Delegated properties

With some common kinds of properties, even though you can implement them manually every time you need them, it is more helpful to implement them once, add them to a library, and reuse them later. For example:

Lazy properties: the value is computed only on first access.
Observable properties: listeners are notified about changes to this property.
Storing properties in a map instead of a separate field for each property.

To cover these (and other) cases, Kotlin supports delegated properties:

class Example {
    var p: String by Delegate()
}

The syntax is: val/var <property name>: <Type> by <expression>. The expression after by is a delegate, because the get() (and set()) that correspond to the property will be delegated to its getValue() and setValue() methods. Property delegates don't have to implement an interface, but they have to provide a getValue() function (and setValue() for vars).

For example:

import kotlin.reflect.KProperty

class Delegate {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): String {
        return "$thisRef, thank you for delegating '${property.name}' to me!"
    }
 
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
        println("$value has been assigned to '${property.name}' in $thisRef.")
    }
}

When you read from p, which delegates to an instance of Delegate, the getValue() function from Delegate is called. Its first parameter is the object you read p from, and the second parameter holds a description of p itself (for example, you can take its name).

val e = Example()
println(e.p)

This prints:

Example@33a17727, thank you for delegating 'p' to me!

Similarly, when you assign to p, the setValue() function is called. The first two parameters are the same, and the third holds the value being assigned:

e.p = "NEW"

This prints:

NEW has been assigned to 'p' in Example@33a17727.

The specification of the requirements for the delegated object can be found below.

You can declare a delegated property inside a function or code block; it doesn't have to be a member of a class. Below you can find an example.

Standard delegates

The Kotlin standard library provides factory methods for several useful kinds of delegates.

Lazy properties

lazy() is a function that takes a lambda and returns an instance of Lazy<T>, which can serve as a delegate for implementing a lazy property. The first call to get() executes the lambda passed to lazy() and remembers the result. Subsequent calls to get() simply return the remembered result.

val lazyValue: String by lazy {
    println("computed!")
    "Hello"
}

fun main() {
    println(lazyValue)
    println(lazyValue)
}

By default, the evaluation of lazy properties is synchronized: the value is computed only in one thread, but all threads will see the same value. If the synchronization of the initialization delegate is not required to allow multiple threads to execute it simultaneously, pass LazyThreadSafetyMode.PUBLICATION as a parameter to lazy().

If you're sure that the initialization will always happen in the same thread as the one where you use the property, you can use LazyThreadSafetyMode.NONE. It doesn't incur any thread-safety guarantees and related overhead.

Observable properties

Delegates.observable() takes two arguments: the initial value and a handler for modifications.

The handler is called every time you assign to the property (after the assignment has been performed). It has three parameters: the property being assigned to, the old value, and the new value:

import kotlin.properties.Delegates

class User {
    var name: String by Delegates.observable("<no name>") {
        prop, old, new ->
        println("$old -> $new")
    }
}

fun main() {
    val user = User()
    user.name = "first"
    user.name = "second"
}

If you want to intercept assignments and veto them, use vetoable() instead of observable(). The handler passed to vetoable will be called before the assignment of a new property value.

Delegating to another property

A property can delegate its getter and setter to another property. Such delegation is available for both top-level and class properties (member and extension). The delegate property can be:

A top-level property
A member or an extension property of the same class
A member or an extension property of another class

To delegate a property to another property, use the :: qualifier in the delegate name, for example, this::delegate or MyClass::delegate.

var topLevelInt: Int = 0
class ClassWithDelegate(val anotherClassInt: Int)

class MyClass(var memberInt: Int, val anotherClassInstance: ClassWithDelegate) {
    var delegatedToMember: Int by this::memberInt
    var delegatedToTopLevel: Int by ::topLevelInt
    
    val delegatedToAnotherClass: Int by anotherClassInstance::anotherClassInt
}
var MyClass.extDelegated: Int by ::topLevelInt

This may be useful, for example, when you want to rename a property in a backward-compatible way: introduce a new property, annotate the old one with the @Deprecated annotation, and delegate its implementation.

class MyClass {
   var newName: Int = 0
   @Deprecated("Use 'newName' instead", ReplaceWith("newName"))
   var oldName: Int by this::newName
}
fun main() {
   val myClass = MyClass()
   // Notification: 'oldName: Int' is deprecated.
   // Use 'newName' instead
   myClass.oldName = 42
   println(myClass.newName) // 42
}

Storing properties in a map

One common use case is storing the values of properties in a map. This comes up often in applications for things like parsing JSON or performing other dynamic tasks. In this case, you can use the map instance itself as the delegate for a delegated property.

class User(val map: Map<String, Any?>) {
    val name: String by map
    val age: Int     by map
}

In this example, the constructor takes a map:

val user = User(mapOf(
    "name" to "John Doe",
    "age"  to 25
))

Delegated properties take values from this map through string keys, which are associated with the names of properties:

class User(val map: Map<String, Any?>) {
    val name: String by map
    val age: Int     by map
}

fun main() {
    val user = User(mapOf(
        "name" to "John Doe",
        "age"  to 25
    ))
//sampleStart
    println(user.name) // Prints "John Doe"
    println(user.age)  // Prints 25
//sampleEnd
}

This also works for var's properties if you use a MutableMap instead of a read-only Map:

class MutableUser(val map: MutableMap<String, Any?>) {
    var name: String by map
    var age: Int     by map
}

Local delegated properties

You can declare local variables as delegated properties. For example, you can make a local variable lazy:

fun example(computeFoo: () -> Foo) {
    val memoizedFoo by lazy(computeFoo)

    if (someCondition && memoizedFoo.isValid()) {
        memoizedFoo.doSomething()
    }
}

The memoizedFoo variable will be computed on first access only. If someCondition fails, the variable won't be computed at all.

Property delegate requirements

For a read-only property (val), a delegate should provide an operator function getValue() with the following parameters:

thisRef must be the same type as, or a supertype of, the property owner (for extension properties, it should be the type being extended).
property must be of type KProperty<*> or its supertype.

getValue() must return the same type as the property (or its subtype).

class Resource

class Owner {
    val valResource: Resource by ResourceDelegate()
}

class ResourceDelegate {
    operator fun getValue(thisRef: Owner, property: KProperty<*>): Resource {
        return Resource()
    }
}

For a mutable property (var), a delegate has to additionally provide an operator function setValue() with the following parameters:

thisRef must be the same type as, or a supertype of, the property owner (for extension properties, it should be the type being extended).
property must be of type KProperty<*> or its supertype.
value must be of the same type as the property (or its supertype).

class Resource

class Owner {
    var varResource: Resource by ResourceDelegate()
}

class ResourceDelegate(private var resource: Resource = Resource()) {
    operator fun getValue(thisRef: Owner, property: KProperty<*>): Resource {
        return resource
    }
    operator fun setValue(thisRef: Owner, property: KProperty<*>, value: Any?) {
        if (value is Resource) {
            resource = value
        }
    }
}

getValue() and/or setValue() functions can be provided either as member functions of the delegate class or as extension functions. The latter is handy when you need to delegate a property to an object that doesn't originally provide these functions. Both of the functions need to be marked with the operator keyword.

You can create delegates as anonymous objects without creating new classes, by using the interfaces ReadOnlyProperty and ReadWriteProperty from the Kotlin standard library. They provide the required methods: getValue() is declared in ReadOnlyProperty; ReadWriteProperty extends it and adds setValue(). This means you can pass a ReadWriteProperty whenever a ReadOnlyProperty is expected.

fun resourceDelegate(resource: Resource = Resource()): ReadWriteProperty<Any?, Resource> =
    object : ReadWriteProperty<Any?, Resource> {
        var curValue = resource 
        override fun getValue(thisRef: Any?, property: KProperty<*>): Resource = curValue
        override fun setValue(thisRef: Any?, property: KProperty<*>, value: Resource) {
            curValue = value
        }
    }

val readOnlyResource: Resource by resourceDelegate()  // ReadWriteProperty as val
var readWriteResource: Resource by resourceDelegate()

Translation rules for delegated properties

Under the hood, the Kotlin compiler generates auxiliary properties for some kinds of delegated properties and then delegates to them.

For example, for the property prop it generates the hidden property prop$delegate, and the code of the accessors simply delegates to this additional property:

class C {
    var prop: Type by MyDelegate()
}

// this code is generated by the compiler instead:
class C {
    private val prop$delegate = MyDelegate()
    var prop: Type
        get() = prop$delegate.getValue(this, this::prop)
        set(value: Type) = prop$delegate.setValue(this, this::prop, value)
}

The Kotlin compiler provides all the necessary information about prop in the arguments: the first argument this refers to an instance of the outer class C, and this::prop is a reflection object of the KProperty type describing prop itself.

Optimized cases for delegated properties

The $delegate field will be omitted if a delegate is:

A referenced property:
class C<Type> { private var impl: Type = ... var prop: Type by ::impl }
A named object:
object NamedObject { operator fun getValue(thisRef: Any?, property: KProperty<*>): String = ... } val s: String by NamedObject
A final val property with a backing field and a default getter in the same module:
val impl: ReadOnlyProperty<Any?, String> = ... class A { val s: String by impl }
A constant expression, enum entry, this, null. The example of this:
class A { operator fun getValue(thisRef: Any?, property: KProperty<*>) ... val s by this }

Translation rules when delegating to another property

When delegating to another property, the Kotlin compiler generates immediate access to the referenced property. This means that the compiler doesn't generate the field prop$delegate. This optimization helps save memory.

Take the following code, for example:

class C<Type> {
    private var impl: Type = ...
    var prop: Type by ::impl
}

Property accessors of the prop variable invoke the impl variable directly, skipping the delegated property's getValueand setValue operators, and thus the KProperty reference object is not needed.

For the code above, the compiler generates the following code:

class C<Type> {
    private var impl: Type = ...

    var prop: Type
        get() = impl
        set(value) {
            impl = value
        }
    
    fun getProp$delegate(): Type = impl // This method is needed only for reflection
}

Providing a delegate

By defining the provideDelegate operator, you can extend the logic for creating the object to which the property implementation is delegated. If the object used on the right-hand side of by defines provideDelegate as a member or extension function, that function will be called to create the property delegate instance.

One of the possible use cases of provideDelegate is to check the consistency of the property upon its initialization.

For example, to check the property name before binding, you can write something like this:

class ResourceDelegate<T> : ReadOnlyProperty<MyUI, T> {
    override fun getValue(thisRef: MyUI, property: KProperty<*>): T { ... }
}
    
class ResourceLoader<T>(id: ResourceID<T>) {
    operator fun provideDelegate(
            thisRef: MyUI,
            prop: KProperty<*>
    ): ReadOnlyProperty<MyUI, T> {
        checkProperty(thisRef, prop.name)
        // create delegate
        return ResourceDelegate()
    }

    private fun checkProperty(thisRef: MyUI, name: String) { ... }
}

class MyUI {
    fun <T> bindResource(id: ResourceID<T>): ResourceLoader<T> { ... }

    val image by bindResource(ResourceID.image_id)
    val text by bindResource(ResourceID.text_id)
}

The parameters of provideDelegate are the same as those of getValue:

thisRef must be the same type as, or a supertype of, the property owner (for extension properties, it should be the type being extended);
property must be of type KProperty<*> or its supertype.

The provideDelegate method is called for each property during the creation of the MyUI instance, and it performs the necessary validation right away.

Without this ability to intercept the binding between the property and its delegate, to achieve the same functionality you'd have to pass the property name explicitly, which isn't very convenient:

// Checking the property name without "provideDelegate" functionality
class MyUI {
    val image by bindResource(ResourceID.image_id, "image")
    val text by bindResource(ResourceID.text_id, "text")
}

fun <T> MyUI.bindResource(
        id: ResourceID<T>,
        propertyName: String
): ReadOnlyProperty<MyUI, T> {
    checkProperty(this, propertyName)
    // create delegate
}

In the generated code, the provideDelegate method is called to initialize the auxiliary prop$delegate property. Compare the generated code for the property declaration val prop: Type by MyDelegate() with the generated code above (when the provideDelegate method is not present):

class C {
    var prop: Type by MyDelegate()
}

// this code is generated by the compiler 
// when the 'provideDelegate' function is available:
class C {
    // calling "provideDelegate" to create the additional "delegate" property
    private val prop$delegate = MyDelegate().provideDelegate(this, this::prop)
    var prop: Type
        get() = prop$delegate.getValue(this, this::prop)
        set(value: Type) = prop$delegate.setValue(this, this::prop, value)
}

Note that the provideDelegate method affects only the creation of the auxiliary property and doesn't affect the code generated for the getter or the setter.

With the PropertyDelegateProvider interface from the standard library, you can create delegate providers without creating new classes.

val provider = PropertyDelegateProvider { thisRef: Any?, property ->
    ReadOnlyProperty<Any?, Int> {_, property -> 42 }
}
val delegate: Int by provider

Last modified: 25 September 2024