StaveVIEW的轴变化给布局误差

首先,viewDidLayoutSubviews不是处理oriantion更改的好选项。苹果推荐的方法是viewWillTransitionOnSize。 有关详细信息: What is the "right" way to handle orientation changes in iOS 8?

第一次运行代码时,堆栈视图显示为垂直分布,第一个视图和第二个视图都有以下约束

第一视图->

         Leading =  StackView Leading,
         Trailing =  StackView Trailing,
         Top      = StackView Top,
         height   =   80

第二视图->

            Leading =  StackView Leading    ,
              Top      = FirstView Top,
              Trailing =  StackView Trailing,
              height   =   80

当您将设备从纵向旋转到横向时,堆栈视图以水平分布显示,并使以下约束处于活动状态。

FirstView.Width=StackView.Width*1/3

生成的firstview与上一个约束(stackview leading,stackview trailing)冲突。因为现在有两个宽度约束

1. (firstview.leading=stackview leading&firstview.trailing=stackview trailing)=(firstview width==stackview.width)
2. FirstView.Width=StackView.Width*1/3

为什么stackview不自动删除冲突约束之一?

-->因为这两个约束都具有高优先级(1000)。如果我们将firstviewwidth更改为较低的(999),并删除secondviewwidth约束,则解决方案将起作用

尝试用以下代码替换您的代码

   import UIKit

class ViewController: UIViewController {

    enum DisplayMode {
        case regular
        case compact
    }

    private let stackView = UIStackView(frame: .zero)
    private let firstView = UIView(frame: .zero)
    private let secondView = UIView(frame: .zero)
    private var firstViewWidth: NSLayoutConstraint?
    private var secondViewWidth: NSLayoutConstraint?

    override func viewDidLoad() {
        super.viewDidLoad()

        stackView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(stackView)
        NSLayoutConstraint.activate([
            stackView.topAnchor.constraint(equalTo: view.topAnchor),
            stackView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            stackView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            ])

        firstView.backgroundColor = .red
        stackView.addArrangedSubview(firstView)
        firstView.heightAnchor.constraint(equalToConstant: 80).isActive = true
        firstViewWidth = firstView.widthAnchor.constraint(equalTo: stackView.widthAnchor, multiplier: 1/3)
        firstViewWidth?.priority = UILayoutPriority(rawValue: 999)
        firstViewWidth?.isActive = true

        secondView.backgroundColor = .black
        stackView.addArrangedSubview(secondView)
        secondView.heightAnchor.constraint(equalToConstant: 80).isActive = true


        if self.view.bounds.width < 400 {
            self.switchMode(.compact)
        } else {
            self.switchMode(.regular)
        }
    }

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()

    }

    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
        super.viewWillTransition(to: size, with: coordinator)


        coordinator.animate(alongsideTransition: { (context) in

        }) { (context) in
            if self.view.bounds.width < 400 {
                self.switchMode(.compact)
            } else {
                self.switchMode(.regular)
            }
        }

    }

}

private extension ViewController {

    func switchMode(_ mode: DisplayMode) {
        switch mode {
        case .regular:
            stackView.axis = .horizontal
            firstViewWidth?.isActive = true

        case .compact:
            stackView.axis = .vertical
            firstViewWidth?.isActive = false
        }
    }
}