1, constant: CGFloat(column))

                    view.addConstraints([constraint2,constraint3,constraintL.constraintH])
                }
            }
        }
}

在我的项目中，我的用户可以移动这些按钮，然后我尝试为识别的按钮添加更多约束，但我得到了一个无穷无尽的错误约束

  "<NSLayoutConstraint:0x7f8e5a4e9450 UIButton:0x7f8e5a644f60'Button6'.centerX == UIView:0x7f8e5a648bc0.centerX - 120>",
  "<NSLayoutConstraint:0x7f8e5a5be040 'UIView-Encapsulated-Layout-Width' H:[UIView:0x7f8e5a648bc0(736)]>",
  "<NSAutoresizingMaskLayoutConstraint:0x7f8e5a5be1f0 h=-&- v=-&- 'UIView-Encapsulated-Layout-Left' H:|-(0)-[UIView:0x7f8e5a648bc0]   (Names: '|':UITransitionView:0x7f8e5a5a8190 )>",
  "<NSAutoresizingMaskLayoutConstraint:0x7f8e5a5b53b0 h=--& v=--& UIButton:0x7f8e5a644f60'Button6'.midX == + 200>"

将尝试通过打破约束来恢复

"<NSLayoutConstraint:0x7f8e5a4e9450 UIButton:0x7f8e5a644f60'Button6'.centerX == UIView:0x7f8e5a648bc0.centerX - 120>"

"<NSLayoutConstraint:0x7f8e5a4e9510 UIButton:0x7f8e5a644f60'Button6'.centerY == UIView:0x7f8e5a648bc0.centerY - 40>",
"<NSLayoutConstraint:0x7f8e5a5be1a0 'UIView-Encapsulated-Layout-Height' V:[UIView:0x7f8e5a648bc0(414)]>",
"<NSAutoresizingMaskLayoutConstraint:0x7f8e5a5be240 h=-&- v=-&- 'UIView-Encapsulated-Layout-Top' V:|-(0)-[UIView:0x7f8e5a648bc0]   (Names: '|':UITransitionView:0x7f8e5a5a8190 )>",
"<NSAutoresizingMaskLayoutConstraint:0x7f8e5a5b82d0 h=--& v=--& UIButton:0x7f8e5a644f60'Button6'.midY == + 189>"

将尝试通过打破约束来恢复

<NSLayoutConstraint:0x7f8e5a4e9510 UIButton:0x7f8e5a644f60'Button6'.centerY == UIView:0x7f8e5a648bc0.centerY - 40>

...我应该更新 newButtonConstraintX / Y 但我不明白我该怎么做吗？

Answer 1

问题是您正在添加一个与现有约束冲突的新约束。

你有几个选择：

1. 在 iOS 8 中生效，您可以在添加新约束之前将约束的 active 属性设置为 false。

2. 在 iOS 8 之前的版本中，您可能希望在添加新约束之前删除旧约束。

3. 理想情况下，最好不必激活/停用（或者，更糟糕的是，添加和删除）约束，而只需修改单个约束的 constant 属性。例如在 Swift 3/4 中：

   class ViewController: UIViewController {
   
       private var xConstraint: NSLayoutConstraint!
       private var yConstraint: NSLayoutConstraint!
   
       override func viewDidLoad() {
           super.viewDidLoad()
   
           let label = UILabel()
           label.text = "x"
           label.translatesAutoresizingMaskIntoConstraints = false
           view.addSubview(label)
   
           // I don't really need to save references to these, so these are local variables
   
           let widthConstraint = label.widthAnchor.constraint(equalToConstant: 50)
           let heightConstraint = label.heightAnchor.constraint(equalToConstant: 50)
   
           // but since I'll be modifying these later, these are class properties
   
           xConstraint = label.centerXAnchor.constraint(equalTo: view.centerXAnchor)
           yConstraint = label.centerYAnchor.constraint(equalTo: view.centerYAnchor)
   
           NSLayoutConstraint.activate([widthConstraint, heightConstraint, xConstraint, yConstraint])
   
           let pan = UIPanGestureRecognizer(target: self, action: #selector(handlePan(_:)))
           view.addGestureRecognizer(pan)
       }
   
       private var originalCenter: CGPoint!
   
       @objc func handlePan(_ gesture: UIPanGestureRecognizer) {
           if gesture.state == .began {
               originalCenter = CGPoint(x: xConstraint.constant, y: yConstraint.constant)
           }
   
           let translation = gesture.translation(in: gesture.view!)
   
           xConstraint.constant = originalCenter.x + translation.x
           yConstraint.constant = originalCenter.y + translation.y
       }
   
   }
   

   当通过修改约束的constant可以达到想要的效果时，一般是最好的。

有关 Swift 2 语法，请参阅previous revision of this answer。

Answer 2

看起来你只是在添加越来越多的约束。你不能这样做，因为它们显然相互冲突。您基本上是在说“将视图放在位置 x = 1、x = 2、x = 3、x = 4 和 x = 5”。

您必须删除旧的约束。您有两种选择。

1. 将约束保存在数组中，并在添加新约束之前从视图中删除这些约束。

2. 或者您保留对更改和调整其属性的约束的引用。

由于您的约束只是常量值不同，您应该选择选项 2。

将newButtonConstraintX 和newButtonConstraintY 设为viewController 的变量。

例如

class ViewController: UIViewController {
    var newButtonConstraintX: NSLayoutConstraint!
    var newButtonConstraintY: NSLayoutConstraint!


    func CreateButtonWithIndex(index:Int) {

        newButtonConstraintX = NSLayoutConstraint(item: newButton, attribute: .CenterX, relatedBy: .Equal, toItem: view, attribute: .CenterX, multiplier: 1, constant: CGFloat(riga))

        newButtonConstraintY = NSLayoutConstraint(item: newButton, attribute: .CenterY, relatedBy: .Equal, toItem: view, attribute: .CenterY, multiplier: 1, constant: CGFloat(colonna))

        /* ... */
    }

    func pan(rec:UIPanGestureRecognizer) {  
        /* ... */
        newButtonConstraintX.constant = line
        newButtonConstraintY.constant = column
        button.layoutIfNeeded()
    }

Answer 3

更新 Rob 对 Swift 3 的回答：

class ViewController: UIViewController {

    private var xConstraint: NSLayoutConstraint!
    private var yConstraint: NSLayoutConstraint!

    override func viewDidLoad() {
    super.viewDidLoad()

    let label = UILabel()
    label.text = "x"
    label.setTranslatesAutoresizingMaskIntoConstraints(false)
    view.addSubview(label)

    // I don't really need to save references to these, so these are local variables


    let widthConstraint = NSLayoutConstraint(item: drugToDrugView, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 50)
    let heightConstraint = NSLayoutConstraint(item: drugToDrugView, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 50)

    // but since I'll be modifying these later, these are class properties

    xConstraint = NSLayoutConstraint(item: drugToDrugView, attribute: .centerX, relatedBy: .equal, toItem: view, attribute: .centerX, multiplier: 1.0, constant: 0)
    yConstraint = NSLayoutConstraint(item: drugToDrugView, attribute: .centerY, relatedBy: .equal, toItem: view, attribute: .centerY, multiplier: 1.0, constant: 0)

    drugToDrugView.addConstraints([widthConstraint, heightConstraint, xConstraint, yConstraint])

    let pan = UIPanGestureRecognizer(target: self, action: #selector(handlePan))
    view.addGestureRecognizer(pan)

 }

 private var originalCenter: CGPoint!

 func handlePan(gesture: UIPanGestureRecognizer) {
    if gesture.state == .began {
        originalCenter = CGPoint(x: xConstraint.constant, y: yConstraint.constant)
    }

    let translation = gesture.translation(in: gesture.view!)

    xConstraint.constant = originalCenter.x + translation.x
    yConstraint.constant = originalCenter.y + translation.y
    view.setNeedsLayout()
}