在Autolayout下设置其图层的anchorPoint后如何使uiview不移动

问题描述

我有一个需求，需要更改UIView层的anchorPoint，但是更改anchorPoint后视图无法移动。 我知道当视图由框架（CGRect：...）定义时是可能的。像这样：

let width = SCREEN_WIDTH - 40
let view2 = UIView(frame: CGRect(x: 20,y: 300,width: width,height: 200))
view2.backgroundColor = .blue
self.view.addSubview(view2)
let oldFrame2 = view2.frame
view2.layer.anchorPoint = CGPoint(x: 0,y: 0.5)
view2.frame = oldFrame2

这有效。

但是我的视图是由 Autolayout 定义的，我尝试了上面代码的解决方案，但它不起作用。代码：

let view1 = UIView()
view1.backgroundColor = .orange
self.view.addSubview(view1)
view1.snp.makeConstraints { (maker) in
     maker.top.equalToSuperview().offset(50)
     maker.leading.equalToSuperview().offset(20)
     maker.trailing.equalToSuperview().offset(-20)
     maker.height.equalTo(200)
}
let oldFrame1 = view1.frame
view1.layer.anchorPoint = CGPoint(x: 0,y: 0.5)
view1.frame = oldFrame1

结果是：橙色的view1被移动了，改变anchorPoint后应该和蓝色的view2一样。

谁能给我一些建议？

------------------------------更新答案-------------- --------------

正如@DonMag 的回答，我们可以通过在使用 Autolayout 时更新视图而非框架的约束来实现这一要求。这是 SnapKit 的代码：

let view1 = UIView()
view1.backgroundColor = .orange
view1.translatesAutoresizingMaskIntoConstraints = false
self.view.addSubview(view1)
view1.snp.makeConstraints { (maker) in
    maker.top.equalToSuperview().offset(100)
    maker.leading.equalToSuperview().offset(20)
    maker.trailing.equalToSuperview().offset(-20)
    maker.height.equalTo(200)
}
        
// important!!!
view1.layoutIfNeeded()

let oldFrame1 = view1.frame
view1.layer.anchorPoint = CGPoint(x: 0,y: 0.5)

// update constraints by updateConstraints function
// if you use @IBOutlet NSLayoutConstraint from xib,// you can also just set xxx.constant = yyy to update the constraints.
view1.snp.updateConstraints { (maker) in
    let subOffset = oldFrame1.width * 0.5
    maker.leading.equalToSuperview().offset(20 - subOffset)
    maker.trailing.equalToSuperview().offset(-20 - subOffset)
}

let width = SCREEN_WIDTH - 40
let view2 = UIView(frame: CGRect(x: 20,y: 0.5)
view2.frame = oldFrame2

另一种解决方案是通过设置 translatesAutoresizingMaskIntoConstraints = true 将自动布局视图更改为框架，如下所示：

let width = SCREEN_WIDTH - 40
let view1 = UIView()
view1.backgroundColor = .orange

self.view.addSubview(view1)
// Autolayout
view1.snp.makeConstraints { (maker) in
    maker.top.equalToSuperview().offset(100)
    maker.leading.equalToSuperview().offset(20)
    maker.trailing.equalToSuperview().offset(-20)
    maker.height.equalTo(200)
}

// change autolayout to frame
view1.translatesAutoresizingMaskIntoConstraints = true
view1.frame = CGRect(x: 20,y: 100,height: 200)
let oldFrame1 = view1.frame
view1.layer.anchorPoint = CGPoint(x: 0,y: 0.5)
view1.frame = oldFrame1

let view2 = UIView(frame: CGRect(x: 20,y: 0.5)
view2.frame = oldFrame2

解决方法

首先，当使用自动布局/约束时，直接设置视图的 .frame 不会给出预期的结果。自动布局更新 UI 后，将重新应用约束。

当您更改 .anchorPoint 时，您会更改视图的几何形状。因此，最好使用 .frame 而不是自动布局。

如果您确实需要/想要使用自动布局，则需要更新约束的 .constant 值以应对几何变化。

我不知道如何使用 SnapKit 做到这一点，但这里有一个使用“标准”约束语法的示例。

• 声明前导和尾随约束变量
• 分配和激活约束
• 告诉自动布局计算框架
• 改变锚点
• 更新 Lead 和 Trailing 约束常量以反映几何变化

注意：这只是示例代码！：

class ViewController: UIViewController {
    
    // these will have their .constant values changed
    //  to account for layer.anchorPoint change
    var leadingConstraint: NSLayoutConstraint!
    var trailingConstraint: NSLayoutConstraint!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // NOT using auto-layout constraints
        let width = view.frame.width - 40
        let view2 = UIView(frame: CGRect(x: 20,y: 300,width: width,height: 200))
        view2.backgroundColor = .blue
        self.view.addSubview(view2)
        let oldFrame2 = view2.frame
        view2.layer.anchorPoint = CGPoint(x: 0,y: 0.5)
        view2.frame = oldFrame2
        
        // USING auto-layout constraints
        let view1 = UIView()
        view1.backgroundColor = .orange
        view1.translatesAutoresizingMaskIntoConstraints = false
        self.view.addSubview(view1)

        // create leading and trailing constraints
        leadingConstraint = view1.leadingAnchor.constraint(equalTo: view.leadingAnchor,constant: 20.0)
        trailingConstraint = view1.trailingAnchor.constraint(equalTo: view.trailingAnchor,constant: -20.0)
        
        // activate constraints
        NSLayoutConstraint.activate([
            view1.topAnchor.constraint(equalTo: view.topAnchor,constant: 80.0),view1.heightAnchor.constraint(equalToConstant: 200.0),leadingConstraint,trailingConstraint,])

        // auto-layout has not run yet,so force it to layout
        //  the view frame
        view1.layoutIfNeeded()
        
        // get the auto-layout generated frame
        let oldFrame1 = view1.frame
        
        // change the anchorPoint
        view1.layer.anchorPoint = CGPoint(x: 0,y: 0.5)
        
        // we've move the X anchorPoint from 0.5 to 0.0,so
        //  we need to adjust the leading and trailing constants
        //  by 0.5 * the frame width
        leadingConstraint.constant -= oldFrame1.width * 0.5
        trailingConstraint.constant -= oldFrame1.width * 0.5

    }
    
}

结果：