符合 Equatable for Diffing 的协议

Question

我这里有一个小聊天应用。

我可以有两种类型的消息： - 文本 - 视频

我在解码 JSON 时使用多态性，如下所示：

import Foundation

enum MessageType: Int, Decodable {
    case text
    case video
}

protocol Message: Decodable {
    static var type: MessageType { get }
    var id: String { get }
    var user: User { get}
    var timestamp: String { get }
}

struct TextMessage: Message {
    static var type: MessageType = .text
    var id: String
    var user: User
    var timestamp: String
    let text: String
}

struct VideoMessage: Message {
    static var type: MessageType = .video
    var id: String
    var user: User
    var timestamp: String
    let text: String
    let link: String
    let poster: String
}

enum MessageWrapper: Decodable {

    enum CodingKeys: String, CodingKey {
        case type
    }

    case text(TextMessage)
    case video(VideoMessage)

    var item: Message {
        switch self {
        case .text(let item): return item
        case .video(let item): return item
        }
    }

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        let type = try values.decode(Int.self, forKey: .type)
        switch type {
        case MessageType.text.rawValue: self = .text(try TextMessage(from: decoder))
        case MessageType.video.rawValue: self = .video(try VideoMessage(from: decoder))
        default:
            throw DecodingError.dataCorruptedError(forKey: .type,
                                                   in: values,
                                                   debugDescription: "Invalid type")
        }
    }
}

我也在像这样使用 MVVM 方法：

struct ChatViewModel {

    enum ViewModelType {
        case loading
        case text(TextMessageViewModel)
        case video(VideoMessageViewModel)
        case failure(ErrorViewModel)
    }

    enum State {
        case initialized
        case loading
        case loaded([Message])
        case failed(Error)
    }

    let state: State
    let viewModels: [ViewModelType]

    init(with state: State) {
        self.state = state
        switch state {
        case .initialized:
            viewModels = []
        case .loading:
            viewModels = [
                .loading,
            ]
        ......
    }
}

为了能够使用像 Differ 这样的 Diffing 库，ChatViewModel 应该符合 Equatable 协议。

extension ChatViewModel: Equatable {
    static func == (lhs: ChatViewModel, rhs: ChatViewModel) -> Bool {
        return lhs.state == rhs.state
    }
}

extension ChatViewModel.State: Equatable {
    static func == (lhs: ChatViewModel.State, rhs: ChatViewModel.State) -> Bool {
        switch (lhs, rhs) {
            case (.initialized, .initialized): return true
            case (.loading, .loading): return true
            case let (.loaded(l), .loaded(r)): return l == r
            case let (.failed(l), .failed(r)): return l.localizedDescription == r.localizedDescription
            default: return false
        }
    }
}

这里的问题是 case let (.loaded(l), .loaded(r)): return l == r、Message 作为一个协议，不符合 Equatable。

使其符合 Equatable 就像

protocol Message: Decodable, Equatable {
    static var type: MessageType { get }
    var id: String { get }
    var user: User { get}
    var timestamp: String { get }
}

为 MessageWrapper:

产生错误 Protocol 'Message' can only be used as a generic constraint because it has Self or associated type requirements

enum MessageWrapper: Decodable {

    ...

    var item: Message { // Protocol 'Message' can only be used as a generic constraint because it has Self or associated type requirements
        switch self {
        case .text(let item): return item
        case .video(let item): return item
        }
    }

    ...
}

有什么想法或建议可以用干净的方法来解决这个问题吗？我看到一些关于 Type Erasure 的 post 但经过一些测试后我不确定它是否真的解决了问题。

Answer 1

您不必遵守 Equatable 即可使用 == 运算符。你可以自己定义一个这样的运算符，而不符合 Equatable.

为方便起见，我假设 TextMessage 和 VideoMessage 已经符合 Equatable。

首先写一个比较Messages的方法：

func messageEqual(m1: Message, m2: Message) -> Bool {
    if let textMessage1 = m1 as? TextMessage, let textMessage2 = m2 as? TextMessage {
        return textMessage1 == textMessage2
    }
    if let videoMessage1 = m1 as? VideoMessage, let videoMessage2 = m2 as? VideoMessage {
        return videoMessage1 == videoMessage2
    }
    return false
}

然后 [Message] 的 == 运算符：

func ==(messages1: [Message], messages2: [Message]) -> Bool {
    return messages1.count == messages2.count && 
        zip(messages1, messages2).allSatisfy(messageEqual)
}

现在 l == r 应该编译了。

符合 Equatable for Diffing 的协议

Protocol conforming to Equatable for Diffing

ios

swift

equatable