Swift：使用泛型将子类类型的对象分配给基类类型的变量会产生错误

Question

我想分配一个 子类 (TapWhatYouHearViewController)，它继承自它的父类 (BaseLectureViewController ) 使用继承的类型 (TapWhatYouHear) 将泛型实现到父类 (BaseLectureViewController 类型的变量) 具有 基类 (Game) 类型的泛型。

Swift 产生以下错误：

Swift:: 错误：无法将类型“TapWhatYouHearViewController”的值分配给类型“BaseLectureViewController？”

我读过，一个解决方案，例如在 Java 中将使用通配符泛型，但这在 Swift 中是不可能的。 还有其他解决方案吗？

---

这是我的设置：

/** GAME TYPES **/

enum GameType {
    case TapWhatYouHear,
    case ChooseTranslation
}

class Game {
    var title: String?
    var type: GameType?
}

class TapWhatYouHear : Game {
    var audio: String?
    var type: GameType = GameType.TapWhatYouHear
}

class ChooseTranslation : Game {
    var video: String?
    var type: GameType = GameType.ChooseTranslation
}

/** VIEW CONTROLLERS **/

class BaseLectureViewController<T: Game> {
   var game: T?
}

class TapWhatYouHearViewController : BaseLectureViewController<TapWhatYouHear> {

}

class ChooseTranslationViewController : BaseLectureViewController<ChooseTranslation> {

}

我想做以下事情：

var game: Game = TapWhatYouHear() // <-- Depending on the user interaction, this can be any sub class of "Game"
var viewController: BaseLectureViewController<Game>

switch game.type {
    case .ChooseTranslation:
        viewController = TapWhatYouHearViewController() // <--Swift:: Error: cannot assign value of type 'TapWhatYouHearViewController' to type 'BaseLectureViewController<Game>?'
    case .TapWhatYouHear:
        viewController = ChooseTranslationViewController()
}

viewController.game = TapWhatYouHear()

---

我要做的是在运行时确定BaseLectureViewController 的哪个子类应该分配给viewController 变量。为了节省代码，我想将Game 类型的游戏分配给BaseLectureViewController 的game 属性。

Answer 1

将变量声明改为

var viewController: BaseLectureViewController<TapWhatYouHear>

或者跳过泛型并将你的超类声明为

class BaseLectureViewController {
    var game: Game?
}

Answer 2

如果您只想为game 属性分配Game 的特定子类的实例，那么您可以使用这样的东西，省略泛型

class Game {
    var title: String?

    required init() {}
}

class TapWhatYouHear : Game {
    var audio: String?
}

class ChooseTranslation : Game {
    var video: String?
}

/** VIEW CONTROLLERS **/

class BaseLectureViewController : UIViewController {
    var game: Game

    init(gameType: Game.Type)
    {
        game = gameType.init()
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

class TapWhatYouHearViewController : BaseLectureViewController{
    init()
    {
        super.init(gameType: TapWhatYouHear.self)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

class ChooseTranslationViewController : BaseLectureViewController{
    init()
    {
        super.init(gameType: ChooseTranslation.self)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

用法

var viewController : BaseLectureViewController
viewController = TapWhatYouHearViewController()

Answer 3

您想到的通用方法是不可能的。我的建议是使用一个简单的继承解决方案，您可以在具体的 ViewController 中覆盖 getGame()（即 TapWhatYouHearViewController）并将游戏转换为您的特定类型。

import UIKit

enum GameType {
    case TapWhatYouHear
    case ChooseTranslation
}

class Game {
    var title: String?
    var type: GameType?
}

class TapWhatYouHear : Game {
    var audio: String?

    override init() {
        super.init()
        type = GameType.TapWhatYouHear
    }
}

class ChooseTranslation : Game {
    var video: String?

    override init() {
        super.init()
        type = GameType.ChooseTranslation
    }
}


/** VIEW CONTROLLERS **/



protocol Lecture {
    var game: Game? { get set }
}

class BaseLectureViewController: UIViewController, Lecture {

    var game: Game?
    func getGame() -> Game? {
        fatalError("Must be implemented in subclass")
    }
}

class TapWhatYouHearViewController : BaseLectureViewController {
    override func getGame() -> TapWhatYouHear? {
        return game as? TapWhatYouHear
    }
}

class ChooseTranslationViewController : BaseLectureViewController {
    override func getGame() -> ChooseTranslation? {
        return game as? ChooseTranslation
    }
}

var game: Game = TapWhatYouHear() // <-- Depending on the user interaction, this can be any sub class of "Game"
var viewController: BaseLectureViewController?

if let gameType = game.type {
    switch gameType {
    case GameType.ChooseTranslation:
        viewController = TapWhatYouHearViewController() // <--Swift:: Error: cannot assign value of type 'TapWhatYouHearViewController' to type 'BaseLectureViewController<Game>?'
    case GameType.TapWhatYouHear:
        viewController = ChooseTranslationViewController()
    }

    viewController?.game = TapWhatYouHear()
}