如何从开关柜中获取NSLayoutAnchor？

Question

我想做一些类似的事情：

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。在anchorForLayoutEdge中失败。Xcode建议将返回类型更改为NSLayoutAnchor，这似乎是错误的。如何才能使它根据指定的边返回正确的NSLayoutXAxisAnchor或NSLayoutYAxisAnchor？

Answer 1

Swift需要能够在编译时确定类型，但是您尝试根据传递的edge参数返回NSLayoutAnchor<NSLayoutXAxisAnchor>或NSLayoutAnchor<NSLayoutYAxisAnchor>对象。

你可以做的是将你的边分割成与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，因为你也必须考虑NSLayoutDimension。您可以玩弄泛型，但最终可能会以某种方式复制Apple已经为您准备好的东西:)。

这就是为什么我认为你在这里与系统背道而驰。后退一步，为什么不直接使用锚点呢？

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

如果你想编写你自己的便利函数，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)
    }
}