Strategy.swift

//MARK: Strategy protocol

protocol StrategyAny {
    func performAny(input: Any) -> Any?
}

extension StrategyAny where Self: Strategy {
    func performAny(input: Any) -> Any? {
        if let input = input as? Input {
            return perform(input: input)
        }
        return nil
    }
}

protocol Strategy: StrategyAny {
    associatedtype Input
    associatedtype Output
    
    func perform(input: Input) -> Output
}

//MARK: Strategy manager

class StrategyManager {
    static let shared = StrategyManager()
    var strategies = [String: StrategyAny]()
    
    func register<T: Strategy>(strategy: T, for identifier: String) {
        strategies[identifier] = strategy
    }
    
    func performStrategy<T, U>(for identifier: String, with input: T) -> U? {
        guard let strategy = strategies[identifier] else { return nil }
        return strategy.performAny(input: input) as? U
    }
}

//MARK: Demo

struct FooStrategy: Strategy {
    typealias Input = Double
    typealias Output = String
    
    func perform(input: Double) -> String {
        return "\(input * 10)"
    }
}

StrategyManager.shared.register(strategy: FooStrategy(), for: "foo")
let result: String? = StrategyManager.shared.performStrategy(for: "foo", with: 20.0)
print("\(result)")