具有關聯值的枚舉的多個鍵路徑的動態成員查找

Question

當存在多個鍵路徑 function 時，在具有關聯值的枚舉上實現動態成員查找會導致構建錯誤。

我有一個包含多個關聯值的枚舉，如下所述：

@dynamicMemberLookup
enum Media {
    case movie(Movie)
    case book(Book)
    
    subscript<T>(dynamicMember keyPath: KeyPath<Entertainment, T>) -> T {
        switch self {
            case .movie(let movie):
                return movie[keyPath: keyPath]
            case .book(let book):
                return book[keyPath: keyPath]
        }
    }
}

結構Movie和Book繼承協議Filmed和Written ，它們本身繼承自更通用的協議Entertainment ：

protocol Entertainment {
    var isbn: Int { get }
    var title: String { get }
    var sales: Int { get set }
}

protocol Written: Entertainment {
    var author: String { get }
}

protocol Filmed: Entertainment {
    var actors: [String] { get set }
}

struct Movie: Filmed {
    var isbn: Int
    var title: String
    var sales: Int
    var actors: [String]
}

struct Book: Written {
    var isbn: Int
    var title: String
    var sales: Int
    var author: String
}

到目前為止，這可行，並允許我通過下標 keypath 查找從頂級Media object 訪問在Entertainment中定義的變量，例如isbn和title 。

let media: Media = .movie(Movie(isbn: 1011, title: "Top Gun", sales: 1000, actors: ["Tom Cruise", "Miles Teller"]))

print(media.title)
print(media.sales)

現在，當我嘗試在下面的Media中添加下標 function 以便我可以選擇訪問符合Filmed或Written的變量時，我收到上述打印語句的錯誤，說明Ambiguous use of 'subscript(dynamicMember:)'

subscript<T>(dynamicMember keyPath: KeyPath<Filmed, T>) -> T? {
    switch self {
        case .movie(let movie):
            return movie[keyPath: keyPath]
        default:
            return nil
    }
}

在此示例中，構建錯誤如下：

let media: Media = .movie(Movie(isbn: 1011, title: "Top Gun", sales: 1000, actors: ["Tom Cruise", "Miles Teller"]))

print(media.title) // BUILD ERROR: Ambiguous use of 'subscript(dynamicMember:)'
print(media.sales) // BUILD ERROR: Ambiguous use of 'subscript(dynamicMember:)'
print(media.actors) // This doesn't throw an error and successfully logs

當我刪除第二個下標 function 時，構建錯誤消失了。

如果我在此處的實施中做錯了什么，請告訴我。

Answer 1

想出了答案：

通過將Filmed和Written更改為不再符合Entertainment ，構建錯誤被刪除。 以下代碼現在可以工作：

protocol Entertainment {
    var isbn: Int { get }
    var title: String { get }
    var sales: Int { get set }
}

protocol Written {
    var author: String { get }
}

protocol Filmed {
    var actors: [String] { get set }
}

struct Movie: Entertainment, Filmed {
    var isbn: Int
    var title: String
    var sales: Int
    var actors: [String]
}

struct Book: Entertainment, Written {
    var isbn: Int
    var title: String
    var sales: Int
    var author: String
}

@dynamicMemberLookup
enum Media {
    case movie(Movie)
    case book(Book)
    
    subscript<T>(dynamicMember keyPath: KeyPath<Entertainment, T>) -> T {
        switch self {
            case .movie(let movie):
                return movie[keyPath: keyPath]
            case .book(let book):
                return book[keyPath: keyPath]
        }
    }

    subscript<T>(dynamicMember keyPath: KeyPath<Filmed, T>) -> T? {
        switch self {
            case .movie(let movie):
                return movie[keyPath: keyPath]
            default:
                return nil
        }
    }
}

這是打印語句的 output：

let media: Media = .movie(Movie(isbn: 1011, title: "Top Gun", sales: 1000, actors: ["Tom Cruise", "Miles Teller"]))

print(media.title) // OUTPUT: "Tom Cruise"
print(media.sales) // OUTPUT: 1000
print(media.actors) // OUTPUT: Optional<[String]>: ["Tom Cruise", "Miles Teller"]