Swift 4：设备旋转后 UI 无法更新

Posted 2023-02-23

【中文标题】Swift 4：设备旋转后 UI 无法更新

【英文标题】：Swift 4: UI fails to update after device rotation

【发布时间】：2019-07-07 07:51:01

【问题描述】：

我想将我的bearImageView 的宽度和高度从纵向 -> 横向更改为 100，在相反方向更改为 200。

在setupLayout 内部，我正在调用停用然后激活widthAnchor 和heightAnchor 上的约束。所以我期待它改变宽度和高度。

问题：变为 100，但不会变回 200。 为什么会这样？

这是代码。

class ViewController: UIViewController 

    // closure objects
    let bearImageView: UIImageView = 
        let imageView = UIImageView(image: #imageLiteral(resourceName: "bear_first")) // type `image literal` and double click
        imageView.translatesAutoresizingMaskIntoConstraints = false // enable autolayout
        return imageView
    ()

    override func viewDidLoad() 
        super.viewDidLoad()            
        view.addSubview(bearImageView) // display image        
        setupLayout(imageView: bearImageView) // apply constraints
    

    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) 
        super.viewWillTransition(to: size, with: coordinator)
        if UIDevice.current.orientation.isLandscape 
            print("landscape")
            setupLayout(imageView: bearImageView) // apply constraints                
         else 
            print("portrait")
            setupLayout(imageView: bearImageView) // apply constraints                
        
    

    private func setupLayout(imageView: UIImageView)

        if UIDevice.current.orientation.isLandscape == true 
            imageView.widthAnchor.constraint(equalToConstant: 200).isActive = false
            imageView.heightAnchor.constraint(equalToConstant: 200).isActive = false
            imageView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
            imageView.topAnchor.constraint(equalTo: view.topAnchor, constant: 100).isActive = true
            imageView.widthAnchor.constraint(equalToConstant: 100).isActive = true
            imageView.heightAnchor.constraint(equalToConstant: 100).isActive = true
            print("changed to landscape")

         else 
            imageView.widthAnchor.constraint(equalToConstant: 100).isActive = false
            imageView.heightAnchor.constraint(equalToConstant: 100).isActive = false
            imageView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
            imageView.topAnchor.constraint(equalTo: view.topAnchor, constant: 100).isActive = true
            imageView.widthAnchor.constraint(equalToConstant: 200).isActive = true
            imageView.heightAnchor.constraint(equalToConstant: 200).isActive = true
            print("changed to portrait")
        

    

这是错误。

[LayoutConstraints] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. 
    Try this: 
        (1) look at each constraint and try to figure out which you don't expect; 
        (2) find the code that added the unwanted constraint or constraints and fix it. 
(
    "<NSLayoutConstraint:0x6000031923a0 UIImageView:0x7fbf72508060.width == 200   (active)>",
    "<NSLayoutConstraint:0x6000031e4730 UIImageView:0x7fbf72508060.width == 100   (active)>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x6000031923a0 UIImageView:0x7fbf72508060.width == 200   (active)>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKitCore/UIView.h> may also be helpful.

【参考方案1】：

请注意，constraint 方法会为您的图像视图创建 new。

当您第一次在viewDidLoad 中调用setupLayout 时，会将200 的宽度和高度约束添加到图像视图中。然后旋转设备以更改为横向。 setupLayout 再次被调用。这次它添加了 100 的宽度和高度约束，但它不会停用您之前添加的常量 200 的约束。这样做：

imageView.widthAnchor.constraint(equalToConstant: 200).isActive = false

创建一个新的非活动约束，不停用一个旧约束。

你应该做的是将宽度和高度约束存储为ViewController的属性：

var widthConstraint: NSLayoutConstraint!
var heightConstraint: NSLayoutConstraint!

在setupLayout 中，分配给这些属性：

private func setupLayout(imageView: UIImageView)
    widthConstraint = imageView.widthAnchor.constraint(equalToConstant: 200)
    heightConstraint = imageView.heightAnchor.constraint(equalToConstant: 200)
    widthConstraint.isActive = true
    heightConstraint.isActive = true
    imageView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
    imageView.topAnchor.constraint(equalTo: view.topAnchor, constant: 100).isActive = true

而且您应该只调用一次setupLayout。

然后您创建另一个名为 updateConstraints 的方法来更新 widthConstraint 和 heightConstraint 的常量：

private func updateConstraints() 
    if UIDevice.current.orientation.isLandscape 
        heightConstraint.constant = 100
        widthConstraint.constant = 100
     else 
        heightConstraint.constant = 200
        widthConstraint.constant = 200
    

用viewWillTransitionToSize 代替setupLayout 调用它。

【讨论】：

谢谢！我想应该是这样的！顺便说一句，您的 setupLayout() 中缺少这两行。 `widthConstraintImage.isActive = true heightConstraintImage.isActive = true`