使用泛型将符合协议的对象和泛型存储在类型数组中 [英] Storing objects conforming to a protocol with generics in a typed array

问题描述

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)
}

和一个符合它的结构:

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，该包装类的行为类似于Set，以处理这些属性的有序列表:

And I'm building a wrapper class that behaves like a Set to handle an ordered list of these properties:

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))

但是那当然是行不通的，出现以下错误:

But that of course doesn't work, erroring with:

错误:协议可调整"只能用作一般约束，因为它具有自我"或相关联的类型要求 var AdjustmentSet:[可调] = []

error: protocol 'Adjustable' can only be used as a generic constraint because it has Self or associated type requirements var adjustmentSet: [Adjustable] = []

环顾四周，我起初以为是因为该协议不符合Equatable，但是后来我添加了它，但它仍然不起作用(或者我做错了).

Looking around, I thought at first this was because the protocol doesn't conform to Equatable, but then I added it, and it still doesn't work (or I did it wrong).

此外，我希望能够在此处使用通用名称，以便可以执行以下操作:

Moreover, I would like to be able to use a generic here, so that I can do something like:

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，以便最终我可以做到:

And still be able to store an array of [Adjustable] types, so that eventually I can do:

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

或

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

推荐答案

您不能拥有类型为Adjustable的项目数组，因为Adjustable并不是真正的类型.这是一张蓝图，描述了一组类型，每个T的可能值中的一个.

You can't have an array of items of type Adjustable, because Adjustable isn't really a type. It's a blue print that describes a set of types, one per every possible value of T.

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

To get around this, you need to use a type eraser https://medium.com/dunnhumby-data-science-engineering/swift-associated-type-design-patterns-6c56c5b0a73a

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