如何将 map 一个可编码的 JSON 结构转换为 Data 以解码响应？

Question

我在编译以下代码时遇到问题 - 它失败并显示“'无法将'CheckUserNameAvailability'类型的值转换为闭包结果类型'JSONDecoder.Input'（又名'Data'）”。

我如何 map 服务器返回的CheckUserNameAvailability JSON 到Data ？

// This is the JSON returned by the server
// {
//    "Username" : "Foo1Bar2",
//    "Available" : true
// }
struct CheckUserNameAvailability: Codable {
    let Username: String
    let Available: Bool
}

enum tvAPI {
    static func checkUserName(username: String) -> AnyPublisher<CheckUserNameAvailability, Error> {
        // ...
    }
}

func testUserName() {
    var cancellable: AnyCancellable?
    let me = "Foo1Bar2"
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .useDefaultKeys
    cancellable = tvAPI.checkUserName(username: me)
        .map { $0 } // compilation fails with “Cannot convert value of type 'CheckUserNameAvailability' to closure result type 'JSONDecoder.Input' (aka 'Data')”
        .decode(type: [CheckUserNameAvailability].self, decoder: JSONDecoder())
        // .print()
        .sink(receiveCompletion: {completion in
            print(completion)
        },
        receiveValue: { availability in
            print("\(availability)")
        })

    RunLoop.main.run(until: Date(timeIntervalSinceNow: 10))

    withExtendedLifetime(cancellable, {})
}

Answer 1

tvAPI.checkUserName(username:)返回具有 output 类型CheckUserNameAvailability的发布者。 发布者上的decode方法要求发布者的 output 与解码器的输入相匹配：

func decode<Item, Coder>(type: Item.Type, decoder: Coder) ->
    Publishers.Decode<Self, Item, Coder> where
      Item : Decodable,
      Coder : TopLevelDecoder,
      Self.Output == Coder.Input

但是， CheckUserNameAvailability与JSONDecoder.Input （又名Data ）不兼容。

您不需要解码数据：

cancellable = tvAPI.checkUserName(username: me)
    // .print()
    .sink(receiveCompletion: { completion in
        print(completion)
    }, receiveValue: { availability in
        print("\(availability)")
    })