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Kotlin Delegation
Philippe Breault
@pbreault
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The Speaker
• Philippe Breault
• Android since 2010
• Sr. Software Engineer @ American Express
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What is Delegation
In software engineering, the delegation pattern is an object-oriented design pattern that
allows object composition to achieve the same code reuse as inheritance. — Wikipedia
https://en.wikipedia.org/wiki/Delegation_pattern
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What is Delegation
Ask someone else to do the work for you — Phil
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What is Delegation
https://en.wikipedia.org/wiki/Delegation_pattern
class Rectangle(val width: Int, val height: Int) {
fun area() = width * height
}
class Window(val bounds: Rectangle) {
// Delegation
fun area() = bounds.area()
}
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Class Delegation
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Class Delegation
Code sharing without inheritance
✴ So let’s start with inheritance !
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Square
Rectangle
area = width * height
Height
Width
Width == Height Width != Height
Side
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Inheritance
Square
Rectangle
Shape
+ area() : Int
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Inheritance
Square
Rectangle
Shape
+ area() : Int
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Inheritance
interface Shape {
fun area(): Float
}
open class Rectangle(val width: Float, val height: Float) : Shape {
override fun area() = width * height
}
class Square(val side: Float) : Rectangle(side, side)
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Inheritance
interface Shape {
fun area(): Float
}
open class Rectangle(val width: Float, val height: Float) : Shape {
override fun area() = width * height
}
class Square(val side: Float) : Rectangle(side, side)
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Inheritance
interface Shape {
fun area(): Float
}
open class Rectangle(val width: Float, val height: Float) : Shape {
override fun area() = width * height
}
class Square(val side: Float) : Rectangle(side, side)
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Inheritance - Problems?
open class Rectangle(val width: Float, val height: Float) : Shape {
override fun area() = width * height
}
class Square(val side: Float) : Rectangle(side, side)
Square
+ side : Int
+ height : Int
+ width : Int
Is this the api that we want?
Redundant properties?
Rectangle can be extended by any class
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Inheritance - Quiz?
fun main() {
println(Square().toName())
}
fun Shape.toName() = when (this) {
is Rectangle -> "Rectangle"
is Square -> "Square"
else -> "?"
}
open class Rectangle(…) : Shape
class Square(…) : Rectangle(…)
Will the answer be
A. “Square”
B. “Rectangle”
C. “Rectangle” and “Square”
D. “?”
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Inheritance - Quiz?
fun main() {
println(Square().toName())
}
fun Shape.toName() = when (this) {
is Rectangle -> "Rectangle"
is Square -> "Square"
else -> "?"
}
open class Rectangle(…) : Shape
class Square(…) : Rectangle(…)
Will the answer be
A. “Rectangle”
B. “Square”
C. “Rectangle” and “Square”
D. “?”
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Class Delegation
interface Shape {
fun area(): Float
}
open class Rectangle(val width: Float, val height: Float) : Shape {
override fun area() = width * height
}
class Square(val side: Float) : Rectangle(side, side)
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Class Delegation
interface Shape {
fun area(): Float
}
class Rectangle(val width: Float, val height: Float) : Shape {
override fun area() = width * height
}
class Square(val side: Float) : Shape by Rectangle(side, side)
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Class Delegation
interface Shape {
fun area(): Float
}
class Rectangle(val width: Float, val height: Float) : Shape {
override fun area() = width * height
}
class Square(val side: Float) : Shape by Rectangle(side, side)
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Class Delegation
class Square(val side: Float) : Shape by Rectangle(side, side)
// Is the same as:
class Square(val side: Float) : Shape {
private val delegate = Rectangle(side, side)
override fun area() = delegate.area()
}
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Class Delegation public final class Square implements Shape {
private final float side;
// $FF: synthetic field
private final Rectangle $$delegate_0;
public final float getSide() {
return this.side;
}
public Square(float side) {
this.$$delegate_0 = new Rectangle(side, side);
this.side = side;
}
public float area() {
return this.$$delegate_0.area();
}
}
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class MyLoggingList(val delegate: MutableList = mutableListOf())
: MutableList by delegate {
override fun add(element: T): Boolean {
println("Adding $element")
return delegate.add(element)
}
// override fun add(index: Int, element: T) {...}
// override fun addAll(index: Int, elements: Collection): Boolean {...}
// override fun addAll(elements: Collection): Boolean {...}
}
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class MyLoggingList(val delegate: MutableList = mutableListOf())
: MutableList by delegate {
override fun add(element: T): Boolean {
println("Adding $element")
return delegate.add(element)
}
// override fun add(index: Int, element: T) {...}
// override fun addAll(index: Int, elements: Collection): Boolean {...}
// override fun addAll(elements: Collection): Boolean {...}
}
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Recap - Class Delegates
• Compile time operation
• Hides complexity
• Keeps classes final
The Good
The Bad
The Ugly
• Can’t change the delegate after it being set up
• Delegates can’t interact with overridden methods
• Overriding a method can break the internal state of the delegate
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Property Delegation
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What is Property Delegation
• Replacement for get()/set()
var age: Int = 100
set(value) {
if (value > 0) field = value
}
var age: Int by vetoable(100) { _, _, newValue ->
newValue > 0
}
• Shareable logic using the “by” keyword
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Lazy
val lazyDelegate by lazy {
computeSomethingExpensive()
}
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Observable
var spied by observable(10) { property, oldValue, newValue ->
println("changed from $oldValue to $newValue")
}
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Vetoable
var age by vetoable(initialValue = 10) { _, _, newValue ->
newValue > 0
}
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NotNull
var someInt by notNull()
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Custom Property Delegates
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Custom Delegates
• ReadOnlyProperty ( val )
• ReadWriteProperty ( var )
• ObservableProperty ( var)
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ReadOnlyProperty
// Declaration
class LogOnRead(private val initialValue: T) : ReadOnlyProperty {
override fun getValue(thisRef: Any, property: KProperty<*>): T {
println("accessing $initialValue")
return initialValue
}
}
// Usage
class MyClass {
val w by LogOnRead(initialValue = 10)
//...
}
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ReadOnlyProperty
// Declaration
class LogOnRead(private val initialValue: T) : ReadOnlyProperty {
override fun getValue(thisRef: Any, property: KProperty<*>): T {
println("accessing $initialValue")
return initialValue
}
}
// Usage
class MyClass {
val w by LogOnRead(initialValue = 10)
//...
}
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ReadOnlyProperty
// Declaration
class LogOnRead(private val initialValue: T) : ReadOnlyProperty {
override fun getValue(thisRef: Any, property: KProperty<*>): T {
println("accessing $initialValue")
return initialValue
}
}
// Usage
class MyClass {
val w by LogOnRead(initialValue = 10)
//...
}
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ReadOnlyProperty
// Declaration
class LogOnRead(private val initialValue: T) : ReadOnlyProperty {
override fun getValue(thisRef: Any, property: KProperty<*>): T {
println("accessing $initialValue")
return initialValue
}
}
// Usage
class MyClass {
val w by LogOnRead(initialValue = 10)
//...
}
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ReadOnlyProperty - Generated Code
public final class LogOnRead implements ReadOnlyProperty {
private final Object initialValue;
public Object getValue(@NotNull Object thisRef, @NotNull KProperty property) {
Intrinsics.checkParameterIsNotNull(thisRef, "thisRef");
Intrinsics.checkParameterIsNotNull(property, "property");
System.out.println("accessing " + this.initialValue);
return this.initialValue;
}
public LogOnRead(Object initialValue) {
this.initialValue = initialValue;
}
}
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ReadOnlyProperty - Generated Code
// Kotlin
val myProp by LogOnRead(initialValue = “Hello”)
// Java
@NotNull
private final LogOnRead myProp$delegate = new LogOnRead("Hello");
@NotNull
public final String getMyProp() {
return (String)this.myProp$delegate.getValue(this, $$delegatedProperties[0]);
}
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ReadOnlyProperty - Generated Code
// Java
@NotNull
private final LogOnRead myProp$delegate = new LogOnRead("Hello");
@NotNull
public final String getMyProp() {
return (String)this.myProp$delegate.getValue(this, $$delegatedProperties[0]);
}
// $FF: synthetic field
static final KProperty[] $$delegatedProperties = new KProperty[]{
(KProperty) Reflection.property1(new PropertyReference1Impl(
Reflection.getOrCreateKotlinClass(MyClass.class), "myProp", "getMyProp()I"
))
};
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ReadWriteProperty
class LogOnReadWrite(initialValue: T) : ReadWriteProperty {
private var value = initialValue
override fun getValue(thisRef: Any, property: KProperty<*>): T {
println("accessing $value")
return value
}
override fun setValue(thisRef: Any, property: KProperty<*>, value: T) {
println("changing ${this.value} to $value ")
this.value = value
}
}
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ReadWriteProperty
class LogOnReadWrite(initialValue: T) : ReadWriteProperty {
private var value = initialValue
override fun getValue(thisRef: Any, property: KProperty<*>): T {
println("accessing $value")
return value
}
override fun setValue(thisRef: Any, property: KProperty<*>, value: T) {
println("changing ${this.value} to $value ")
this.value = value
}
}
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ReadWriteProperty
class LogOnReadWrite(initialValue: T) : ReadWriteProperty {
private var value = initialValue
override fun getValue(thisRef: Any, property: KProperty<*>): T {
println("accessing $value")
return value
}
override fun setValue(thisRef: Any, property: KProperty<*>, value: T) {
println("changing ${this.value} to $value ")
this.value = value
}
}
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ReadWriteProperty
class LogOnReadWrite(initialValue: T) : ReadWriteProperty {
private var value = initialValue
override fun getValue(thisRef: Any, property: KProperty<*>): T {
println("accessing $value")
return value
}
override fun setValue(thisRef: Any, property: KProperty<*>, value: T) {
println("changing ${this.value} to $value ")
this.value = value
}
}
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ReadWriteProperty
class LogOnReadWrite(initialValue: T) : ReadWriteProperty {
private var value = initialValue
override fun getValue(thisRef: Any, property: KProperty<*>): T {
println("accessing $value")
return value
}
override fun setValue(thisRef: Any, property: KProperty<*>, value: T) {
println("changing ${this.value} to $value ")
this.value = value
}
}
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ObservableProperty
class LogOnWrite(initialValue: T) : ObservableProperty(initialValue) {
override fun afterChange(property: KProperty<*>, oldValue: T, newValue: T) {
println("Changed from $oldValue to $newValue")
}
override fun beforeChange(property: KProperty<*>, oldValue: T, newValue: T)
= true // Accept change
// = false // Reject change
}
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ObservableProperty
class LogOnWrite(initialValue: T) : ObservableProperty(initialValue) {
override fun afterChange(property: KProperty<*>, oldValue: T, newValue: T) {
println("Changed from $oldValue to $newValue")
}
override fun beforeChange(property: KProperty<*>, oldValue: T, newValue: T)
= true // Accept change
// = false // Reject change
}
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ObservableProperty
class LogOnWrite(initialValue: T) : ObservableProperty(initialValue) {
override fun afterChange(property: KProperty<*>, oldValue: T, newValue: T) {
println("Changed from $oldValue to $newValue")
}
override fun beforeChange(property: KProperty<*>, oldValue: T, newValue: T)
= true // Accept change
// = false // Reject change
}
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Will the answer be
val p: String?
by observable("Hello") {/* ... */}
fun main() {
if (!p.isNullOrEmpty()) {
val firstChar = p[0]
}
}
A. “H”
B. Won’t compile
C. Crash
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Will the answer be
A. “H”
B. Won’t compile
C. Crash
val p: String?
by observable("Hello") {/* ... */}
fun main() {
if (!p.isNullOrEmpty()) {
val firstChar = p[0]
}
}
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Recap
• Compile time operation
• Hides complexity
• Code reuse for properties
The Good
The Bad
The Ugly
• Doesn’t play well with smart cast
• Reflection call during class loading
• Breaks expectations of what “val” means
• Potential side-effects on read & writes
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Recap
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Class Delegates
• An alternative to inheritance
• Compile time
• Invisible to the end user (mostly)
class Square(val side: Float) : Shape by Rectangle(side, side)
• Reuse logic for Properties
• Built-in delegates for
• lazy
• vetoable
• observable
• notNull
• Can build your own
• The compiler generates the underlying code
Property Delegates
val lazyDelegate by lazy { expensive() }
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Should you use them?
• Does it make the code cleaner?
• Is it simple to understand?
• Is it free of side-effects ( or are they explicit ) ?
• Is it worth the additional complexity?
A few questions to ask
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Thank You
Philippe Breault
@pbreault