为了帮助理解为什么我们不能将约束与.center（帧）更改混合...
下面是两个几乎相同的视图控制器。
每个都添加一个按钮，并将按钮的.centerXAnchor和.centerYAnchor设置为视图的.centerXAnchor和.centerYAnchor。
我们将平移手势添加到按钮，以便可以四处拖动它。
我们还实现了touchesBegan(...)，我们所做的只是改变按钮标题，这样做将触发一个自动布局通道。
在第一个示例的平移函数中，我们使用了一个典型的“获取平移位置并更新.center属性：

@objc func pan(_ sender: UIPanGestureRecognizer) {
    guard let v = sender.view, let sv = v.superview else { return }
    if sender.state == .changed {
        let pt: CGPoint = sender.location(in: sv)
        // update the button's .center property (changes the frame)
        v.center = pt
    }
}

这样做很好，直到我们点击按钮的任何位置。在这一点上，我们改变按钮的标题和自动布局移动按钮回到它的中心X和Y约束。
在第二个示例中，我们将X和Y约束作为var /属性添加到控制器：

// btn center constraints
//  we will modify the .constants when panning
var xConstraint: NSLayoutConstraint!
var yConstraint: NSLayoutConstraint!

在viewDidLoad()中设置它们，然后在平移手势中 * 移动按钮 *，如下所示：

@objc func pan(_ sender: UIPanGestureRecognizer) {
    guard let v = sender.view, let sv = v.superview else { return }
    if sender.state == .changed {
        let pt: CGPoint = sender.location(in: sv)
        let xOff = pt.x - sv.center.x
        let yOff = pt.y - sv.center.y
        // update the .constant values for the btn center x/y
        xConstraint.constant = xOff
        yConstraint.constant = yOff
    }
}

现在，点击其他任何地方将更改按钮标题，但它将保持不变，因为我们已经更改了X和Y约束的.constant值。

设置.center--问题

class DemoVC: UIViewController {

    let btnToMove = UIButton()

    var tapCounter: Int = 0

    override func viewDidLoad() {
        super.viewDidLoad()

        btnToMove.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(btnToMove)

        btnToMove.setTitle("Test", for: [])
        btnToMove.backgroundColor = .red

        NSLayoutConstraint.activate([
            btnToMove.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            btnToMove.centerYAnchor.constraint(equalTo: view.centerYAnchor),
        ])

        let p = UIPanGestureRecognizer(target: self, action: #selector(pan(_:)))
        btnToMove.addGestureRecognizer(p)
    }

    @objc func pan(_ sender: UIPanGestureRecognizer) {
        guard let v = sender.view, let sv = v.superview else { return }
        if sender.state == .changed {
            let pt: CGPoint = sender.location(in: sv)
            // update the button's .center property (changes the frame)
            v.center = pt
        }
    }

    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        // update the button title - which will trigger an auto-layout pass
        tapCounter += 1
        btnToMove.setTitle("Test \(tapCounter)", for: [])
    }

}

更新约束.constant值--没有问题

class DemoVC: UIViewController {

    let btnToMove = UIButton()

    var tapCounter: Int = 0

    // btn center constraints
    //  we will modify the .constants when panning
    var xConstraint: NSLayoutConstraint!
    var yConstraint: NSLayoutConstraint!

    override func viewDidLoad() {
        super.viewDidLoad()

        btnToMove.setTitle("Test", for: [])
        btnToMove.backgroundColor = .red

        btnToMove.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(btnToMove)

        xConstraint = btnToMove.centerXAnchor.constraint(equalTo: view.centerXAnchor, constant: 0.0)
        yConstraint = btnToMove.centerYAnchor.constraint(equalTo: view.centerYAnchor, constant: 0.0)

        NSLayoutConstraint.activate([
            xConstraint, yConstraint,
        ])

        let p = UIPanGestureRecognizer(target: self, action: #selector(pan(_:)))
        btnToMove.addGestureRecognizer(p)
    }

    @objc func pan(_ sender: UIPanGestureRecognizer) {
        guard let v = sender.view, let sv = v.superview else { return }
        if sender.state == .changed {
            let pt: CGPoint = sender.location(in: sv)
            let xOff = pt.x - sv.center.x
            let yOff = pt.y - sv.center.y
            // update the .constant values for the btn center x/y
            xConstraint.constant = xOff
            yConstraint.constant = yOff
        }
    }

    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        // update the button title - which will trigger an auto-layout pass
        tapCounter += 1
        btnToMove.setTitle("Test \(tapCounter)", for: [])
    }

}