Kotlin
Custom delegates
Learn to write your own property delegates.
Custom delegates
You can write your own delegate class. For a read-only property define operator fun getValue(thisRef: Any?, property: KProperty<*>): T. Example: class Single(val v: Int) { operator fun getValue(thisRef: Any?, property: KProperty<*>): Int = v }. Then use it with val x by Single(42).
For a read-write property add operator fun setValue too. You may implement the ReadOnlyProperty or ReadWriteProperty interfaces, but writing the operator functions is enough. This is a powerful way to fully control behavior.
The getValue operator
When a delegate class defines operator fun getValue, it can be bound to a property with the by keyword. Example: class Single(val v: Int) { operator fun getValue(thisRef: Any?, property: KProperty<*>): Int = v }.
Usage: val x by Single(42). Every access to x calls getValue and returns 42. thisRef is the property owner (may be null), property is the property reference. This simple example returns a constant; more complex delegates can cache, log or compute.
Exercise Define the Single delegate
Add import kotlin.reflect.KProperty. Define class Single(val v: Int) with operator fun getValue(thisRef: Any?, property: KProperty<*>): Int = v inside. In main define val x by Single(42) and call println(x).
Show example solution
import kotlin.reflect.KProperty
class Single(val v: Int) {
operator fun getValue(thisRef: Any?, property: KProperty<*>): Int = v
}
fun main() {
val x by Single(42)
println(x)
}
Test yourself
-
Match the custom delegate parts
- ReadOnlyProperty
- Read-only
- ReadWriteProperty
- Read-write
- getValue
- Read
- setValue
- Write
-
Complete the getValue body
class Single(val v: Int) {
operator fun getValue(thisRef: Any?, p: KProperty<*>): Int = ____
}
Show answer
class Single(val v: Int) {
operator fun getValue(thisRef: Any?, p: KProperty<*>): Int = v
}
-
Which operator does a read-only custom delegate require?
- getValue
- setValue
- both
- neither
Show answer
getValue — A read-only delegate requires only getValue.
Next lesson: Summary - Delegation
