Kotlin Delegates: Reduce the boilerplate

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.

Transcript

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 instead public 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