Delegation in Kotlin by Philippe Breault

Delegation in Kotlin
Level: Beginner
by Philippe Breault, American Express

Ever heard of the phrase “Prefer composition over inheritance”? One of the better patterns to achieve this is “Delegation”. And thankfully, Kotlin has built-in support for this pattern.

This talk will cover class delegation and delegated properties as a means to make your code more readable and reusable. We’ll take a look at both approaches’ strengths, weaknesses, and when you should use them. We will also take a look at the code that is generated under the hood to discuss performance implications.

https://gdgmontreal.com/2019/07/26/kotlin-everywhere/

Transcript

1. Kotlin Delegation
   Philippe Breault
   @pbreault
   

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2. The Speaker
   • Philippe Breault
   • Android since 2010
   • Sr. Software Engineer @ American Express
   

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3. 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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4. What is Delegation
   Ask someone else to do the work for you — Phil
   

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5. 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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6. Class Delegation
   

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7. Class Delegation
   Code sharing without inheritance
   ✴ So let’s start with inheritance !
   

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8. Square
   Rectangle
   area = width * height
   Height
   Width
   Width == Height Width != Height
   Side
   

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9. Inheritance
   Square
   Rectangle
   Shape
   + area() : Int
   

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10. Inheritance
    Square
    Rectangle
    Shape
    + area() : Int
    

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11. 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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12. 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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13. 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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14. 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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15. 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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16. 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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17. 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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18. 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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19. 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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20. 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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21. 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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22. 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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23. 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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24. 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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25. Property Delegation
    

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26. 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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27. Lazy
    val lazyDelegate by lazy {
    computeSomethingExpensive()
    }
    

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28. Observable
    var spied by observable(10) { property, oldValue, newValue ->
    println("changed from $oldValue to $newValue")
    }
    

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29. Vetoable
    var age by vetoable(initialValue = 10) { _, _, newValue ->
    newValue > 0
    }
    

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30. NotNull
    var someInt by notNull()
    

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31. Custom Property Delegates
    

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32. Custom Delegates
    • ReadOnlyProperty ( val )
    • ReadWriteProperty ( var )
    • ObservableProperty ( var)
    

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33. 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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34. 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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35. 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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36. 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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37. 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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38. 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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39. 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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40. 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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41. 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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42. 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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43. 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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44. 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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45. 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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46. 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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47. 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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48. 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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49. 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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50. 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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51. Recap
    

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52. 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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53. 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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54. Thank You
    Philippe Breault
    @pbreault
    

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