Question

我想做类似的事情：

public enum LayoutEdge
{
    case top
    case right
    ...
}

func anchorForLayoutEdge(_ edge : LayoutEdge) -> NSLayoutAnchor {
    switch edge
    {
    case .top:      return topAnchor
    case .right:    return rightAnchor
    ... 
    }
}

public func constrain_edge(_ edge : LayoutEdge,
                           toEdge : LayoutEdge,
                           view : UIView) -> NSLayoutConstraint{
    let a1 = anchorForLayoutEdge(edge)
    let a2 = anchorForLayoutEdge(toEdge)
    return a1.constraint(equalTo: a2))
}

但是这不能编译。它在anchorForLayoutEdge中失败了。 Xcode建议将返回类型更改为NSLayoutAnchor，这似乎是错误的。我如何才能使其返回正确的NSLayoutXAxisAnchor或NSLayoutYAxisAnchor，具体取决于指定的边缘？

Answer 1

Swift需要能够在编译时确定类型，但是你尝试返回 NSLayoutAnchor<NSLayoutXAxisAnchor>或NSLayoutAnchor<NSLayoutYAxisAnchor>个对象取决于传递的edge参数。

你可以做的是将你的边缘分成与x轴和y轴相关的边缘：

extension UIView
{
    public enum XLayoutEdge {
        case right
        // ...
    }

    public enum YLayoutEdge {
        case top
        // ...
    }

    func anchor(for edge: XLayoutEdge) -> NSLayoutAnchor<NSLayoutXAxisAnchor> {
        switch edge
        {
        case .right: return rightAnchor
        // ...
        }
    }

    func anchor(for edge: YLayoutEdge) -> NSLayoutAnchor<NSLayoutYAxisAnchor> {
        switch edge
        {
        case .top: return topAnchor
        // ...
        }
    }

    public func constrain(edge edge1: XLayoutEdge, to edge2: XLayoutEdge, of view: UIView) -> NSLayoutConstraint {
        return anchor(for: edge1).constraint(equalTo: view.anchor(for: edge2))
    }

    public func constrain(edge edge1: YLayoutEdge, to edge2: YLayoutEdge, of view: UIView) -> NSLayoutConstraint {
        return anchor(for: edge1).constraint(equalTo: view.anchor(for: edge2))
    }

    func useEdges(view: UIView)
    {
        _ = constrain(edge: .right, to: .right, of: view)
        _ = constrain(edge: .top, to: .top, of: view)
    }
}

它会变得更糟，因为你也必须考虑NSLayoutDimension。你可以玩仿制药但你最终可能会以某种方式复制苹果已经为你准备的内容：）。

这就是我认为你在这里反对系统的原因。退后一步，为什么不直接使用锚点？

extension UIView
{
    func useAnchors(view: UIView)
    {
        _ = rightAnchor.constraint(equalTo: view.rightAnchor)
        _ = topAnchor.constraint(equalTo: view.bottomAnchor)
    }
}

如果您想编写自己的便利功能，可以这样做：

extension UIView
{
    public func constrain<T>(_ anchor1: NSLayoutAnchor<T>, to anchor2: NSLayoutAnchor<T>) -> NSLayoutConstraint {
        return anchor1.constraint(equalTo: anchor2)
    }

    func useYourOwnFunctions(view: UIView)
    {
        _ = constrain(rightAnchor, to: view.rightAnchor)
        _ = constrain(topAnchor, to: view.bottomAnchor)
    }
}

如何从交换机案例中获取NSLayoutAnchor？

1 个答案: