CODESAMPLE

Visitor - Swift

Visitor Swift

The Visitor pattern allows you to define a new operation without modifying the classes of the objects on which it operates. It’s achieved by moving the operation to a separate “visitor” class that accepts different object types as arguments. This is useful when you have a complex object structure and want to add operations that depend on the specific types within that structure.

This Swift implementation defines a Vehicle protocol with concrete types Car and Bike. The VehicleVisitor protocol declares visit methods for each vehicle type. CarVisitor and BikeVisitor are concrete visitors implementing specific actions (e.g., calculating cost). The client code accepts a visitor and each vehicle, calling the appropriate visit method on the visitor. This adheres to Swift’s protocol-oriented programming style, leveraging protocols for defining the interface and concrete types for implementation.

// Vehicle Protocol
protocol Vehicle {
    func accept(visitor: VehicleVisitor)
}

// Concrete Vehicles
class Car: Vehicle {
    let model: String
    init(model: String) {
        self.model = model
    }
    func accept(visitor: VehicleVisitor) {
        visitor.visit(car: self)
    }
}

class Bike: Vehicle {
    let style: String
    init(style: String) {
        self.style = style
    }
    func accept(visitor: VehicleVisitor) {
        visitor.visit(bike: self)
    }
}

// Visitor Protocol
protocol VehicleVisitor {
    func visit(car: Car)
    func visit(bike: Bike)
}

// Concrete Visitors
class CarCostVisitor: VehicleVisitor {
    var cost: Int = 0
    func visit(car: Car) {
        cost += 100 + (car.model.count * 10)
    }
    func visit(bike: Bike) {
        // Do nothing for bikes in this visitor
    }
}

class BikeCostVisitor: VehicleVisitor {
    var cost: Int = 0
    func visit(car: Car) {
        // Do nothing for cars in this visitor
    }
    func visit(bike: Bike) {
        cost += 50 + (bike.style.count * 5)
    }
}

// Client Code
let car = Car(model: "Tesla Model S")
let bike = Bike(style: "Mountain Bike")

let carCostVisitor = CarCostVisitor()
car.accept(visitor: carCostVisitor)
print("Car cost: \(carCostVisitor.cost)")

let bikeCostVisitor = BikeCostVisitor()
bike.accept(visitor: bikeCostVisitor)
print("Bike cost: \(bikeCostVisitor.cost)")