ios - swift 数组 : subscript with IndexPath - unable to mutate

Question

我想为数组的数组添加一个扩展，以检索具有大小为 2 的 IndexPath 的 Element:

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

我在实现以下扩展时遇到错误无法通过下标分配下标是只获取:

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？如何向下标添加突变选项？

Answer 1

为了改变嵌套数组你必须要求

Element : MutableCollection

代替 Element : Collection。

你还可以定义两个扩展名:一个只读的下标用于只读 集合，以及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
        }
    }
}