swift - 如何从开关盒中获取 NSLayoutAnchor？

Question

我想做这样的事情:

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))
}

但这不编译。它在 anchorForLayoutEdge 中失败。 Xcode 建议将返回类型更改为 NSLayoutAnchor，这似乎是错误的。我怎样才能让它根据指定的边缘返回正确的 NSLayoutXAxisAnchor 或 NSLayoutYAxisAnchor？

Answer 1

Swift 需要能够在编译时确定类型，但您尝试返回 NSLayoutAnchor<NSLayoutXAxisAnchor>或 NSLayoutAnchor<NSLayoutYAxisAnchor>对象取决于通过了edge参数。

您可以做的是将您的边拆分为与 x 轴和 y 轴相关的边:

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)
    }
}

情况会变得更糟，因为您必须考虑 NSLayoutDimension ， 也。你可以玩泛型但你最终可能会以某种方式复制 Apple 已经为你准备好的东西 :)。

这就是为什么我认为您在这里与系统作对。退一步说，为什么不直接使用 anchor 呢？

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

如果你想编写自己的便利函数，你可以这样做:

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)
    }
}