Observable property delegation in Kotlin - Mini Project: Build & Apply

Observable Property Delegation in Kotlin

📖 Scenario: Imagine you are creating a simple app that tracks a person's mood. You want to know whenever the mood changes so you can show a message or take action.

🎯 Goal: Build a Kotlin program that uses observable property delegation to watch changes in a person's mood and print a message every time the mood changes.

📋 What You'll Learn

Create a variable with observable property delegation

Use a lambda to react to changes in the property

Print a message showing the old and new mood values

💡 Why This Matters

🌍 Real World

Observable properties are useful in apps where you want to track changes and update the UI or perform actions automatically.

💼 Career

Many Kotlin developers use property delegation to write cleaner and more reactive code, especially in Android app development.

1

Create a mood variable

Create a var called mood of type String and set its initial value to "Happy".

Kotlin

// Create a var called mood with initial value "Happy"
// Your code here

Need a hint?

Use var mood: String = "Happy" to create the variable.

2

Add observable property delegation

Import kotlin.properties.Delegates and change the mood variable to use Delegates.observable with initial value "Happy".

Kotlin

import kotlin.properties.Delegates

// Change mood to use Delegates.observable with initial value "Happy"
// Your code here

Need a hint?

Use var mood: String by Delegates.observable("Happy") { prop, old, new -> }.

3

Add reaction to mood changes

Inside the observable lambda, add a println statement that prints: "Mood changed from $old to $new".

Kotlin

import kotlin.properties.Delegates

var mood: String by Delegates.observable("Happy") { prop, old, new ->
    // Print the mood change message here
    // Your code here
}

Need a hint?

Use println("Mood changed from $old to $new") inside the lambda.

4

Change mood and print output

Change the value of mood to "Excited" and then to "Calm". This should print the mood change messages.

Kotlin

import kotlin.properties.Delegates

var mood: String by Delegates.observable("Happy") { prop, old, new ->
    println("Mood changed from $old to $new")
}

// Change mood to "Excited" and then to "Calm"
// Your code here

Need a hint?

Assign mood = "Excited" and then mood = "Calm" to see the messages.