Class và Object

1. Giới thiệu

Class là reference type, hỗ trợ kế thừa và deinitializers.

2. Khai báo Class

class Person {
    var name: String
    var age: Int

    init(name: String, age: Int) {
        self.name = name
        self.age = age
    }

    func introduce() {
        print("Tôi là \(name), \(age) tuổi")
    }
}

let person = Person(name: "Alice", age: 25)
person.introduce()

3. Reference Type Semantics

class Point {
    var x: Int
    var y: Int

    init(x: Int, y: Int) {
        self.x = x
        self.y = y
    }
}

let point1 = Point(x: 10, y: 20)
let point2 = point1  // Same reference

point2.x = 100

print(point1.x)  // 100 (changed!)
print(point2.x)  // 100

4. Deinitializer

class FileHandle {
    let filename: String

    init(filename: String) {
        self.filename = filename
        print("Opening \(filename)")
    }

    deinit {
        print("Closing \(filename)")
    }
}

var handle: FileHandle? = FileHandle(filename: "test.txt")
handle = nil  // Prints: Closing test.txt

5. Computed Properties

class Circle {
    var radius: Double

    init(radius: Double) {
        self.radius = radius
    }

    var area: Double {
        get { .pi * radius * radius }
    }

    var diameter: Double {
        get { radius * 2 }
        set { radius = newValue / 2 }
    }
}

6. Property Observers

class StepCounter {
    var steps: Int = 0 {
        willSet {
            print("About to set to \(newValue)")
        }
        didSet {
            print("Changed from \(oldValue) to \(steps)")
        }
    }
}

let counter = StepCounter()
counter.steps = 100

7. Lazy Properties

class DataLoader {
    lazy var data: [String] = {
        print("Loading data...")
        return ["a", "b", "c"]
    }()
}

let loader = DataLoader()
// Data chưa được load
print(loader.data)  // Now loading...

8. Static vs Class Members

class Parent {
    static var staticProp = "Static"
    class var classProp: String { "Class" }

    static func staticMethod() {}
    class func classMethod() {}  // Can be overridden
}

class Child: Parent {
    override class var classProp: String { "Overridden" }
    override class func classMethod() {}
    // static không thể override
}

9. Identity Operators

let a = Person(name: "Alice", age: 25)
let b = a
let c = Person(name: "Alice", age: 25)

print(a === b)  // true (same instance)
print(a === c)  // false (different instances)
print(a !== c)  // true

10. Ví dụ thực tế

class ViewController: UIViewController {
    private let viewModel: ViewModel

    lazy var tableView: UITableView = {
        let tv = UITableView()
        tv.delegate = self
        tv.dataSource = self
        return tv
    }()

    init(viewModel: ViewModel) {
        self.viewModel = viewModel
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) not implemented")
    }

    deinit {
        print("ViewController deallocated")
    }
}

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📝 Tóm tắt

• Class là reference type
• Bắt buộc có initializer
• deinit cho cleanup
• === và !== so sánh identity
• lazy cho delayed initialization
• class keyword cho overridable statics