Static Properties & Methods

1. Giới thiệu

Static properties và methods thuộc về type (class/struct), không phải instance.

2. Static trong Struct

struct AppConfig {
    static let appName = "MyApp"
    static let version = "1.0.0"
    static var isDebugMode = false

    static func printInfo() {
        print("\(appName) v\(version)")
    }
}

// Truy cập qua type, không qua instance
print(AppConfig.appName)     // MyApp
print(AppConfig.version)     // 1.0.0
AppConfig.isDebugMode = true
AppConfig.printInfo()        // MyApp v1.0.0

3. Static trong Class

class Counter {
    static var totalCount = 0
    var instanceCount = 0

    init() {
        Counter.totalCount += 1
        instanceCount = Counter.totalCount
    }

    static func reset() {
        totalCount = 0
    }
}

let c1 = Counter()  // totalCount = 1
let c2 = Counter()  // totalCount = 2
let c3 = Counter()  // totalCount = 3

print(Counter.totalCount)  // 3
print(c1.instanceCount)    // 1
print(c3.instanceCount)    // 3

4. Class vs Static (trong Class)

class Animal {
    // static: không thể override
    static var species = "Animal"

    // class: có thể override
    class var description: String {
        "An animal"
    }

    class func makeSound() {
        print("...")
    }
}

class Dog: Animal {
    // Override class property
    override class var description: String {
        "A dog"
    }

    // Override class method
    override class func makeSound() {
        print("Woof!")
    }
}

print(Animal.description)  // An animal
print(Dog.description)     // A dog
Dog.makeSound()            // Woof!

5. Singleton Pattern

class DatabaseManager {
    static let shared = DatabaseManager()

    private init() {
        // Private init ngăn tạo instance mới
        print("Database initialized")
    }

    func query(_ sql: String) -> [String] {
        print("Executing: \(sql)")
        return []
    }
}

// Sử dụng
let db = DatabaseManager.shared
db.query("SELECT * FROM users")

// DatabaseManager()  // Error: init is private

6. Factory Methods

struct Color {
    var red: Double
    var green: Double
    var blue: Double

    // Static factory methods
    static let white = Color(red: 1, green: 1, blue: 1)
    static let black = Color(red: 0, green: 0, blue: 0)
    static let red = Color(red: 1, green: 0, blue: 0)

    static func gray(brightness: Double) -> Color {
        Color(red: brightness, green: brightness, blue: brightness)
    }

    static func random() -> Color {
        Color(
            red: Double.random(in: 0...1),
            green: Double.random(in: 0...1),
            blue: Double.random(in: 0...1)
        )
    }
}

let bg = Color.white
let text = Color.black
let dimGray = Color.gray(brightness: 0.3)
let randomColor = Color.random()

7. Utility Classes

struct MathUtils {
    static let pi = 3.14159
    static let e = 2.71828

    static func square(_ x: Double) -> Double {
        x * x
    }

    static func cube(_ x: Double) -> Double {
        x * x * x
    }

    static func clamp(_ value: Double, min: Double, max: Double) -> Double {
        Swift.min(Swift.max(value, min), max)
    }
}

print(MathUtils.square(5))            // 25.0
print(MathUtils.clamp(150, min: 0, max: 100))  // 100.0

8. Configuration và Constants

struct APIEndpoints {
    static let baseURL = "https://api.example.com"

    static var users: String { baseURL + "/users" }
    static var posts: String { baseURL + "/posts" }
    static var comments: String { baseURL + "/comments" }

    static func user(id: Int) -> String {
        "\(users)/\(id)"
    }
}

print(APIEndpoints.users)       // https://api.example.com/users
print(APIEndpoints.user(id: 5)) // https://api.example.com/users/5

struct Theme {
    static let primaryColor = Color.blue
    static let secondaryColor = Color.gray
    static let fontSize: CGFloat = 16
    static let cornerRadius: CGFloat = 8
}

9. Trong Enum

enum Direction: String {
    case north, south, east, west

    static let all: [Direction] = [.north, .south, .east, .west]

    static func random() -> Direction {
        all.randomElement()!
    }

    static var defaultDirection: Direction { .north }
}

print(Direction.all)        // [north, south, east, west]
print(Direction.random())   // Random direction

10. Ví dụ thực tế: Analytics

class Analytics {
    static let shared = Analytics()
    private init() {}

    private(set) static var eventCount = 0

    static func track(event: String, properties: [String: Any] = [:]) {
        eventCount += 1
        print("📊 Event #\(eventCount): \(event)")
        for (key, value) in properties {
            print("   \(key): \(value)")
        }
    }

    static func reset() {
        eventCount = 0
    }
}

Analytics.track(event: "app_open")
Analytics.track(event: "button_tap", properties: ["button": "login"])
print("Total events: \(Analytics.eventCount)")

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📝 Tóm tắt

• static - thuộc về type, không thể override
• class - thuộc về type, có thể override (chỉ trong class)
• Truy cập qua TypeName.property hoặc TypeName.method()
• Dùng cho: constants, factory methods, singleton, utilities
• Self (viết hoa) để refer đến type hiện tại