协议类型数组

我已经在 stackoverflow 上检查了有关此问题的所有答案，但仍然不知道如何解决此问题。 我的模型看起来像这样

protocol Commandable: Equatable {
    var condition: Condition? {get set}

    func execute() -> SKAction
}

以及实现该协议的 3 个结构体

struct MoveCommand: Commandable {

    var movingVector: CGVector!

    //MARK: - Commandable

    var condition: Condition?
    func execute() -> SKAction {
       ...
    }
}

extension MoveCommand {
    // MARK:- Equatable

    static func ==(lhs: MoveCommand, rhs: MoveCommand) -> Bool {
        return lhs.movingVector == rhs.movingVector && lhs.condition == rhs.condition
    }
}

struct RotateCommand: Commandable {
    var side: RotationSide!

    // MARK: - Commandable

    var condition: Condition?
    func execute() -> SKAction {
        ...
    }
}

extension RotateCommand {
    // MARK: - Equatable
    static func ==(lhs: RotateCommand, rhs: RotateCommand) -> Bool {
        return lhs.side == rhs.side && lhs.condition == rhs.condition
    }
}

当我尝试创建具有 [Commandable] 数组的第三个结构时，问题就开始了：

struct FunctionCommand: Commandable {
    var commands = [Commandable]()

编译器输出：Protocol 'Commandable' can only be used as a generic constraint because it has Self or associated type requirements。然后我用这种方式重写了我的结构：

struct FunctionCommand<T : Equatable>: Commandable {
    var commands = [T]()

我解决了这个问题，但又出现了新的问题。现在我无法创建FunctionCommand使用“旋转”和“移动”命令的实例，仅使用其中之一的实例 :( :

let f = FunctionCommand(commands: [MoveCommand(movingVector: .zero, condition: nil), 
RotateCommand(side: .left, condition: nil)], condition: nil)

任何帮助，将不胜感激。

Update：那篇文章帮助我弄清楚了 -https://krakendev.io/blog/generic-protocols-and-their-shortcomings https://krakendev.io/blog/generic-protocols-and-their-shortcomings

您需要做的是使用类型擦除，就像AnyHashable在 Swift 标准库中。

你不能这样做：

var a: [Hashable] = [5, "Yo"]
// error: protocol 'Hashable' can only be used as a generic constraint because it has Self or associated type requirements

你要做的就是使用类型擦除类型AnyHashable:

var a: [AnyHashable] = [AnyHashable(5), AnyHashable("Yo")]
a[0].hashValue // => shows 5 in a playground

所以你的解决方案是首先将协议分成更小的部分并推广Equatable to Hashable（重复使用AnyHashable)

protocol Conditionable {
    var condition: Condition? { get set }
}

protocol Executable {
    func execute() -> SKAction
}

protocol Commandable: Hashable, Executable, Conditionable {}

然后创建一个AnyCommandable结构体，像这样：

struct AnyCommandable: Commandable, Equatable {
    var exeBase: Executable
    var condBase: Conditionable
    var eqBase: AnyHashable

    init<T: Commandable>(_ commandable: T) where T : Equatable {
        self.condBase = commandable
        self.exeBase = commandable
        self.eqBase = AnyHashable(commandable)
    }

    var condition: Condition? {
        get {
            return condBase.condition
        }
        set {
            condBase.condition = condition
        }
    }

    var hashValue: Int {
        return eqBase.hashValue
    }

    func execute() -> SKAction {
        return exeBase.execute()
    }

    public static func ==(lhs: AnyCommandable, rhs: AnyCommandable) -> Bool {
        return lhs.eqBase == rhs.eqBase
    }
}

然后你可以像这样使用它：

var a = FunctionCommand()
a.commands = [AnyCommandable(MoveCommand()), AnyCommandable(FunctionCommand())]

并且您可以轻松访问的属性commands， 因为AnyCommandable实施Commandable

a.commands[0].condition

您需要记住现在添加Hashable and Equatable听从您的所有命令。 我使用这些实现进行测试：

struct MoveCommand: Commandable {

    var movingVector: CGVector!

    var condition: Condition?
    func execute() -> SKAction {
        return SKAction()
    }

    var hashValue: Int {
        return Int(movingVector.dx) * Int(movingVector.dy)
    }

    public static func ==(lhs: MoveCommand, rhs: MoveCommand) -> Bool {
        return lhs.movingVector == rhs.movingVector
    }
}

struct FunctionCommand: Commandable {
    var commands = [AnyCommandable]()

    var condition: Condition?

    func execute() -> SKAction {
        return SKAction.group(commands.map { $0.execute() })
    }

    var hashValue: Int {
        return commands.count
    }

    public static func ==(lhs: FunctionCommand, rhs: FunctionCommand) -> Bool {
        return lhs.commands == rhs.commands
    }
}