Kotlin delegates examples

A-Kotlin-Delegates.md

Some Kotlin delegates examples

• property delegates and class delegates

ListWithTrash.kt

class ListWithTrash<T>(
    private val innerList: MutableList<T> = ArrayList()
) : MutableList<T> by innerList {
    var deletedItem: T? = null

    fun recover() = deletedItem

    override fun remove(element: T): Boolean {
        deletedItem = element
        return innerList.remove(element)
    }

    override fun removeAt(index: Int): T {
        return innerList.removeAt(index).also {
            deletedItem = it
        }
    }
}

fun main() {
    val lista = ListWithTrash<String>(arrayListOf("One","Two","Three"))

    lista.removeAt(1)
    println(lista.recover())
}

PropertyDelegates.kt

import java.net.NetworkInterface
import kotlin.properties.Delegates
import kotlin.properties.ReadWriteProperty
import kotlin.reflect.KProperty


class PropertyDelegates {
    val stream: NetworkInterface by lazy {
        NetworkInterface.getNetworkInterfaces().asSequence().first { it.isUp }
    }

    var subscibers: Number by Delegates.observable(0) { property, oldValue, newValue ->
        println("${property.name} has been changed from $oldValue to $newValue")
    }

    var counter: Int by Delegates.vetoable(0){ property, oldValue, newValue ->
        return@vetoable if (newValue > oldValue) true else throw IllegalArgumentException("New value must be larger than old value")
    }

    var id: Int by Delegates.notNull()

    fun initialize(_id: Int){
        id = _id
    }
}

class Person(_firstName: String, _lastName: String){
    var firstName: String by NameFormatter(_firstName)
    var lastName: String by NameFormatter(_lastName)

}

class NameFormatter(initialValue: String): ReadWriteProperty<Person,String>{
    private var formattedValue: String = format(initialValue)
    override fun getValue(thisRef: Person, property: KProperty<*>): String = formattedValue

    override fun setValue(thisRef: Person, property: KProperty<*>, value: String) {
        formattedValue = format(value)
    }
    private fun format(value: String): String{
        return value.trim().lowercase().capitalize()
    }

}

fun main() {
    val person = Person("jOhnnY ","depp")

    println("${person.firstName} ${person.lastName}")
}

Reposetories.kt

data class Author(val id: String,val name: String,val books: List<String>)

data class Book(val id: String,val name: String, val author: String)

interface AuthorsRepository{
    fun getAuthorById(id: String): Author
    fun getAuthorByName(id: String): Author
    fun getAllAuthors(): List<Author>
}

interface BooksRepository{
    fun getBookById(id: String): Book
    fun getBookByName(id: String): Book
    fun getAllBooks(): List<Book>
}

class AuthorsRepositoryImpl: AuthorsRepository{
    val someAuthor = Author(id = "23rfdfsd",name = "Herman Smallville", listOf("4ffssds3"))
    override fun getAuthorById(id: String): Author {
        return someAuthor
    }

    override fun getAuthorByName(id: String): Author {
        return someAuthor
    }

    override fun getAllAuthors(): List<Author> {
        return listOf(someAuthor)
    }

}

class BooksRepositoryImpl: BooksRepository{
    val someBook = Book(id = "4ffssds3",name = "Bobby Dick", "23rfdfsd")

    override fun getBookById(id: String): Book {
        return someBook
    }

    override fun getBookByName(id: String): Book {
        return someBook
    }

    override fun getAllBooks(): List<Book> {
        return listOf(someBook)
    }
}

class CatalogRepository(
    private val booksRepository: BooksRepository,
    private val authorsRepository: AuthorsRepository
): BooksRepository by booksRepository, AuthorsRepository by authorsRepository{

    fun getBookAndAuthor(bookId: String): Pair<Book,Author>{
        val book = booksRepository.getBookById(bookId)
        val author = authorsRepository.getAuthorById(book.author)
        return book to author
    }
    fun getAllBooksByAuthorName(name: String): List<Book>{
        val author = authorsRepository.getAuthorByName(name)
        return author.books.map { booksRepository.getBookById(it) }
    }
}

fun main(){
    val catalogRepository = CatalogRepository(BooksRepositoryImpl(),AuthorsRepositoryImpl())

    catalogRepository.getAllBooksByAuthorName("")
}