arrays - 不会为集合的元素推断协议(protocol)一致性

Question

我有一组协议(protocol)可以在 UITableView 中显示一个元素:

protocol TableRepresentableRow {
  var title: String { get }
  var subtitle: String { get }
}

extension TableRepresentableRow {
  var title: String {return ""}
  var subtitle: String {return ""}
}

protocol TableRepresentableSection {
  var title: String { get }
  var count: Int { get }
  subscript(index: Int) -> TableRepresentableRow {get}
}

extension TableRepresentableSection {
  var title: String {
    return ""
  }
}

单个元素符合TableRepresentableRow 协议(protocol):

extension ServicesSummary.Service: TableRepresentableRow {
  var title: String {
    return serviceNumber
  }
  var subtitle: String {
    return serviceUserName
  }
}

我希望协议(protocol)一致性也能在 TableRepresentableSection 中推断出来，因为 ServicesSummary.Service 是 TableRepresentableRow 但是，这不会发生:

extension Array: TableRepresentableSection where Element == ServicesSummary.Service {
  // Error: the compiler requires me to add subscript too, while it should be inferred
  subscript(index: Int) -> TableRepresentableRow {
    <#code#>
  }

  var title: String {
    return first?.businessType.rawValue.uppercased() ?? ""
  }
}

为什么会出现此错误？

更新:协议(protocol)组合也不起作用:

Answer 1

要解决此问题，请将元素类型检查更改为仅协议(protocol):

改变:

where Element == ServicesSummary.Service

到:

where Element == TableRepresentableRow

整个扩展:

extension Array: TableRepresentableSection where Element == TableRepresentableRow {
    var title: String {
        return first?.businessType.rawValue.uppercased() ?? ""
    }
}