使用 ib action-Swift 删除尾随约束

Question

我创建了一个视图控制器，顶部有一个分段控制器。当您点击分段控制器时，它只是充当一个按钮，并通过调用将其更改为相应尺寸的函数来更改控制器内部的 imageView 是纵向模式还是横向模式。

我的问题是我改变它的方式是我只是向 imageView 添加了约束，但是当更改为横向模式时，尾随约束并没有被删除。

而且，这是将 imageView 更改为纵向的代码（imageView 已经具有顶部和底部约束）：

func portraitContraints() {
    
    NSLayoutConstraint.activate ([
    // the trailing contraint
    imageView.trailingAnchor.constraint(equalTo: viewContainer.safeAreaLayoutGuide.trailingAnchor, constant: -75), // this is the contraints that doesn't get removed
    // the leading contraints
    imageView.leadingAnchor.constraint(equalTo: viewContainer.safeAreaLayoutGuide.leadingAnchor, constant: 75
    ])
   
    // the aspect ratio contraints
    imageView.addConstraint(NSLayoutConstraint(item: self.imageView as Any,attribute: .height,relatedBy: .equal,toItem: self.imageView,attribute: .width,multiplier: (4.0 / 3.0),constant: 0))
}

这是将 imageView 更改为横向的代码：

func landscapeContraints() {
    
    NSLayoutConstraint.activate([
    // the trailing contraints
    imageView.trailingAnchor.constraint(equalTo: viewContainer.safeAreaLayoutGuide.trailingAnchor, constant: 0),
    // the leading contraint
    imageView.leadingAnchor.constraint(equalTo: viewContainer.safeAreaLayoutGuide.leadingAnchor, constant: 0)
    ])
    // the aspect ratio contraint
    imageView.addConstraint(NSLayoutConstraint(item: self.imageView as Any,attribute: .height,relatedBy: .equal,toItem: self.imageView,attribute: .width,multiplier: (9.0 / 16.0),constant: 0))
}

这段代码就像一个魅力，除了唯一的问题是纵向模式的尾随约束将保留在视图上（-75 常量约束）。

风景看起来像这样（注意右边的常数是-75）：

人像是这样的：

Answer 1

您可以通过添加具有不同优先级的两个比率约束来做到这一点。更改优先级以使所需的比率“活跃”。

并且，创建前导和尾随约束变量，以便您可以更改它们的常量。

这是一个简单的例子：

class ToggleConstraintsViewController: UIViewController {
    
    let imageView = UIImageView()
    
    var isPortrait: Bool = true
    
    var portraitConstraint: NSLayoutConstraint!
    var landscapeConstraint: NSLayoutConstraint!
    var leadingConstraint: NSLayoutConstraint!
    var trailingConstraint: NSLayoutConstraint!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        imageView.backgroundColor = .blue
        
        imageView.translatesAutoresizingMaskIntoConstraints = false
        
        view.addSubview(imageView)
        
        let g = view.safeAreaLayoutGuide
        
        portraitConstraint = imageView.heightAnchor.constraint(equalTo: imageView.widthAnchor, multiplier: 4.0/3.0)
        portraitConstraint.priority = .defaultHigh
        landscapeConstraint = imageView.heightAnchor.constraint(equalTo: imageView.widthAnchor, multiplier: 9.0/16.0)
        landscapeConstraint.priority = .defaultLow
        
        leadingConstraint = imageView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 75.0)
        trailingConstraint = imageView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -75.0)
        
        NSLayoutConstraint.activate([
            // y-position constraint does not change
            imageView.centerYAnchor.constraint(equalTo: g.centerYAnchor),
            
            // these will have their priorities changed when desired
            portraitConstraint,
            landscapeConstraint,
            
            // these will have their constants changed when desired
            leadingConstraint,
            trailingConstraint,
        ])
        
        let t = UITapGestureRecognizer(target: self, action: #selector(self.toggleOrientation(_:)))
        view.addGestureRecognizer(t)
    }

    @objc func toggleOrientation(_ g: UITapGestureRecognizer) -> Void {
        isPortrait.toggle()
        
        portraitConstraint.priority = isPortrait ? .defaultHigh : .defaultLow
        landscapeConstraint.priority = isPortrait ? .defaultLow : .defaultHigh
        
        leadingConstraint.constant = isPortrait ? 75.0 : 0.0
        trailingConstraint.constant = isPortrait ? -75.0 : 0.0
        
    }
    
}

每次点击视图时，imageView 的宽高比都会发生变化，前导/尾随约束常量也会发生变化。

Answer 2

尝试在添加新约束之前删除旧约束。这是 Apples 关于如何做到这一点的文档：https://developer.apple.com/documentation/uikit/uiview/1622593-removeconstraints