[英]Storing objects conforming to a protocol with generics in a typed array
我有一個協議:
protocol Adjustable: Equatable {
associatedtype T
var id: String { get set }
var value: T { get set }
init(id: String, value: T)
}
以及符合它的結構:
struct Adjustment: Adjustable {
static func == (lhs: Adjustment, rhs: Adjustment) -> Bool {
return lhs.id == rhs.id
}
typealias T = CGFloat
var id: String
var value: T
}
我正在構建一個包裝器 class ,其行為類似於Set
來處理這些屬性的有序列表:
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))
但這當然行不通,會出錯:
錯誤:協議 'Adjustable' 只能用作通用約束,因為它具有 Self 或關聯的類型要求 var adjustSet: [Adjustable] = []
環顧四周,起初我以為這是因為協議不符合Equatable
,但后來我添加了它,它仍然不起作用(或者我做錯了)。
此外,我希望能夠在這里使用泛型,以便我可以執行以下操作:
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)
並且仍然能夠存儲[Adjustable]
類型的數組,因此最終我可以做到:
var set = AdjustmentSet()
if set.contains(.brightness) {
// Do something!
}
或者
var brightness = ...
brightness.value = 1.5
set.append(.brightness)
您不能具有Adjustable
類型的項目數組,因為Adjustable
實際上不是一種類型。 這是一個藍圖,描述了一組類型,每個可能的T
值一個。
要解決此問題,您需要使用類型橡皮擦https://medium.com/dunnhumby-data-science-engineering/swift-associated-type-design-patterns-6c56c5b0a73a
利用亞歷山大的建議取得了很大的進步; 我能夠使用一些嵌套的類類型來繼承基本類型的擦除類,並使用符合AnyHashable
的通用協議,因此可以將其與集合一起使用!
// 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
特別感謝本文: https : //medium.com/@chris_dus/type-erasure-in-swift-84480c807534 ,其中提供了有關如何實現通用類型擦除器的簡單示例。
使用 Swift 5.7,您將能夠通過在您的協議前加上any
前綴,而不會出現任何編譯器錯誤,因此您的設置變為:
struct AdjustmentSet {
var adjustmentSet: [any Adjustable] = []
func contains(_ item: some Adjustable) -> Bool {
return adjustmentSet.first { $0.id == item.id } != nil
}
}
請注意,自編譯時 swift 無法確定存在類型Adjustable
的大小,因為實現它的類型將具有可變大小,所以您的adjustmentSet
數組中的所有項目都將在堆上分配。
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