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 ""

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
}

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

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

Hint

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

Practice

1. What is the main purpose of using custom delegated properties in Kotlin?

easy

A. To define extension functions for a class

B. To create multiple constructors for a class

C. To control how a property's value is stored and retrieved

D. To override the default toString() method

Solution

1. Step 1: Understand property delegation concept

   Delegated properties let you customize how a property's value is handled behind the scenes.

2. Step 2: Identify the purpose of custom delegation

   Custom delegated properties specifically allow control over getting and setting property values.

3. Final Answer:

   To control how a property's value is stored and retrieved -> Option C

4. Quick Check:

   Custom delegation = control property access [OK]

Hint: Delegation controls property get/set behavior [OK]

Common Mistakes:

• Confusing delegation with constructors
• Thinking delegation defines functions
• Mixing delegation with method overriding

2. Which of the following is the correct syntax to declare a custom delegated property in Kotlin?

easy

A. val prop = Delegate()

B. val prop by Delegate()

C. val prop: Delegate by this

D. val prop delegate Delegate()

Solution

1. Step 1: Recall Kotlin delegation syntax

   Delegated properties use the by keyword followed by a delegate instance.

2. Step 2: Match correct syntax

   val prop by Delegate() correctly declares a delegated property.

3. Final Answer:

   val prop by Delegate() -> Option B

4. Quick Check:

   Delegation uses 'by' keyword [OK]

Hint: Look for 'by' keyword with delegate instance [OK]

Common Mistakes:

• Using '=' instead of 'by' for delegation
• Incorrect delegate type declaration
• Missing parentheses for delegate constructor

3. What will be the output of the following Kotlin code?

class Delegate {
    operator fun getValue(thisRef: Any?, property: kotlin.reflect.KProperty<*>): String {
        return "Hello from delegate"
    }
}

class Example {
    val message: String by Delegate()
}

fun main() {
    val e = Example()
    println(e.message)
}

medium

A. Hello from delegate

B. null

C. Compilation error

D. Hello from Example

Solution

1. Step 1: Understand getValue operator

   The getValue function returns the string "Hello from delegate" when the property is accessed.

2. Step 2: Trace property access in main()

   Accessing e.message calls getValue and prints the returned string.

3. Final Answer:

   Hello from delegate -> Option A

4. Quick Check:

   Delegate getValue returns string [OK]

Hint: getValue defines returned property value [OK]

Common Mistakes:

• Expecting default property value instead of delegate's
• Confusing getValue with setValue
• Assuming compilation error due to missing parameters

4. Identify the error in this Kotlin code using a custom delegated property:

class Delegate {
    operator fun getValue(thisRef: Any?, property: kotlin.reflect.KProperty<*>): Int {
        return 42
    }
}

class Example {
    var number: Int by Delegate()
}

fun main() {
    val e = Example()
    e.number = 10
    println(e.number)
}

medium

A. Delegate class must be an interface

B. Property number should be val, not var

C. getValue must return String, not Int

D. Delegate class lacks setValue operator function

Solution

1. Step 1: Check property mutability and delegation

   The property number is var, so delegate must support both get and set.

2. Step 2: Verify delegate methods

   The Delegate class only defines getValue, missing setValue needed for var.

3. Final Answer:

   Delegate class lacks setValue operator function -> Option D

4. Quick Check:

   var needs getValue and setValue [OK]

Hint: var properties need both getValue and setValue [OK]

Common Mistakes:

• Forgetting setValue for var properties
• Assuming getValue return type must be String
• Thinking delegate must be interface

5. You want to create a custom delegated property that logs every time the property is read or written. Which of the following delegate implementations correctly achieves this?

class LoggingDelegate<T>(var value: T) {
    operator fun getValue(thisRef: Any?, property: kotlin.reflect.KProperty<*>): T {
        println("Getting value of ${property.name}")
        return value
    }

    operator fun setValue(thisRef: Any?, property: kotlin.reflect.KProperty<*>, newValue: T) {
        println("Setting value of ${property.name} to $newValue")
        value = newValue
    }
}

class Example {
    var data: String by LoggingDelegate("initial")
}

fun main() {
    val e = Example()
    e.data = "new"
    println(e.data)
}

hard

A. Logs on get and set, prints 'Setting value of data to new' then 'Getting value of data' and 'new'

B. Only logs on get, no output on set

C. Throws error because delegate class is not generic

D. Does not compile due to missing constructor parameters

Solution

1. Step 1: Analyze delegate methods

   The delegate defines getValue and setValue with print statements logging property access.

2. Step 2: Trace main function output

   Setting e.data = "new" prints setting log, then printing e.data triggers get log and prints value.

3. Final Answer:

   Logs on get and set, prints 'Setting value of data to new' then 'Getting value of data' and 'new' -> Option A

4. Quick Check:

   Delegate logs on get/set correctly [OK]

Hint: Check both getValue and setValue print logs [OK]

Common Mistakes:

• Missing setValue causes no set logging
• Assuming delegate must be non-generic
• Forgetting to pass initial value to delegate