Swift 完成处理程序中的泛型

Question

我正在尝试重构 dat 获取函数以使其适用于多种 Decodable 结构类型。

    func fetchData<T: Decodable>(_ fetchRequest: FetchRequestType, completion: @escaping ((Result<T, Error>) -> Void)) {
    guard !isFetching else { return }
    isFetching = true
    
    guard let url = getURL(fetchRequest) else { assertionFailure("Could not compose URL"); return }
    let urlRequest = URLRequest(url: url)
    
    self.session.dataTask(with: urlRequest) { [unowned self] (data, response, error) in
        guard let response = response as? HTTPURLResponse,
            response.statusCode == 200 else {
                self.isFetching = false
                completion(.failure(NSError()))
                return
        }

        guard let data = data else { assertionFailure("No data"); return }
        
        if let jsonData = try? JSONDecoder().decode(T.self, from: data) {
            self.isFetching = false
            completion(.success(jsonData))
        } else {
            assertionFailure("Could not decode JSON data"); return
        }

        
    }.resume()
    
}

但是当我使用一种 Decodable 类型从控制器调用 func 时，出现编译错误

Generic parameter 'T' could not be inferred

        networkClient.fetchData(.accountsSearch(searchLogin: text, pageNumber: 1)) { [unowned self] result in
        switch result {
            case .success(let dataJSON):
                let accountsListJSON = dataJSON as! AccountsListJSON
                let fetchedAccounts = accountsListJSON.items
                    .map({ AccountGeneral(login: [=12=].login, id: [=12=].id, avatarURLString: [=12=].avatarURL, type: [=12=].type) })
                self.accounts = fetchedAccounts
            
            case .failure(_):
                assertionFailure("Fetching error!")
            
        }
    }

请帮我找出发生了什么并解决问题。

Answer 1

如果该方法没有可以指定静态类型的 return 类型，您必须添加一个参数

func fetchData<T: Decodable>(_ fetchRequest: FetchRequestType, type: T.Type, completion: @escaping (Result<T, Error>) -> Void) { ...

并调用它

networkClient.fetchData(.accountsSearch(searchLogin: text, pageNumber: 1), type: AccountsListJSON.self) { [unowned self] result in

并删除沮丧 as! AccountsListJSON

Answer 2

通常可以通过提供 result 类型来帮助编译器推断 T 类型，当您像这样调用 fetchData(_:completion:) 函数时：

networkClient.fetchData(
    .accountsSearch(searchLogin: text, pageNumber: 1)
) { [unowned self] (result: Result<AccountsListJSON, Error>) in
    ...
}

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