Custom delegated properties in Kotlin

Introduction

Custom delegated properties let you control how a property's value is stored and retrieved. This helps you add special behavior when getting or setting a value.

You want to log every time a property changes.

You need to validate a value before saving it.

You want to store a property's value in a different place, like a map or database.

You want to add lazy loading or caching behavior to a property.

Syntax

Kotlin

import kotlin.reflect.KProperty

class MyDelegate {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Type {
        // return the property value
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Type) {
        // set the property value
    }
}

class MyClass {
    var myProperty: Type by MyDelegate()
}

The delegate class must define getValue and setValue operator functions.

The by keyword connects the property to its delegate.

Examples

This delegate stores a string but always converts it to uppercase when set.

Kotlin

import kotlin.reflect.KProperty

class StringDelegate {
    private var storedValue = ""

    operator fun getValue(thisRef: Any?, property: KProperty<*>): String {
        return storedValue
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
        storedValue = value.uppercase()
    }
}

class Example {
    var name: String by StringDelegate()
}

This delegate logs every time the property is read or changed.

Kotlin

import kotlin.reflect.KProperty

class LoggingDelegate<T>(private var value: T) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
        println("Getting value of '${property.name}': $value")
        return value
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, newValue: T) {
        println("Setting value of '${property.name}' from $value to $newValue")
        value = newValue
    }
}

class User {
    var age: Int by LoggingDelegate(0)
}

Sample Program

This program uses a custom delegate to store the name always in uppercase. When we set p.name to "kotlin", it stores "KOTLIN". Printing p.name shows the uppercase string.

Kotlin

import kotlin.reflect.KProperty

class UppercaseDelegate {
    private var storedValue = ""

    operator fun getValue(thisRef: Any?, property: KProperty<*>): String {
        return storedValue
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
        storedValue = value.uppercase()
    }
}

class Person {
    var name: String by UppercaseDelegate()
}

fun main() {
    val p = Person()
    p.name = "kotlin"
    println(p.name)  // Should print uppercase version
}

Important Notes

Delegated properties can be used with var (read-write) or val (read-only) properties. For val, only getValue is needed.

The thisRef parameter refers to the object that owns the property. It can be useful if the delegate needs context.

Delegates can be generic to work with any type.

Summary

Custom delegated properties let you add special behavior when getting or setting a property.

You create a delegate class with getValue and setValue operator functions.

Use the by keyword to connect a property to its delegate.

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