Protocols

Protocol is a contract that says what properties/methods a class, struct, or enum must have. Protocol-oriented programming is used heavily in Swift development.

Using protocols lets you write code based on behavior instead of being tied to one concrete type. They're extremely useful for testing, dependency injection, and reusable components.

protocol Payable {
    var amount: Double { get }
    func pay()
}

struct CreditCardPayment: Payable {
    let amount: Double

    func pay() {
        print("Paid $\(amount) with credit card")
    }
}

let payment = CreditCardPayment(amount: 29.99)
payment.pay()

Payable protocol requires an amount property and a pay() method. CreditCardPayment struct follows that contract.

Paid $29.99 with credit card