Object Declaration và Singleton trong Kotlin

object DatabaseConnection {
    private var isConnected = false
    
    fun connect() {
        isConnected = true
        println("Connected to database")
    }
    
    fun disconnect() {
        isConnected = false
        println("Disconnected")
    }
    
    fun status() = if (isConnected) "Connected" else "Disconnected"
}
 
fun main() {
    DatabaseConnection.connect()
    println(DatabaseConnection.status())
}

object Config {
    const val API_URL = "https://api.example.com"
    const val TIMEOUT = 30000
    
    var debug = false
    
    val version: String
        get() = "1.0.0"
}
 
fun main() {
    println(Config.API_URL)
    Config.debug = true
}

interface Logger {
    fun log(message: String)
}
 
object ConsoleLogger : Logger {
    override fun log(message: String) {
        println("[LOG] $message")
    }
}
 
fun main() {
    ConsoleLogger.log("Hello!")
}

open class Animal(val name: String) {
    open fun speak() = println("$name speaks")
}
 
object Parrot : Animal("Polly") {
    override fun speak() = println("$name says: Hello!")
}
 
fun main() {
    Parrot.speak()  // Polly says: Hello!
}

interface ClickListener {
    fun onClick()
}
 
fun main() {
    val listener = object : ClickListener {
        override fun onClick() {
            println("Clicked!")
        }
    }
    
    listener.onClick()
}

interface Drawable {
    fun draw()
}
 
interface Clickable {
    fun click()
}
 
fun main() {
    val button = object : Drawable, Clickable {
        override fun draw() = println("Drawing button")
        override fun click() = println("Button clicked")
    }
    
    button.draw()
    button.click()
}

fun createCounter() = object {
    var count = 0
    
    fun increment() {
        count++
        println("Count: $count")
    }
}
 
fun main() {
    val counter = createCounter()
    counter.increment()  // Count: 1
    counter.increment()  // Count: 2
}

// Object Declaration - singleton, có tên
object Singleton {
    fun doSomething() {}
}
 
// Object Expression - không phải singleton, có thể anonymous
val myObject = object {
    val x = 10
    val y = 20
}

object AppState {
    private val observers = mutableListOf<() -> Unit>()
    private var _count = 0
    
    val count: Int get() = _count
    
    fun increment() {
        _count++
        notifyObservers()
    }
    
    fun addObserver(observer: () -> Unit) {
        observers.add(observer)
    }
    
    private fun notifyObservers() {
        observers.forEach { it() }
    }
}
 
fun main() {
    AppState.addObserver { println("Count changed: ${AppState.count}") }
    AppState.increment()  // Count changed: 1
}