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"
    // property get
    println(example1.value)
}

Output:

property set value from null to example1
property get value: example1
example1

Please note that the field here is an implicit field. To learn more about Properties and Fields in Kotlin, refer to this article.

Understand Fields and Properties in Kotlin

The problem of this is I need to implement this custom get() and set() for all the properties that I would like to monitor. For example, anotherValue above is the boilerplate code. To reduce the boilerplate code, we use by operator - delegated properties.

“by” Delegated Properties