Understand “by” Delegated Properties in Kotlin

Simple way to understand “by” operator (used for delegated properties) in Kotlin and the reasons to use it.

When I first learned Kotlin, the by operator is an alien to me. What the hell is that? In this article, I'm going to provide a simple example to show the reasons why we want to use this by operator.

Custom Property get() and set()

Let’s say I want to create a custom property get() and set() to print out something when the property value is read and set. I will do something like this.

class PropertyAccessExample {  
    var value: String? = null  
        get() {  
            println("property get value: $field")  
            return field  
        }  
        set(value: String?) {  
            println("property set value from $field to $value")  
            field = value  
        }  

    var anotherValue: String? = null  
        get() {  
            println("property get value: $field")  
            return field  
        }  
        set(value: String?) {  
            println("property set value from $field to $value")  
            field = value  
        }  
}

fun main() {
    val example1 = PropertyAccessExample()
    // property set
    example1.value = "example1"
    //…