Exercise: Mutable lazy delegate
The lazy delegate can only be used to val variables. Implement a MutableLazy delegate that can be used to var variables. It should behave like a lazy delegate but supporting read-write properties. If the property getter is called before the setter, the MutableLazy delegate should initialize the property using its lambda expression. If the property getter is called after the setter, it should return the value that was set.
fun calculate(): Int {
print("Calculating... ")
return 42
}
var a by mutableLazy { calculate() }
println(a) // Calculating... 42
println(a) // 42
a = 1
println(a) // 1
var b by mutableLazy { calculate() }
b = 2
println(b) // 2
fun <T> mutableLazy(
initializer: () -> T
): ReadWriteProperty<Any?, T> = TODO()
This problem can either be solved in the below playground or you can clone kotlin-exercises project and solve it locally. In the project, you can find code template for this exercise in advanced/delegates/MutableLazy.kt. You can find there starting code and unit tests.
Once you are done with the exercise, you can check your solution here.
import org.junit.Assert
import org.junit.Test
import kotlin.properties.ReadWriteProperty
import kotlin.reflect.KProperty
import kotlin.system.measureTimeMillis
fun <T> mutableLazy(
initializer: () -> T
): ReadWriteProperty<Any?, T> = TODO()
class MutableLazyTest {
@Test
fun `Do not initialize if initialized`() {
val time = measureTimeMillis {
var game: Game? by mutableLazy { readGameFromSave() }
game = Game()
print(game)
}
assert(time in 0..100)
}
@Test
fun `Initializes if not initialized`() {
val time = measureTimeMillis {
val game: Game? by mutableLazy { readGameFromSave() }
print(game)
}
assert(time in 450..550)
}
@Test
fun `Do not initialize again if already initialized`() {
val time = measureTimeMillis {
val game: Game? by mutableLazy { readGameFromSave() }
print(game)
print(game)
print(game)
}
assert(time in 450..550)
}
@Test
fun `MutableLazy should accept nullable values`() {
val lazy by mutableLazy<String?> { null }
Assert.assertNull(lazy)
var lazy2 by mutableLazy<String?> { "A" }
lazy2 = null
Assert.assertNull(lazy2)
}
private class Game()
private fun readGameFromSave(): Game? {
Thread.sleep(500)
return Game()
}
}
Marcin Moskala is a highly experienced developer and Kotlin instructor as the founder of Kt. Academy, an official JetBrains partner specializing in Kotlin training, Google Developers Expert, known for his significant contributions to the Kotlin community. Moskala is the author of several widely recognized books, including "Effective Kotlin," "Kotlin Coroutines," "Functional Kotlin," "Advanced Kotlin," "Kotlin Essentials," and "Android Development with Kotlin."
Beyond his literary achievements, Moskala is the author of the largest Medium publication dedicated to Kotlin. As a respected speaker, he has been invited to share his insights at numerous programming conferences, including events such as Droidcon and the prestigious Kotlin Conf, the premier conference dedicated to the Kotlin programming language.
