Slides from my talk at Kyiv Kotlin User Group: First Blood, 2017-12-12 Video: https://youtu.be/s2m1w8t7k3Y?t=5730 The Delegation pattern is an OOD pattern that allows object composition to achieve the same code reuse as inheritance. Let's find out what Kotlin offers us in a world where we have no control over the creation of many objects to save our time in many cases.

1. Kotlin Delegates:
reduce the boilerplate

2. Hello!
I am Dmytro Zaitsev
Mobile Team Leader at Lóhika
You can find me at @DmitriyZaitsev

3. Delegation

4. “the delegation
pattern allows object
composition to
achieve the same code
reuse as inheritance.”

5. External
Can be implemented in
any object-oriented
language
Two types of Delegation
Internal
Requires language
support

6. External Delegation

7. External Delegation
interface Area {
fun area(): Int
}

8. External Delegation
class Rectangle(val width: Int, val height: Int) : Area {
override fun area(): Int = width * height
}

9. External Delegation
class Window(val rect: Rectangle) : Area {
// Delegation
override fun area(): Int = rect.area()
}

10. Internal Delegation

11. Interface Delegation
class Window: Area by Rectangle(10, 10)

12. Interface Delegation
class Window: Area by Rectangle(10, 10)

13. Translation rule
public final class Window implements Area {
// $FF: synthetic field
private final Rectangle $$delegate_0 = new Rectangle(10, 10);
public int area() {
return this.$$delegate_0.area();
}
}

14. Delegated Properties

15. Example
class Person {
val name: String by Delegate()
}

16. When I should care?
When you want to extend the
behavior of classes that you cannot
or don’t want to subclass because…

17. Encapsulation, thou art a heartless bitch
×Classes are final (e.g. File)
×You offer a limited API, using 98% of the existing code
×You want to hide the implementation from calling
code, so clients cannot cast to the superclass

18. DelegateD
Properties

19. What properties would we like to have?
×Lazy (computed only upon first access)
×Observable (listeners get notified about changes)
×Storing in a map, not in separate field each
×<Whatever I want property semantics>

20. Or?..

21. Val / Var: define an operator
// For val
operator fun getValue(thisRef: R, prop: KProperty<*>): T
// For var
operator fun getValue(thisRef: R, prop: KProperty<*>): T
operator fun setValue(thisRef: R, prop: KProperty<*>, value: T)
Property
metadata
Property owner

22. Val / Var: or implement an interface
// For val
interface ReadOnlyProperty<in R, out T> {
operator fun getValue(thisRef: R, prop: KProperty<*>): T
}
// For var
interface ReadWriteProperty<in R, T> {
operator fun getValue(thisRef: R, prop: KProperty<*>): T
operator fun setValue(thisRef: R, prop: KProperty<*>, value: T)
}

23. Read Only Properties
class Person {
val name: String by NameVal()
}
class NameVal: ReadOnlyProperty<Person, String> {
override fun getValue(thisRef: Person, prop: KProperty<*>): String {
return "$thisRef, thank you for delegating '${prop.name}' to me!"
}
}

24. Read-Write Properties
class Person {
var name: String by NameVar()
}
class NameVar: ReadWriteProperty<Person, String> {
override fun getValue(…): { … }
override fun setValue(thisRef: Person, prop: KProperty<*>, value: String) {
println("$value has been assigned to '${prop.name} in $thisRef.'")
}
}

25. This is generated by the compiler insteadpublic final class Person {
// $FF: synthetic field
static final KProperty[] $$delegatedProperties = new KProperty[]{ /* Nightmare */ }
@NotNull private final NameVar name$delegate = new NameVar();
@NotNull public final String getName() {
return this.name$delegate.getValue(this, $$delegatedProperties[0]);
}
public final void setName(@NotNull String var1) {
Intrinsics.checkParameterIsNotNull(var1, "<set-?>");
this.name$delegate.setValue(this, $$delegatedProperties[0], var1);
}
}

26. KProperty<*>
×Provides meta information about property:
×name
×annotations
×parameters
×returnType
×… etc
Depends on
kotlin-reflect 😭

27. Standard Delegates
Useful kinds of delegates
from stdlib

28. Lazy
class Person {
val name by lazy { "Dmytro" }
}

29. observable
class Person {
var name by Delegates.observable("Dmytro") {
prop: KProperty<*>, old, new ->
println("$old -> $new")
}
}

30. Vetoable
class Person {
var name by Delegates.vetoable("Dmytro") {
prop: KProperty<*>, old, new ->
new.startsWith("D")
}
}

31. Vetoable
class Person {
var name by Delegates.vetoable("Dmytro") {
prop: KProperty<*>, old, new ->
new.startsWith("D")
}
}

32. NotNull
class Person {
var name by Delegates.notNull<String>()
}

33. Map
class Person(map: Map<String, Any?>) {
val name: String by map
val age: Int by map
}
class PersonWrapper(map: Map<String, Any?>) {
val person: Person by map
}

34. Real-life Use cases

36. Vanilla Android
class PersonActivity : Activity() {
private lateinit var firstName: TextView
private var lastName: TextView by Delegates.notNull()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
firstName = findViewById(R.id.first_name) as TextView
lastName = findViewById(R.id.last_name) as TextView
}
}

37. Butterknife/Kotterknife
class PersonActivity : Activity() {
private val firstName: TextView by bindView(R.id.first_name)
private val lastName: TextView by bindView(R.id.last_name)
}

38. Butterknife/Kotterknife
fun <V : View> Activity.bindView(id: Int): ReadOnlyProperty<Activity, V> {
return object : ReadOnlyProperty<View, V> {
private var value: Any? = EMPTY
override fun getValue(thisRef: Activity, property: KProperty<*>): V {
if (value == EMPTY) {
value = thisRef.findViewById(id) as V? ?: throw IllegalStateException()
}
return value as V
}
}
}

39. What else?

40. For example
×Build type-safe APIs on top of unsafe APIs:
SharedPreferences, Bundle, Intent, Cursor
×DI without annotation processing: KOIN,
injekt etc

41. Links
× https://kotlinlang.org/docs/reference/
delegation.html
× https://kotlinlang.org/docs/reference/
delegated-properties.html

42. THANKS!
Any questions?
You can find me at @DmitriyZaitsev
😉