lateinit और lazy initialization

class Repository {
    lateinit var db: String   // assigned later by DI framework
    fun query() = "querying $db"
}
fun main() {
    val r = Repository()
    r.db = "postgres://localhost"
    println(r.query())
}

class Foo {
    lateinit var bar: String
    fun status() = if (::bar.isInitialized) "set to $bar" else "unset"
}
fun main() {
    val f = Foo()
    println(f.status()) // unset
    f.bar = "hi"
    println(f.status())
}

// VALID:
lateinit var name: String
// INVALID:
// lateinit var count: Int       // primitives not allowed
// lateinit val constant: String // val not allowed
// lateinit var maybe: String?   // nullable not allowed
fun main() { name = "ok"; println(name) }

val expensive: String by lazy {
    println("Computing...")
    "result"
}
fun main() {
    println(expensive) // Computing... then result
    println(expensive) // result (cached)
}

class Config {
    val parsed: Map<String, String> by lazy {
        println("parsing config...")
        mapOf("host" to "localhost", "port" to "8080")
    }
}
fun main() {
    val c = Config()
    println("config object created — not parsed yet")
    println(c.parsed["host"]) // now parses
    println(c.parsed["port"]) // cached
}

val safe: String by lazy { "thread-safe" }
val fast: String by lazy(LazyThreadSafetyMode.NONE) { "fast, single-thread" }
fun main() {
    println(safe)
    println(fast)
}

class Demo {
    lateinit var injected: String         // assigned by caller
    val computed: String by lazy { "computed once" }
}
fun main() {
    val d = Demo()
    d.injected = "from outside"
    println(d.injected)
    println(d.computed)
}

class User(val first: String, val last: String) {
    val fullName: String by lazy { "$first $last" }
}
fun main() {
    val u = User("Ada", "Lovelace")
    println(u.fullName)
}

class App {
    lateinit var config: Map<String, String>
    val host: String by lazy { config["host"] ?: "localhost" }
}
fun main() {
    val app = App()
    app.config = mapOf("host" to "api.example.com")
    println(app.host) // api.example.com
}