枚举关联值令人困惑答案

【问题标题】：Enum associated value confusing枚举关联值令人困惑
【发布时间】：2018-11-09 15:06:06
【问题描述】：

当我尝试使用 func obj func 时，我得到了错误：

无法使用类型为“（消息：（QueueAddable））”的参数列表调用“obj”

我对 Swift 类型感到困惑。 Obj func 用于获取具体的解码类型。

protocol QueueAddable: Encodable {
    var playlistsCollection:String? { get }
    var playlists: [String]? { get }
}

struct Playlist: QueueAddable {
    var playlistsCollection:String? {
        return "id"
    }
    var playlists: [String]? {
        return ["id", "id2"]
    }
    private enum CodingKeys:String,CodingKey {
        case playlistsCollection
        case playlists
    }

    public func encode(to encoder: Encoder) throws {
        var values = encoder.container(keyedBy: CodingKeys.self)
        try values.encode(playlistsCollection, forKey: Playlist.CodingKeys.playlistsCollection)
        try values.encode(playlists, forKey: .playlists)
    }
}

func obj<Q>(message: Q) where Q: QueueAddable {
    let encoder = JSONEncoder()
    let data = try! encoder.encode(message)
}

enum SomeEnum {
    case playlist(QueueAddable)
    func doSome() throws -> Data {
        switch self {
        case .playlist(let queueAddable):
            let encoder = JSONEncoder()

            // Error on the following line:
            obj(message: queueAddable)
            return Data()
        }
    }
}

let playlist = Playlist()
let data = try SomeEnum.playlist(playlist).doSome()

【问题讨论】：

obj 期待某种实现QueueAddable 的类型，但您发送的值是“类型”QueueAddable。协议不会自己实现，因此不能用message: QueueAddable 调用obj。

标签： swift enums associated-value

【解决方案1】：

我认为问题在于函数需要类型而不是协议。如果您使用实现协议的类型使枚举成为泛型，它将起作用。

像这样更改枚举的前两行：

enum SomeEnum<Q : QueueAddable> {
case playlist(Q)

我在以下操场上进行了测试：

import Foundation

protocol QueueAddable: Encodable {
    var playlistsCollection:String? { get }
    var playlists: [String]? { get }
}

struct Playlist: QueueAddable {
    var playlistsCollection:String? {
        return "id"
    }
    var playlists: [String]? {
        return ["id", "id2"]
    }
    private enum CodingKeys:String,CodingKey {
        case playlistsCollection
        case playlists
    }

    public func encode(to encoder: Encoder) throws {
        var values = encoder.container(keyedBy: CodingKeys.self)
        try values.encode(playlistsCollection, forKey:      Playlist.CodingKeys.playlistsCollection)
        try values.encode(playlists, forKey: .playlists)
    }
}

func obj<Q>(message: Q) where Q: QueueAddable {
    let encoder = JSONEncoder()
    let data = try! encoder.encode(message)
}

enum SomeEnum<Q : QueueAddable> {
    case playlist(Q)
    func doSome() throws -> Data {
        switch self {
        case .playlist(let queueAddable):
            let encoder = JSONEncoder()

            // No longer error on the following line:
            obj(message: queueAddable)
            return Data()
        }
    }
}

let playlist = Playlist()
let data = try SomeEnum.playlist(playlist).doSome()

希望对你有帮助！

【讨论】：