Custom delegated properties in Kotlin - Mini Project: Build & Apply

Custom Delegated Properties in Kotlin

📖 Scenario: You are building a simple Kotlin program to manage a user's profile settings. You want to control how certain properties behave when accessed or changed.

🎯 Goal: Create a custom delegated property that logs every time a property is read or written. Then use this delegate for a user's name property.

📋 What You'll Learn

Create a class called LoggingDelegate that implements the getValue and setValue operator functions.

The delegate should print a message when the property is read and when it is written.

Create a class called User with a property name that uses LoggingDelegate.

Create an instance of User, set the name property, then read it and print the value.

💡 Why This Matters

🌍 Real World

Custom delegated properties help manage how data is accessed or changed, useful in settings, caching, or validation.

💼 Career

Understanding Kotlin delegates is valuable for Android developers and Kotlin backend developers to write clean, reusable, and maintainable code.

1

Create the LoggingDelegate class

Create a class called LoggingDelegate with a mutable property value of type String initialized to an empty string.

Kotlin

# Your code here
// Create class LoggingDelegate with a mutable property value of type String initialized to ""

Need a hint?

Use var value: String = "" inside the class to hold the property value.

2

Add getValue and setValue operator functions

Inside the LoggingDelegate class, add the operator fun getValue and operator fun setValue functions. In getValue, print "Getting value" and return value. In setValue, print "Setting value to" followed by the new value, then update value.

Kotlin

class LoggingDelegate {
    var value: String = ""
    // Add operator fun getValue and setValue here
    # Your code here
}

Need a hint?

Remember to use operator fun getValue(thisRef: Any?, property: KProperty<*>) and operator fun setValue(thisRef: Any?, property: KProperty<*>, newValue: String).

3

Create User class with name property using delegate

Create a class called User with a property name of type String that uses LoggingDelegate() as its delegate.

Kotlin

class LoggingDelegate {
    var value: String = ""

    operator fun getValue(thisRef: Any?, property: kotlin.reflect.KProperty): String {
        println("Getting value")
        return value
    }

    operator fun setValue(thisRef: Any?, property: kotlin.reflect.KProperty, newValue: String) {
        println("Setting value to $newValue")
        value = newValue
    }
}

// Create class User with property name delegated to LoggingDelegate
# Your code here

Need a hint?

Use var name: String by LoggingDelegate() inside the User class.

4

Create User instance, set and get name, then print it

Create a variable user as an instance of User. Set user.name to "Alice". Then print user.name.

Kotlin

class LoggingDelegate {
    var value: String = ""

    operator fun getValue(thisRef: Any?, property: kotlin.reflect.KProperty): String {
        println("Getting value")
        return value
    }

    operator fun setValue(thisRef: Any?, property: kotlin.reflect.KProperty, newValue: String) {
        println("Setting value to $newValue")
        value = newValue
    }
}

class User {
    var name: String by LoggingDelegate()
}

// Create user instance, set name to "Alice", then print user.name
# Your code here

Need a hint?

Remember to create val user = User(), then user.name = "Alice", and finally println(user.name).