Extensions trong Swift

extension Double {
    var km: Double { self * 1000.0 }
    var m: Double { self }
    var cm: Double { self / 100.0 }
}
 
let distance = 5.km
print(distance)  // 5000.0

extension Int {
    func squared() -> Int {
        return self * self
    }
    
    mutating func square() {
        self *= self
    }
}
 
print(5.squared())  // 25
 
var num = 4
num.square()
print(num)  // 16

extension CGRect {
    init(center: CGPoint, size: CGSize) {
        let origin = CGPoint(
            x: center.x - size.width / 2,
            y: center.y - size.height / 2
        )
        self.init(origin: origin, size: size)
    }
}

protocol Describable {
    var description: String { get }
}
 
extension Int: Describable {
    var description: String {
        "Number: \(self)"
    }
}
 
print(42.description)  // Number: 42

extension Array where Element: Numeric {
    var sum: Element {
        reduce(0, +)
    }
}
 
print([1, 2, 3, 4, 5].sum)  // 15

extension Int {
    enum Kind {
        case negative, zero, positive
    }
    
    var kind: Kind {
        switch self {
        case 0: return .zero
        case let x where x > 0: return .positive
        default: return .negative
        }
    }
}
 
print(5.kind)   // positive
print((-3).kind)  // negative

protocol Greetable {
    var name: String { get }
}
 
extension Greetable {
    func greet() {
        print("Hello, \(name)!")
    }
}
 
struct Person: Greetable {
    var name: String
}
 
Person(name: "Alice").greet()  // Hello, Alice!

class ViewController: UIViewController {
    // Core functionality
}
 
// MARK: - UITableViewDataSource
extension ViewController: UITableViewDataSource {
    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return 10
    }
    
    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        return UITableViewCell()
    }
}
 
// MARK: - UITableViewDelegate
extension ViewController: UITableViewDelegate {
    func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        // Handle selection
    }
}

// ❌ Không thể thêm stored properties
// extension String {
//     var cached: String  // Error!
// }
 
// ✅ Chỉ có thể thêm computed properties
extension String {
    var reversed: String {
        String(self.reversed())
    }
}