SwiftUI 列表数据的可识别协议扩展

Question

我正在试验 SwiftUI，并在为我的一个 List 实现数据模型时遇到了一个问题。我的计划是创建一个协议CardProtocol 作为我列表元素的数据协议，然后有一个协议的 CoreData 实现以及一个用于单元测试和 Canvas 使用的虚拟协议。如果您在 SwiftUI List 中使用数据集合，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 }
}

这甚至不会编译，因为Identifiable 协议有 2 个关联类型，如果协议要用于变量定义，则需要指定这些类型。

/// 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 在CoreData 和虚拟实现中本质上是不同的。

我发现唯一合理的解决方案是从协议中删除对Identifiable 的遵从性，然后稍后在视图中使用cardsModel.cards.identified(by: \.id) 重新引入它。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

一个简单的类型橡皮擦解决了这个问题。

final class CardsModel: BindableObject {
    var cards: [AnyCardModel] = [AnyCardModel]()
    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 }
}

class AnyCardModel: CardProtocol {

    var _id: Int
    var _firstName: String?
    var _lastName: String?
    var _email: String?
    var _phone: String?

    init<T: CardProtocol>(card: T) {
        _id = card.id
        _firstName = card.firstName
        _lastName = card.lastName
        _email = card.email
        _phone = card.phone
    }

    var id: Int {
        get { return _id }
        set { _id = newValue}
    }
    var firstName: String? {
        get { return _firstName }
        set { _firstName = newValue}
    }
    var lastName: String? {
        get { return _lastName }
        set { _lastName = newValue}
    }
    var email: String?{
        get { return _email }
        set { _email = newValue}
    }
    var phone: String?{
        get { return _phone }
        set { _phone = newValue}
    }
}

Answer 2

除了通过 .identified(by: \.id) 添加 Identifiable 之外，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的类型，然后允许声明一个 AnyCard 对象数组。该实现非常庞大，对于我的问题可能不值得。但它是这样的：

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 {}