Extensions

1. Giới thiệu

Extensions thêm functionality vào types có sẵn.

2. Thêm Computed Properties

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

3. Thêm Methods

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

4. Thêm Initializers

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)
    }
}

5. Conform to Protocol

protocol Describable {
    var description: String { get }
}

extension Int: Describable {
    var description: String {
        "Number: \(self)"
    }
}

print(42.description)  // Number: 42

6. Extension với where

extension Array where Element: Numeric {
    var sum: Element {
        reduce(0, +)
    }
}

print([1, 2, 3, 4, 5].sum)  // 15

7. Nested Types

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

8. Extension cho Protocol

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!

9. Tổ chức code với Extension

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
    }
}

10. Giới hạn

// ❌ 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())
    }
}

---

📝 Tóm tắt

• Extension thêm functionality
• Computed properties, methods, initializers
• Conform protocols qua extension
• where cho constrained extensions
• Tổ chức code theo protocols
• Không thể thêm stored properties