arrays - 具有通用项的 Swift 数组扩展

Question

我正在使用 Swift 3.1 并且有一些通用结构:

struct Section<InfoT: Equatable, ItemsT: Equatable> {
    let info: InfoT?
    let items: [ItemsT]
}

并希望使用自定义方法对这种泛型类型的元素进行数组扩展:

extension Array where Element == Section<Equatable, Equatable> {
    // Just dummy example function:
    func debugDummy() {
        for section in self {
            let info = section.info
            print("section info: \(info).")
            for item in section.items {
                print("item: \(item).")
            }
        }
    }
}

这给我编译错误:

error: using 'Equatable' as a concrete type conforming to protocol 'Equatable' is not supported
extension Array where Element == Section<Equatable, Equatable> {
                                 ^

error: GenericsListPlayground.playground:6:24: error: value of type 'Element' has no member 'info'
            let info = section.info
                       ^~~~~~~ ~~~~

error: GenericsListPlayground.playground:8:25: error: value of type 'Element' has no member 'items'
            for item in section.items {
                        ^~~~~~~ ~~~~~

如何正确声明这样的扩展？我尝试了几种声明此扩展的变体，例如:

extension Array where Element == Section (no arguments)

产生:

error: reference to generic type 'Section' requires arguments in <...>

等等...他们都不想编译。

Answer 1

试试这个:

import Foundation

protocol Section {
    associatedtype InfoT: Equatable
    associatedtype ItemsT: Equatable
    var info: InfoT? { get }
    var items: [ItemsT] { get }
}

extension Array where Element: Section {
    // Just dummy example function:
    func debugDummy() {
        for section in self {
            let info = section.info
            print("section info: \(String(describing: info)).")
            for item in section.items {
                print("item: \(item).")
            }
        }
    }
}