Observable Delegate in Kotlin

Introduction

Observable Delegate is a fundamental concept every Kotlin developer should understand. Property delegates move repetitive getter/setter logic into reusable classes — lazy initialization and observable properties are common examples.

Delegates.observable triggers callback when property value changes. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• Delegates.observable triggers callback when property value changes.
• Callback receives old and new values.
• Useful for UI updates and logging.

Syntax

Delegates.observable(initial) { _, old, new -> }

Observable Delegate in Kotlin Example Program in Kotlin

import kotlin.properties.Delegates

fun main(args: Array<String>) {
    var count by Delegates.observable(1) { _, old, new ->
        println("Changed: $old -> $new")
    }
    count = 5
}

Sample Output

Changed: 1 -> 5

When to use

Use delegates when multiple properties share the same access pattern — lazy init, logging, or validation.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. The println("Changed: $old -> $new") statement writes a line to the console — this produces part of the sample output below.

3. count = 5 assigns or updates a value used later in the program.

4. Delegates.observable triggers callback when property value changes.

5. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

Common Mistakes

Key Points

• Delegates.observable triggers callback when property value changes.
• Callback receives old and new values.
• Useful for UI updates and logging.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.