SwiftUI 列表数据的可识别协议扩展

Question

我正在尝试 SwiftUI，并在为我的一个列表实现数据模型时遇到了问题。我的计划是创建一个协议CardProtocol作为列表元素的数据协议，然后使用该协议的 CoreData 实现以及用于单元测试和 Canvas 使用的虚拟协议。如果您在 SwiftUI 中使用数据集合，List则单个元素需要符合Identifiable协议。

代码如下所示：

import SwiftUI
import Combine


final class CardsModel: BindableObject {
    var cards: [CardProtocol] = []
    let didChange = PassthroughSubject<CardsModel, Never>()
}

protocol CardProtocol: Identifiable {
    var id: Int { get set }
    var firstName: String? { get set }
    var lastName: String? { get set }
    var email: String? { get set }
    var phone: String? { get set }
}

这甚至无法编译，因为Identifiable协议有 2 个关联类型，如果要将协议用于变量定义，则需要指定这两个类型。

/// A type that can be compared for identity equality.
public protocol Identifiable {

    /// A type of unique identifier that can be compared for equality.
    associatedtype ID : Hashable

    /// A unique identifier that can be compared for equality.
    var id: Self.ID { get }

    /// The type of value identified by `id`.
    associatedtype IdentifiedValue = Self

    /// The value identified by `id`.
    ///
    /// By default this returns `self`.
    var identifiedValue: Self.IdentifiedValue { get }
}

确切的错误是error: protocol 'CardProtocol' can only be used as a generic constraint because it has Self or associated type requirements. 现在ID不是问题并且可以修复，但IdentifiedValue它在 CoreData 和虚拟实现中本质上是不同的。

我发现的唯一合理的解决方案是Identifiable从协议中删除对协议的遵从性，并稍后在视图中使用 重新引入它cardsModel.cards.identified(by: \.id)。有没有更好的方法来解决这个问题，让我在协议级别保持可识别的合规性？

Answer 1

除了通过 添加 Identificate 之外.identified(by: \.id)，唯一的解决方案是使用类型擦除模式。这使用 3 个类来装箱和隐藏关联类型，然后允许声明 AnyCard 对象的数组。该实现相当庞大，对于我的问题来说可能不值得。但事情是这样的：

final class CardsModel<IdentifiedValue:CardProtocol>: BindableObject {
    var cards: [AnyCard<IdentifiedValue>] = []
    let didChange = PassthroughSubject<CardsModel, Never>()
}

protocol CardProtocol: Identifiable{
    var id: Int32 { get set }
    var firstName: String? { get set }
    var lastName: String? { get set }
    var email: String? { get set }
    var phone: String? { get set }
}

struct TestCard: CardProtocol {
    var id: Int32
    var firstName: String?
    var lastName: String?
    var email: String?
    var phone: String?
}

extension CardsModel where IdentifiedValue == TestCard {
    convenience init(cards: [TestCard]) {
        self.init()
        self.cards = cards.map({ (card) -> AnyCard<TestCard> in
            return AnyCard(card)
        })
    }
}

private class _AnyCardBase<IdentifiedValue>: CardProtocol {
    init() {
        guard type(of: self) != _AnyCardBase.self else {
            fatalError("_AnyCardBase<Model> instances can not be created; create a subclass instance instead")
        }
    }

    var id: Int32 {
        get { fatalError("Must override") }
        set { fatalError("Must override") }
    }

    var firstName: String? {
        get { fatalError("Must override") }
        set { fatalError("Must override") }
    }

    var lastName: String? {
        get { fatalError("Must override") }
        set { fatalError("Must override") }
    }

    var email: String? {
        get { fatalError("Must override") }
        set { fatalError("Must override") }
    }

    var phone: String? {
        get { fatalError("Must override") }
        set { fatalError("Must override") }
    }
}


private final class _AnyCardBox<Concrete: CardProtocol>: _AnyCardBase<Concrete.IdentifiedValue> {
    var concrete: Concrete

    init(_ concrete: Concrete) {
        self.concrete = concrete
    }

    override var id: Int32 {
        get {
            return concrete.id
        }
        set {
            concrete.id = newValue
        }
    }

    override var firstName: String? {
        get {
            return concrete.firstName
        }
        set {
            concrete.firstName = newValue
        }
    }

    override var lastName: String? {
        get {
            return concrete.lastName
        }
        set {
            concrete.lastName = newValue
        }
    }

    override var email: String? {
        get {
            return concrete.email
        }
        set {
            concrete.email = newValue
        }
    }

    override var phone: String? {
        get {
            return concrete.phone
        }
        set {
            concrete.phone = newValue
        }
    }
}

final class AnyCard<IdentifiedValue>: CardProtocol {
    private let box: _AnyCardBase<IdentifiedValue>

    init<Concrete: CardProtocol>(_ concrete: Concrete) where Concrete.IdentifiedValue == IdentifiedValue {
        box = _AnyCardBox(concrete)
    }

    var id: Int32 {
        get {
            return box.id
        }
        set {
            box.id = newValue
        }
    }

    var firstName: String? {
        get {
            return box.firstName
        }
        set {
            box.firstName = newValue
        }
    }

    var lastName: String? {
        get {
            return box.lastName
        }
        set {
            box.lastName = newValue
        }
    }

    var email: String? {
        get {
            return box.email
        }
        set {
            box.email = newValue
        }
    }

    var phone: String? {
        get {
            return box.phone
        }
        set {
            box.phone = newValue
        }
    }
}

//NSManagedObject extention
extension Card:CardProtocol {}