Array of protocol type

Question

I have checked all answers about this problem on stackoverflow, but still can not figure out how to fix this. My model looks like this

protocol Commandable: Eq         


        

Answer 1

What you need to do is to use type erasure, much like AnyHashable does in the Swift Standard Library.

You can't do:

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

What you have to do is to use the type-erased type AnyHashable:

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

So your solution would be to first split the protocol in smaller parts and promote Equatable to Hashable (to reuse AnyHashable)

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

protocol Executable {
    func execute() -> SKAction
}

protocol Commandable: Hashable, Executable, Conditionable {}

Then create an AnyCommandable struct, like this:

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

    init(_ 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
    }
}

And then you can use it like this:

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

And you can easily access properties of commands, because AnyCommandable implements Commandable

a.commands[0].condition

You need to remember to now add Hashable and Equatable to all your commands. I used those implementations for testing:

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
    }
}