如何在设置图层的anchorPoint后使uiview不移动

Question

我有一个需要更改UIView层的anchorPoint的要求，但是在更改anchorPoint之后不能移动视图。我知道当视图由框架(CGRect：.)定义时是可能的。就像这样：

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

这个很管用。

但是我的视图是由Autolayout定义的，我像上面的代码一样尝试解决方案，但是它不起作用。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，它应该像改变anchorPoint后的蓝色view2。

​

​

有人能给我一些建议吗？

正如@DonMag的回答一样，我们可以通过更新使用Autolayout时视图而不是框架的约束来实现这一要求。下面是SnapKit编写的代码：

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: 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

另一种解决方案是通过设置translatesAutoresizingMaskIntoConstraints = true将autolayout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, width: width, 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: 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

Answer 1

首先，当使用自动布局/约束时，直接设置视图的.frame不会带来预期的结果。一旦自动布局更新用户界面，约束将被重新应用.

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

如果do需要/希望使用自动布局，则需要更新约束的.constant值，以考虑几何变化。

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

variables

• assign

• 声明前导和尾随约束，并激活constraints

• tell自动布局以计算框架

• ，更改anchorPoint

• update前导和尾随约束常量，以反映几何变化

。

注意：，这是示例代码！

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：

​

​