Delegation in Kotlin

Kotlin supports two types of delegation: Implementation by delegation and property delegation. This talk explores what the delegation pattern is and means in object oriented programming and how the programming language Kotlin supports it.

Transcript

1. Delegation in Kotlin
   Ralf Wondratschek
   @vRallev
   

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2. Delegation
   Delegation is a design pattern … and often misunderstood.
   Not part of the Gang of Four design pattern book.
   Many languages don’t support object oriented delegation, e.g. Java
   and C++.
   

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3. Delegation is powerful
   One tool in a box with many other structural design patterns like the
   proxy, adapter or composition patterns.
   Plays nicely with dependency injection.
   Helps breaking down big classes.
   

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4. Delegation pattern
   public class Rectangle {
   private final int width;
   private final int height;
   public Rectangle(int width, int height) {
   this.width = width;
   this.height = height;
   }
   public int area() {
   return width * height;
   }
   }
   https://en.wikipedia.org/wiki/Delegation_pattern
   public class Square {
   private final Rectangle rectangle;
   public Square(int side) {
   this.rectangle = new Rectangle(side, side);
   }
   public int area() {
   return rectangle.area();
   }
   }
   

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5. Delegation
   public class Area {
   public int area(Square square) {
   return square.side * square.side;
   }
   }
   https://en.wikipedia.org/wiki/Delegation_(object-oriented_programming)
   public static class Square {
   private final int side;
   private final Area area;
   public Square(int side) {
   this.side = side;
   this.area = new Area();
   }
   public int area() {
   return area.area(this);
   }
   }
   

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6. Delegation in Kotlin
   Kotlin made delegation first class citizen like other design patterns,
   e.g. Singleton.
   Supports two types of delegation.
   

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7. Implementation by delegation
   class Rectangle(
   private val width: Int,
   private val height: Int
   ) {
   fun area(): Int = width * height
   }
   class Square(side: Int) {
   private val rectangle = Rectangle(side, side)
   fun area(): Int = rectangle.area()
   }
   https://kotlinlang.org/docs/reference/delegation.html
   

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8. Implementation by delegation
   class Rectangle(
   private val width: Int,
   private val height: Int
   ) {
   fun area(): Int = width * height
   }
   class Square(side: Int) {
   private val rectangle = Rectangle(side, side)
   fun area(): Int = rectangle.area()
   }
   interface Shape {
   fun area(): Int
   }
   

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9. Implementation by delegation
   class Rectangle(
   private val width: Int,
   private val height: Int
   ) : Shape {
   override fun area(): Int =
   width * height
   }
   class Square(side: Int) {
   private val rectangle = Rectangle(side, side)
   fun area(): Int = rectangle.area()
   }
   interface Shape {
   fun area(): Int
   }
   

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10. Implementation by delegation
    class Rectangle(
    private val width: Int,
    private val height: Int
    ) : Shape {
    override fun area(): Int =
    width * height
    }
    class Square(side: Int) : Shape {
    private val rectangle = Rectangle(side, side)
    override fun area(): Int = rectangle.area()
    }
    interface Shape {
    fun area(): Int
    }
    

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11. Implementation by delegation
    class Rectangle(
    private val width: Int,
    private val height: Int
    ) : Shape {
    override fun area(): Int =
    width * height
    }
    class Square(side: Int) :
    Shape by Rectangle(side, side)
    interface Shape {
    fun area(): Int
    }
    

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12. Implementation by delegation
    class Rectangle(
    private val width: Int,
    private val height: Int
    ) : Shape {
    override fun area(): Int =
    width * height
    }
    class Square(side: Int) :
    Shape by Rectangle(side, side) {
    override fun area(): Int = 0
    }
    interface Shape {
    fun area(): Int
    }
    

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13. Proxy class
    interface Shape {
    fun area(): Int
    }
    class Unimplemented : Shape {
    override fun area(): Int {
    TODO("not implemented")
    }
    }
    

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14. Proxy class
    inline fun TODO(): T {
    val clazz = T::class.java
    return Proxy.newProxyInstance(clazz.classLoader, arrayOf(clazz)) { _, method, _ ->
    kotlin.TODO("Method ${method.name}() not implemented")
    } as T
    }
    

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15. Proxy class
    interface Shape {
    fun area(): Int
    }
    class Unimplemented : Shape {
    override fun area(): Int {
    TODO("not implemented")
    }
    }
    class Unimplemented : Shape by TODO()
    

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16. Property delegation
    https://kotlinlang.org/docs/reference/delegated-properties.html
    Object oriented delegation.
    You hide implementation details and delegate the knowledge to
    another object.
    Built-in delegates.
    

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17. But why?
    interface Lazy {
    fun get(): T
    }
    class Square(side: Int) : Shape {
    val lazyRectangle = object : Lazy {
    override fun get(): Rectangle = Rectangle(side, side)
    }
    override fun area(): Int = lazyRectangle.get().area()
    }
    

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18. Lazy
    class Square(side: Int) : Shape {
    val rectangle by lazy { Rectangle(side, side) }
    override fun area(): Int = rectangle.area()
    }
    

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19. Observable
    class Square(side: Int) : Shape {
    var side by Delegates.observable(side) { property, oldValue, newValue ->
    println("Side changed from $oldValue to $newValue")
    }
    override fun area(): Int = side * side
    }
    

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20. Interfaces
    // Use for val
    interface ReadOnlyProperty {
    operator fun getValue(thisRef: R, property: KProperty<*>): T
    }
    // Use for var
    interface ReadWriteProperty {
    operator fun getValue(thisRef: R, property: KProperty<*>): T
    operator fun setValue(thisRef: R, property: KProperty<*>, value: T)
    }
    

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21. View delegate
    class MainActivity : AppCompatActivity() {
    private lateinit var button: Button
    override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    button = findViewById(R.id.button)
    }
    }
    

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22. View delegate
    private class ViewDelegate(
    @IdRes private val viewId: Int
    ) : ReadOnlyProperty {
    private var view: T? = null
    override fun getValue(thisRef: Activity, property: KProperty<*>): T {
    return view ?: thisRef.findViewById(viewId).also { view = it }
    }
    }
    fun view(
    @IdRes viewId: Int
    ): ReadOnlyProperty = ViewDelegate(viewId)
    

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23. View delegate
    class MainActivity : AppCompatActivity() {
    private lateinit var button: Button
    override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    button = findViewById(R.id.button)
    }
    }
    

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24. View delegate
    class MainActivity : AppCompatActivity() {
    private val button by view(R.id.button)
    override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    }
    }
    

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25. KProperty
    Represents a property in Kotlin’s reflection API.
    Part of the standard library.
    Provides useful information like name, visibility and other attributes
    

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26. Saving instance state
    private const val COUNT_KEY = "count_key"
    class MainActivity : AppCompatActivity() {
    private var clickCount = 0
    override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    clickCount = savedInstanceState?.getInt(COUNT_KEY, clickCount) ?: clickCount
    }
    override fun onSaveInstanceState(outState: Bundle) {
    outState.putInt(COUNT_KEY, clickCount)
    }
    fun click(view: View) {
    clickCount++
    updateButtonText()
    }
    }
    

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27. Saving instance state
    private sealed class BaseInstanceStateProperty(
    private val defaultValue: T
    ) : ReadWriteProperty, InstanceStateProperty {
    private var value = defaultValue
    private var key: String? = null
    private var savedInstanceState: Bundle? = null
    }
    

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28. Saving instance state
    private sealed class BaseInstanceStateProperty(
    private val defaultValue: T
    ) : ReadWriteProperty, InstanceStateProperty {
    private var key: String? = null
    private fun KProperty<*>.toKey(): String = this.name
    }
    

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29. Saving instance state
    private sealed class BaseInstanceStateProperty(
    private val defaultValue: T
    ) : ReadWriteProperty, InstanceStateProperty {
    abstract fun fromBundle(bundle: Bundle, key: String, defaultValue: T): T
    abstract fun saveInBundle(bundle: Bundle, key: String, value: T)
    }
    

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30. Saving instance state
    private sealed class BaseInstanceStateProperty(
    private val defaultValue: T
    ) : ReadWriteProperty, InstanceStateProperty {
    private var value = defaultValue
    final override fun getValue(thisRef: Activity, property: KProperty<*>): T {
    initializeValue(thisRef, property)
    return value
    }
    final override fun setValue(thisRef: Activity, property: KProperty<*>, value: T) {
    initializeValue(thisRef, property)
    this.value = value
    }
    }
    

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31. Saving instance state
    private sealed class BaseInstanceStateProperty(
    private val defaultValue: T
    ) : ReadWriteProperty, InstanceStateProperty {
    final override fun restore(savedInstanceState: Bundle) {
    key?.let { key ->
    if (savedInstanceState.containsKey(key) &&
    savedInstanceState.getInt(key) != value &&
    value != defaultValue) {
    throw IllegalStateException("The value ...")
    }
    }
    this.savedInstanceState = savedInstanceState
    }
    final override fun save(outState: Bundle) {
    key?.let { saveInBundle(outState, it, value) }
    }
    }
    

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32. Saving instance state
    private sealed class BaseInstanceStateProperty(
    private val defaultValue: T
    ) : ReadWriteProperty, InstanceStateProperty {
    private var value = defaultValue
    private var key: String? = null
    private var savedInstanceState: Bundle? = null
    private fun initializeValue(thisRef: Activity, property: KProperty<*>) {
    key = key ?: property.toKey()
    InstanceStateController.register(thisRef, this)
    val bundle = savedInstanceState
    if (bundle != null) {
    value = fromBundle(bundle, key!!, defaultValue)
    savedInstanceState = null
    }
    }
    }
    

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33. Saving instance state
    private class IntInstanceStateProperty(
    defaultValue: Int
    ) : BaseInstanceStateProperty(defaultValue) {
    override fun fromBundle(bundle: Bundle, key: String, defaultValue: Int): Int =
    bundle.getInt(key, defaultValue)
    override fun saveInBundle(bundle: Bundle, key: String, value: Int) {
    bundle.putInt(key, value)
    }
    }
    

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34. Saving instance state
    private class StringInstanceStateProperty(
    defaultValue: String
    ) : BaseInstanceStateProperty(defaultValue) {
    override fun fromBundle(bundle: Bundle, key: String, defaultValue: String): String =
    bundle.getString(key, defaultValue)
    override fun saveInBundle(bundle: Bundle, key: String, value: String) {
    bundle.putString(key, value)
    }
    }
    

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35. Saving instance state
    fun instanceState(defaultValue: T): ReadWriteProperty {
    return when (defaultValue) {
    is Int -> IntInstanceStateProperty(defaultValue)
    is String -> StringInstanceStateProperty(defaultValue)
    else -> throw NotImplementedError("Missing implementation...")
    } as ReadWriteProperty
    }
    

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36. Saving instance state
    private const val COUNT_KEY = "count_key"
    class MainActivity : AppCompatActivity() {
    private var clickCount = 0
    override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    clickCount = savedInstanceState?.getInt(COUNT_KEY, clickCount) ?: clickCount
    }
    override fun onSaveInstanceState(outState: Bundle) {
    outState.putInt(COUNT_KEY, clickCount)
    }
    fun click(view: View) {
    clickCount++
    updateButtonText()
    }
    }
    

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37. Saving instance state
    class MainActivity : AppCompatActivity() {
    private var clickCount by instanceState(defaultValue = 0)
    fun click(view: View) {
    clickCount++
    updateButtonText()
    }
    }
    

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38. Operator
    // Use for val
    interface ReadOnlyProperty {
    operator fun getValue(thisRef: R, property: KProperty<*>): T
    }
    // Use for var
    interface ReadWriteProperty {
    operator fun getValue(thisRef: R, property: KProperty<*>): T
    operator fun setValue(thisRef: R, property: KProperty<*>, value: T)
    }
    

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39. Operator
    class User(val map: Map) {
    val name: String by map
    val age: Int by map
    }
    val user = User(mapOf(
    "name" to "John Doe",
    "age" to 25
    ))
    

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40. Operator
    operator fun Map.getValue(
    thisRef: Any?,
    property: KProperty<*>
    ): V1 = @Suppress("UNCHECKED_CAST") (getOrImplicitDefault(property.name) as V1)
    

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41. Operator
    inline operator fun Bundle.getValue(
    thisRef: Any?,
    property: KProperty<*>
    ): V = when (V::class) {
    String::class -> this.getString(property.name)
    Int::class -> this.getInt(property.name)
    else -> throw NotImplementedError("Missing for ${V::class}")
    } as V
    

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42. Operator
    class User(val bundle: Bundle) {
    val name: String by bundle
    val age: Int by bundle
    }
    val user = User(bundleOf(
    "name" to "John Doe",
    "age" to 25
    ))
    

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43. Source code
    https://github.com/vRallev/delegate-meetup
    

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44. Summary
    Kotlin supports true object oriented delegation.
    Two types: implementation by delegation and property delegation.
    Two very powerful mechanisms in our Kotlin toolbox.
    

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45. Resources
    Shoulders of giants: Languages Kotlin learnt from (Andrey Breslav)
    https://2018.geekout.ee/andrey-breslav/
    Delegation pattern
    https://en.wikipedia.org/wiki/Delegation_pattern
    https://en.wikipedia.org/wiki/Delegation_(object-oriented_programming)
    Using Prototypical Objects to Implement Shared Behavior in Object
    Oriented Systems (Henry Lieberman)
    http://web.media.mit.edu/~lieber/Lieberary/OOP/Delegation/Delegation.html
    Documentation
    https://kotlinlang.org/docs/reference/delegation.html
    https://kotlinlang.org/docs/reference/delegated-properties.html
    

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46. Thank you!
    Ralf Wondratschek
    @vRallev
    

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