Forgotten traits — Михаил Розумянский (Joom)

В этом докладе Михаил расскажет про подходы к сокращению количества boilerplate-кода при помощи таких средств языка Kotlin, как интерфейсы с частичной реализацией и делегаты классов. Продемонстрирует, как при помощи данных средств можно имитировать примеси (mixins), а также покажет, какие изменения в языке могли бы существенно улучшить текущую ситуацию.

Kotlin Night Moscow
25/11/2017

Transcript

1. FORGOTTEN TRAITS Михаил Розумянский 1
2. ПРИМЕСЬ (MIXIN) Subclass Mixin Base Class Mixin 2
3. TRAIT Отсутствие состояния Ограничение базового класса Переопределение методов базового класса Вызов методов базового класса Модификаторы доступа 3
4. ОБЪЯВЛЕНИЕ trait CollectionWithComparator<E : Comparable<E>> : AbstractCollection<E> { protected val comparator: Comparator<E> override fun contains(element: E): Boolean { return any { comparator.compare(it, element) == 0 } } } 4
5. ИСПОЛЬЗОВАНИЕ class CaseInsensitiveStringCollection : AbstractCollection<String>(), CollectionWithComparator<String> { override val comparator = String.CASE_INSENSITIVE_ORDER // ... } 5
6. ЧЕМ ЗАМЕНИТЬ? 6
7. НИЧЕМ 7
8. А НА САМОМ ДЕЛЕ? 8
9. ЗАДАЧА Manager Worker Employee delegates implements implements 9
10. ДЕЛЕГАТ Manager Worker Employee delegates implements implements 10
12. СОТРУДНИК interface Employee { fun performTask(task: String) } abstract class AbstractEmployee(val name: String) : Employee { fun say(text: String, vararg arguments: String) { Output.write(name, text, *arguments) } } 12
13. JAVA WAY 13
14. WORKER public class Worker extends AbstractEmployee { public Worker(final String name) { super(name); } @Override public void performTask(final String task) { say("I had to %s and I've just done it.", task); } } 14
15. MANAGER public class Manager extends AbstractEmployee { private final Employee delegate = new Worker("Dilbert"); public Manager(final String name) { super(name); } @Override public void performTask(@NotNull final String task) { delegate.performTask(task); } public void manageTask(@NotNull final String task) { say("Hey, %s.", task); performTask(task); say("It's just a piece of cake to %s.", task); } } 15
16. MAIN public class Demo { public static void main(final String[] args) { final Manager manager = new Manager("Boss"); for (final String task : args) { manager.manageTask(task); } } } 16
17. Boss Hey, prepare a presentation. Dilbert I had to prepare a presentation and I've just done it. Boss It's just a piece of cake to prepare a presentation. 17
18. KOTLIN WAY 18
19. WORKER class Worker(name: String) : AbstractEmployee(name) { override fun performTask(task: String) { say("I had to $task and I've just done it.") } } 19
20. MANAGER class Manager( name: String ) : AbstractEmployee(name), Employee by Worker("Dilbert") { fun manageTask(task: String) { say("Hey, $task.") performTask(task) say("It's just a piece of cake to $task.") } } 20
21. ЧТО ВНУТРИ? public final class Manager extends AbstractEmployee { private final Worker $$delegate_0 = new Worker("Dilbert"); public Manager(@NotNull String name) { super(name); } @Override public void performTask(@NotNull String task) { $$delegate_0.performTask(task); } public final void manageTask(@NotNull String task) { say("Hey, " + task + '.'); performTask(task); say("It's just a piece of cake to " + task + '.'); } } 21
22. ДОСТУП К ДЕЛЕГАТУ 22
23. НОВЫЙ MANAGER class Manager private constructor( name: String, private val delegate: Employee ) : AbstractEmployee(name), Employee by delegate { constructor(name: String) : this(name, Worker("Dilbert")) override fun performTask(task: String) { say("Hey, $task.") delegate.performTask(task) say("It's just a piece of cake to $task.") } } 23
24. МОГЛО БЫ БЫТЬ И ЛУЧШЕ — Distinguish syntactically when delegating to a property — Provide a way to reference a delegate in class body KT-83 KT-18427 24
25. ИЗМЕНЕНИЕ ДЕЛЕГАТА Manager Worker 1 Manager Worker 2 25
26. НАИВНЫЙ ПОДХОД private var delegate: Employee fireAndHire() private fun fireAndHire() { say("Seems I need to fire this guy and hire someone else.") delegate = Worker(Names.next()) } class Manager private constructor( name: String, ) : AbstractEmployee(name), Employee by delegate { constructor(name: String) : this(name, Worker(Names.next())) fun manageTask(task: String) { say("Hey, $task.") performTask(task) say("It's just a piece of cake to $task.") } } 26
27. Boss Hey, prepare a presentation. Dilbert I had to prepare a presentation and I've just done it. Boss It's just a piece of cake to prepare a presentation. Seems I need to fire this guy and hire someone else. Hey, write a speech. Dilbert I had to write a speech and I've just done it. Boss It's just a piece of cake to write a speech. Seems I need to fire this guy and hire someone else. 27
28. НО ПОЧЕМУ? 28
29. ДЕКОМПИЛИРУЕМ private Employee delegate; private final Employee $$delegate_0; $$delegate_0 = delegate; delegate = delegate; $$delegate_0.performTask(task); delegate = new Worker(Names.INSTANCE.next()); public final class Manager extends AbstractEmployee { private Manager(String name, Employee delegate) { super(name); } public Manager(@NotNull String name) { this(name, new Worker(Names.INSTANCE.next()); } public void performTask(@NotNull String task) { } private final void fireAndHire() { say("Seems I need to fire this guy and hire someone else."); } } 29
30. ОКАЗЫВАЕТСЯ Для делегата всегда создаётся поле — Delegation to var field KT-5870 30
31. И ЧТО ЖЕ ДЕЛАТЬ? 31
32. ОЧЕРЕДНОЙ MANAGER private var delegate = Worker(Names.next()) delegate.performTask(task) delegate = Worker(Names.next()) class Manager(name: String) : AbstractEmployee(name) { override fun performTask(task: String) { say("Hey, $task.") say("It's just a piece of cake to $task.") fireAndHire() } private fun fireAndHire() { say("Seems I need to fire this guy and hire someone else.") } } 32
33. ПРЯМО КАК В JAVA public class Manager extends AbstractEmployee { private final Employee delegate = new Worker(Names.INSTANCE.next()); public Manager(final String name) { super(name); } @Override public void performTask(task: String) { say("Hey, %s.", task); delegate.performTask(task); say("It's just a piece of cake to %s.", task); fireAndHire(); } private void fireAndHire() { say("Seems I need to fire this guy and hire someone else."); delegate = Worker(Names.INSTANCE.next()); } } 33
34. ИНТЕРФЕЙСЫ С ЧАСТИЧНОЙ РЕАЛИЗАЦИЕЙ interface DelegatingEmployee : Employee { val delegate: Employee override fun performTask(task: String) { delegate.performTask(task) } } 34
35. КАК РАБОТАЮТ? public interface DelegatingEmployee extends Employee { @NotNull Employee getDelegate(); void performTask(@NotNull String task); final class DefaultImpls { public static void performTask( @NotNull DelegatingEmployee self, @NotNull String task) { self.getDelegate().performTask(task); } } } 35
36. ВТОРАЯ ПОПЫТКА , DelegatingEmployee override var delegate: Employee = Worker(Names.next()) super.performTask(task) delegate = Worker(Names.next()) class Manager(name: String) : AbstractEmployee(name) { override fun performTask(task: String) { say("Hey, $task.") say("It's just a piece of cake to $task.") fireAndHire() } private fun fireAndHire() { say("Seems I need to fire this guy and hire someone else.") } } 36
37. Boss Hey, prepare a presentation. Dilbert I had to prepare a presentation and I've just done it. Boss It's just a piece of cake to prepare a presentation. Seems I need to fire this guy and hire someone else. Hey, write a speech. Wally I had to write a speech and I've just done it. Boss It's just a piece of cake to write a speech. Seems I need to fire this guy and hire someone else. 37
38. ПРЯМО КАК В JAVA 8 public class Manager extends AbstractEmployee, DelegatingEmployee { private final String name; private Employee delegate = new Worker(Names.INSTANCE.next()); public Manager(final String name) { super(name); } @Override public Employee getDeleagate() { return delegate; } @Override public void performTask(final String task) { say("Hey, %s.", task); super.performTask(task); say("It's just a piece of cake to %s.", task); fireAndHire(); } private void fireAndHire() { say("Seems I need to fire this guy and hire someone else."); delegate = Worker(Names.INSTANCE.next()); } } 38
39. ТОЛЬКО НЕМНОГО ХУЖЕ — Generate default methods for implementations in interfaces — Introduce JvmDefault annotation KT-4779 KT-19415 39
40. ДЕЛЕГАТ ДЕЛЕГАТА Manager Worker Expert implements uses 40
41. ПОЯВЛЯЕТСЯ ЭКСПЕРТ interface Expert { fun answer(question: String) } 41
42. РАБОТНИК СПРАШИВАЕТ ЭКСПЕРТА private val expert: Expert ask("What is the theme?") ask("When is the deadline?") private fun ask(question: String) { expert.answer(question) } class Worker( name: String, ) : AbstractEmployee(name) { override fun performTask(task: String) { say("I had to $task and I've just done it.") } say(question) } 42
43. ОЧЕВИДНОЕ РЕШЕНИЕ Employee by Worker("Dilbert", this), Expert override fun answer(question: String) { say(MotivationalQuotes.random()) } class Manager( name: String ) : AbstractEmployee(name), { fun manageTask(task: String) { say("Hey, $task.") performTask(task) say("It's just a piece of cake to $task.") } } 43
44. НЕ РАБОТАЕТ Error: 'this' is not defined in this context — Allow passing this to a class delegate KT-13603 44
45. ИСПРАВЛЯЕМ DelegatingEmployee override val delegate: Employee = Worker("Dilbert", this) class Manager( name: String ) : AbstractEmployee(name), , Expert { fun manageTask(task: String) { say("Hey, $task.") performTask(task) say("It's just a piece of cake to $task.") } override fun answer(question: String) { say(MotivationalQuotes.random()) } } 45
46. Boss Hey, prepare a presentation. Dilbert What is the theme? Boss To win in the marketplace you must first win in the workplace. Dilbert When is the deadline? Boss If it scares you, it might be a good thing to try. Dilbert I had to prepare a presentation and I've just done it. Boss It's just a piece of cake to prepare a presentation. 46
47. ОПЯТЬ КАК В JAVA 8 public class Manager extends AbstractEmployee implements DelegatingEmployee, Expert { private final String name; private final Employee delegate = new Worker("Dilbert", this); public Manager(final String name) { super(name); } public void manageTask(final String task) { say("Hey, %s.", task); performTask(task); say("It's just a piece of cake to %s.", task); } @Override public Employee getDelegate() { return delegate; } @Override public void answer(final String question) { say(MotivationalQuotes.INSTANCE.random()); } } 47
48. В РЕЗУЛЬТАТЕ Наличие состояния Ограничение использования Вызов методов контейнера Модификаторы доступа Отсутствие базового класса Переопределение методов базового класса базового класса 48
50. СПАСИБО ЗА ВНИМАНИЕ! 50