private var doctorId: Int by Delegates.notNull()

• Kotlin의 by 키워드?

  • Delegate 패턴을 대신 처리할 수 있도록 제공하는 기능 provided by

  • Delegation for property

    import kotlin.reflect.KProperty
    
    class Delegate {
        // for get() method, ref - a reference to the object from 
        // which property is read. prop - property
        operator fun getValue(ref: Any?, prop: KProperty<*>) = "textA"
        // for set() method, 'v' stores the assigned value
        operator fun setValue(ref: Any?, prop: KProperty<*>, v: String) {
            println("value = $v")
        }
    }
    
    object SampleBy {
        var s: String by Delegate() // delegation for property
    }
    

  • Delegation for class

    interface BaseInterface {
        val value: String
        fun f()
    }
    
    class ClassA: BaseInterface {
        override val value = "property from ClassA"
        override fun f() { println("fun from ClassA") }
    }
    
    // The ClassB can implement the BaseInterface by delegating all public 
    // members from the ClassA.
    class ClassB(classA: BaseInterface): BaseInterface by classA
    

  • Delegation for parameter

    // for val properties Map is used; for var MutableMap is used
    class User(mapA: Map<String, Any?>, mapB: MutableMap<String, Any?>) {
        val name: String by mapA
        val age: Int by mapA
        var address: String by mapB
        var id: Long by mapB
    }
    

  • 레퍼런스

    • https://stackoverflow.com/questions/38250022/what-does-by-keyword-do-in-kotlin
    • https://codechacha.com/ko/kotlin-deligation-using-by/#by를-사용하여-작성한-delegate-pattern-예제

lateinit var something: String

if (::something.isInitialized) {
	// do something
}

• Kotlin의 :: 연산자?
  • 리플렉션 기능
    • Class Reference val myClass = MyClass::class
    • Function Reference this::isEmpty
    • Property Reference ::someVal.isInitialized
    • Constructor Reference ::MyClass
  • 레퍼런스
    • https://kotlinlang.org/docs/reflection.html