TL;DR #

Object A outsources a specific job to Object B, without knowing the details of how it’s done.

Real-Life Example #

Imagine you’re ordering food in a restaurant:

• Waiter (B): Takes your order.
• Chef (A): Actually cooks the food.

The waiter delegates the task of preparing the food to the chef. Similarly, in code, one class can delegate specific responsibilities to another.

Code example #

// Define a protocol that outlines the responsibilities the Chef must handle
protocol KitchenDelegate {
    func prepareFood(order: String) -> String
}

// Chef class (A) - The one who performs the delegated task
class Chef: KitchenDelegate {
    func prepareFood(order: String) -> String {
        return "Chef has prepared your \(order). Enjoy your meal!"
    }
}

// Waiter class (B) - The one who delegates the task
class Waiter {
    var delegate: KitchenDelegate? // The delegate property, optional since it might not always be set
    
    func takeOrder(order: String) {
        print("Waiter: I've taken your order for \(order).")
        
        // Delegate the food preparation to the Chef if available
        if let preparedFood = delegate?.prepareFood(order: order) {
            print("Waiter: Here’s your food. \(preparedFood)")
        } else {
            print("Waiter: Sorry, no chef available to prepare your \(order).")
        }
    }
}

// Example usage
func restaurantScenario() {
    // Create instances
    let chef = Chef()
    let waiter = Waiter()
    
    // Set the chef as the waiter's delegate
    waiter.delegate = chef
    
    // Customer places an order
    waiter.takeOrder(order: "Spaghetti Carbonara")
    
    // Now try without a chef
    let loneWaiter = Waiter() // No delegate assigned
    loneWaiter.takeOrder(order: "Pizza Margherita")
}

// Run the scenario
restaurantScenario()