Swift Array：带IndexPath的下标 - 无法变异

Question

我想为数组数组添加一个扩展来检索IndexPath大小为2的Element ：

let array: [[String]] =  ....
let indexPath = IndexPath(indexes: [0, 0])
let string = array[indexPath]

我得到一个错误无法通过下标下标分配是get-only同时实现以下扩展：

extension Array where Element : Collection, Element.Index == Int {
  subscript(indexPath: IndexPath) -> Element.Iterator.Element {
    get {
      return self[indexPath.section][indexPath.item]
    }
    set {
      self[indexPath.section][indexPath.item] = newValue
    }
  }
}

出现这种错误的原因是什么？ 如何在下标中添加变异选项？

Answer 1

为了改变嵌套数组，你必须要求它

Element : MutableCollection

而不是Element : Collection 。

您还可以定义两个扩展：只读集合的​​只读下标和可变集合的读写下标：

extension Collection where Index == Int, Element : Collection, Element.Index == Int {
    subscript(indexPath: IndexPath) -> Element.Iterator.Element {
        return self[indexPath[0]][indexPath[1]]
    }
}

extension MutableCollection where Index == Int, Element : MutableCollection, Element.Index == Int {
    subscript(indexPath: IndexPath) -> Element.Iterator.Element {
        get {
            return self[indexPath[0]][indexPath[1]]
        }
        set {
            self[indexPath[0]][indexPath[1]] = newValue
        }
    }
}