在类型化数组中存储符合泛型协议的对象

Question

我有一个协议：

protocol Adjustable: Equatable {
    associatedtype T
    var id: String { get set }
    var value: T { get set }
    init(id: String, value: T)
}

还有一个符合它的结构体：

struct Adjustment: Adjustable {
    static func == (lhs: Adjustment, rhs: Adjustment) -> Bool {
        return lhs.id == rhs.id
    }

    typealias T = CGFloat
    var id: String
    var value: T
}

我正在构建一个包装类，其行为类似于 Set 来处理这些属性的有序列表：

struct AdjustmentSet {
    var adjustmentSet: [Adjustable] = []
    func contains<T: Adjustable>(_ item: T) -> Bool {
        return adjustmentSet.filter({ $0.id == item.id }).first != nil
    }
}

let brightness = Adjustment(id: "Brightness", value: 0)

let set = AdjustmentSet()
print(set.contains(brightness))

但这当然行不通，会报错：

错误：协议 'Adjustable' 只能用作通用约束，因为它具有 Self 或关联的类型要求 var 调整集：[可调整] = []

看了一圈，一开始以为是协议不符合Equatable，后来加了，还是不行（或者是我做错了）。

此外，我希望能够在这里使用泛型，以便我可以执行以下操作：

struct Adjustment<T>: Adjustable {
    static func == (lhs: Adjustment, rhs: Adjustment) -> Bool {
        return lhs.id == rhs.id
    }

    var id: String
    var value: T
}

let brightness = Adjustment<CGFloat>(id: "Brightness", value: 0)

或者：

struct FloatAdjustment: Adjustable {
    static func == (lhs: Adjustment, rhs: Adjustment) -> Bool {
        return lhs.id == rhs.id
    }
    typealias T = CGFloat
    var id: String
    var value: T
}

let brightness = FloatAdjustment(id: "Brightness", value: 0)

并且仍然能够存储[Adjustable] 类型的数组，这样最终我可以做到：

var set = AdjustmentSet()
if set.contains(.brightness) {
    // Do something!
}

或者

var brightness = ...
brightness.value = 1.5
set.append(.brightness)

Answer 1

您不能拥有Adjustable 类型的项目数组，因为Adjustable 并不是真正的类型。这是一个描述一组类型的蓝图，每个可能的 T 值都有一个。

要解决这个问题，您需要使用类型橡皮擦https://medium.com/dunnhumby-data-science-engineering/swift-associated-type-design-patterns-6c56c5b0a73a

Answer 2

在亚历山大的建议下取得了很大的进步；我能够使用一些嵌套类类型来继承基类型擦除类，并使用符合AnyHashable 的通用协议，因此我可以将其与集合一起使用！

// Generic conforming protocol to AnyHashable
protocol AnyAdjustmentProtocol {
    func make() -> AnyHashable
}

protocol AdjustmentProtocol: AnyAdjustmentProtocol {
    associatedtype A
    func make() -> A
}

struct AdjustmentTypes {
    internal class BaseType<T>: Hashable {

        static func == (lhs: AdjustmentTypes.BaseType<T>, rhs: AdjustmentTypes.BaseType<T>) -> Bool {
            return lhs.name == rhs.name
        }

        typealias A = T

        var hashValue: Int { return name.hashValue }

        let name: String
        let defaultValue: T
        let min: T
        let max: T
        var value: T

        init(name: String, defaultValue: T, min: T, max: T) {
            self.name = name
            self.defaultValue = defaultValue
            self.min = min
            self.max = max
            self.value = defaultValue
        }
    }

    class FloatType: BaseType<CGFloat> { }

    class IntType: BaseType<Int> { }
}

struct AnyAdjustmentType<A>: AdjustmentProtocol, Hashable {
    static func == (lhs: AnyAdjustmentType<A>, rhs: AnyAdjustmentType<A>) -> Bool {
        return lhs.hashValue == rhs.hashValue
    }

    private let _make: () -> AnyHashable
    private let hashClosure:() -> Int

    var hashValue: Int {
        return hashClosure()
    }

    init<T: AdjustmentProtocol & Hashable>(_ adjustment: T) where T.A == A {
        _make = adjustment.make
        hashClosure = { return adjustment.hashValue }
    }
    func make() -> AnyHashable {
        return _make()
    }
}

struct Brightness: AdjustmentProtocol, Hashable {
    func make() -> AnyHashable {
        return AdjustmentTypes.FloatType(name: "Brightness", defaultValue: 0, min: 0, max: 1)
    }
}
struct WhiteBalance: AdjustmentProtocol, Hashable {
    func make() -> AnyHashable {
        return AdjustmentTypes.IntType(name: "White Balance", defaultValue: 4000, min: 3000, max: 7000)
    }
}

let brightness = Brightness().make()
let whiteBalance = WhiteBalance().make()

var orderedSet = Set<AnyHashable>()

orderedSet.insert(brightness)
print(type(of: orderedSet))
print(orderedSet.contains(brightness))

for obj in orderedSet {
    if let o = obj as? AdjustmentTypes.FloatType {
        print(o.value)
    }
    if let o = obj as? AdjustmentTypes.IntType {
        print(o.value)
    }
}

打印：

Set<AnyHashable>
true
0.0

特别感谢这篇文章：https://medium.com/@chris_dus/type-erasure-in-swift-84480c807534，其中有一个关于如何实现泛型类型橡皮擦的简单明了的示例。