SwiftUI泛型视图中如何正确表达FavouritesArray参数的关联关系？

问题背景 #

先定义了以下协议：

protocol ListableArrayElement: Identifiable, Equatable {
    associatedtype T = Hashable
    
    var id:T { get }
}

protocol Listable: ObservableObject, RandomAccessCollection where Element == ArrayElement {
    associatedtype ArrayElement: ListableArrayElement
    
    var _elements: [ArrayElement] { get set }
    var count: Int { get }
    var isEmpty: Bool { get }
    var endIndex: Int { get }
    var startIndex: Int { get }
    
    subscript(position: Int) -> ArrayElement { get set }

    func index(after i: Int) -> Int
    func append(_ element: ArrayElement)
    func insert(_ newElement: ArrayElement, at i: Int)
    func remove(at index: Int)
    func removeAll()
    func index(of element: ArrayElement) -> Int?
}

protocol FavouritesArray: Listable, Codable where Element: Codable {
        
    // MARK: - PROPERTIES
    var _elements: [Element] { get set }
    var key: String { get set }
    
    init()
    
    init(key: String)
}

FavouritesArray通过关联扩展实现了元素增删、基于key持久化到UserDefaults的功能，Listable协议用于避免核心为元素数组的ViewModel重复编写代码。

现在需要编写泛型SwiftUI视图，通过EnvironmentObject传递遵循FavouritesArray的实例，但编写类型签名时遇到编译错误：

尝试的代码：

struct FavouritesArrayView<Favourites: FavouritesArray, Favourite>: View
   where Favourite == FavouritesArray.Element {
    
    @EnvironmentObject var favourites: Favourites
    @ObservedObject var favourite: Favourite
    // Other properties here

    var body: some View {
        // Layout code here
    }
}

触发错误：Associated type 'Element' can only be used with a concrete type or generic parameter base

解决方案 #

错误原因是FavouritesArray.Element是协议的关联类型，不能直接在泛型约束中使用，应该绑定到泛型参数Favourites的关联类型Element。有两种修正方式：

方式一：使用typealias简化类型声明 #

struct FavouritesArrayView<Favourites: FavouritesArray>: View {
    // 直接关联到泛型参数的Element类型
    typealias Favourite = Favourites.Element
    
    @EnvironmentObject var favourites: Favourites
    @ObservedObject var favourite: Favourite
    // Other properties here

    var body: some View {
        // Layout code here
    }
}

方式二：显式在泛型约束中关联 #

struct FavouritesArrayView<Favourites: FavouritesArray, Favourite>: View
    // 约束Favourite等于具体泛型参数Favourites的Element类型
    where Favourite == Favourites.Element {
    
    @EnvironmentObject var favourites: Favourites
    @ObservedObject var favourite: Favourite
    // Other properties here

    var body: some View {
        // Layout code here
    }
}

关键注意点 #

• 核心修改是将约束中的FavouritesArray.Element替换为Favourites.Element，因为Favourites是遵循协议的具体泛型参数，它的Element关联类型是明确可推导的。
• 如果使用@ObservedObject修饰favourite，需要确保Favourites.Element遵循ObservableObject协议；若元素只是纯数据模型，可改用@State或@Binding管理状态。

内容的提问来源于stack exchange，提问作者Philip Pegden