Custom delegated properties in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom delegated property in Kotlin?

A custom delegated property in Kotlin is a way to define how a property’s getter and setter work by delegating them to another object. This lets you control property behavior without writing boilerplate code in every class.

intermediate

Which operator functions must a delegate class implement for a var property?

For a var property, the delegate class must implement <code>operator fun getValue(thisRef: Any?, property: KProperty&lt;*&gt;): T</code> and <code>operator fun setValue(thisRef: Any?, property: KProperty&lt;*&gt;, value: T)</code>.

beginner

How do you declare a property using a custom delegate in Kotlin?

You declare it using the by keyword followed by an instance of the delegate class. For example: var myProp by MyDelegate().

intermediate

What parameters do the getValue and setValue operator functions receive?

getValue receives two parameters: thisRef (the object containing the property) and property (metadata about the property). setValue receives three parameters: thisRef, property, and the new value to set.

beginner

Why use custom delegated properties instead of normal properties?

Custom delegated properties let you reuse property logic across classes, add extra behavior like validation or lazy loading, and keep your code clean by separating concerns.

Which keyword is used to delegate a property to a custom delegate in Kotlin?

A delegate

B by

C with

D using

The by keyword is used to delegate a property to a custom delegate instance.

What must a delegate class implement to support a read-only property?

A operator fun getValue(...)

B operator fun setValue(...)

C operator fun getValue(...) and setValue(...)

D No operator functions needed

A read-only property requires only getValue operator function.

In the operator function getValue(thisRef, property), what is property?

A The delegate instance

B The property’s value

C The property’s metadata

D The class name

property provides metadata like the property’s name and type.

What is a common use case for custom delegated properties?

A To create new classes automatically

B To replace all functions in a class

C To change Kotlin syntax

D To add validation logic when setting a property

Custom delegates often add behaviors like validation or lazy loading to properties.

Which of these is NOT a parameter of the setValue operator function?

A delegate

B property

C value

D thisRef

setValue receives thisRef, property, and value, but not delegate.

Explain how to create and use a custom delegated property in Kotlin.

Think about how Kotlin lets you move property logic outside the class.

What are the benefits of using custom delegated properties compared to normal properties?

Consider how delegation helps keep your code DRY and organized.