尝试从方法返回协议关联类型时出错

Question

我需要使用 unsplash.com 搜索API搜索实体(照片，相册等(。为了达到可重用性，并且不必在每次需要搜索新实体时都创建新函数，我创建了一个协议：

protocol SearchApiResource {
associatedtype ModelType: Decodable
var methodPath: String { get }
var searchTerm: String { get set }
var pageNumber: Int { get set }
var parameters: [String: String] { get }
var url: URL { get }
}

以及符合该协议的结构：

struct SearchPhotoResource: SearchApiResource {
typealias ModelType = Photo
var methodPath = "/search/photos"
var searchTerm = String()
var pageNumber = Int()
let itemsPerPage = 30
let accessKey = "93e0a185df414cc1d0351dc2238627b7e5af3a64bb228244bc925346485f1f44"
var parameters: [String: String] {
var params = [String: String]()
params["query"] = searchTerm
params["page"] = String(pageNumber)
params["per_page"] = String(itemsPerPage)
params["client_id"] = accessKey
return params
}
var url: URL {
var components = URLComponents()
components.scheme = "https"
components.host = "api.unsplash.com"
components.path = methodPath
components.queryItems = parameters.map {URLQueryItem(name: $0, value: $1)}
return components.url!
}
}

现在我想创建一个函数，该函数将接受符合 SearchApiResource 协议的结构或类：

func searchForItem(resource: SearchApiResource, searchTerm: String, pageNumber: Int, completion: @escaping (SearchApiResource.ModelType) -> Void ) {
}

但是我收到一个错误： "关联类型'模型类型'只能与具体类型或泛型参数库一起使用">

如何修复错误，我做错了什么？

Answer 1

只需按照错误所说的操作 - 使用SearchApiResource作为通用参数库。

func searchForItem<T: SearchApiResource>(resource: T, searchTerm: String, pageNumber: Int, completion: @escaping (T.ModelType) -> Void ) {
}