strategy-pattern.swift

protocol Strategy {
    func execute(a: Int, b: Int)
}

class ConcreteStrategyAdd: Strategy {
    func execute(a: Int, b: Int) {
        print("a + b = \(a + b)")
    }
}

class ConcreteStrategySubtract: Strategy {
    func execute(a: Int, b: Int) {
        print("a - b = \(a - b)")
    }
}

class ConcreteStrategyMultiply: Strategy {
    func execute(a: Int, b: Int) {
        print("a * b = \(a * b)")
    }
}

class Context {
    private var strategy: Strategy?

    func setStrategy(_ strategy: Strategy) {
        self.strategy = strategy
    }

    func executeStrategy(a: Int, b: Int) {
        strategy?.execute(a: a, b: b)
    }
}

let context = Context()
context.setStrategy(ConcreteStrategyAdd())
context.executeStrategy(a: 10, b: 5)            // a + b = 15

context.setStrategy(ConcreteStrategySubtract())
context.executeStrategy(a: 10, b: 5)            // a - b = 5

context.setStrategy(ConcreteStrategyMultiply())
context.executeStrategy(a: 10, b: 5)            // a * b = 50

// Reference: 
// [Dive Into Design Pattern - Alexander Shvets](https://refactoring.guru/design-patterns/book)