Exercise: Observable value

Your task is to implement the Observable class, which should hold a value and allow observing its changes. It should have a value property, which should be settable. It should also have a observe function, which should take a function that will be called whenever the value changes.

val observable = Observable(1)
println(observable.value) // 1
observable.observe { println("Changed to $it") }
observable.value = 2 // Changed to 2
println(observable.value) // 2
observable.observe { println("now it is $it") }
observable.value = 3
// Changed to 3
// now it is 3

Starting code:

class Observable<T>(initial: T) {
    var value: T = initial
}

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 functional/base/Observable.kt. You can find there starting code, example usage and unit tests.

Once you are done with the exercise, you can check your solution here.

Playground

import junit.framework.TestCase.assertTrue
import org.junit.Test
import kotlin.test.assertEquals

class Observable<T>(initial: T) {
    var value: T = initial
}

fun main() {
    val observable = Observable(1)
    println(observable.value) // 1
    observable.observe { println("Changed to $it") }
    observable.value = 2 // Changed to 2
    println(observable.value) // 2
    observable.observe { println("now it is $it") }
    observable.value = 3
    // Changed to 3
    // now it is 3
}

class ObservableTest {

    @Test
    fun `Observable should notify observers`() {
        val observable = Observable(1)
        var notified = false
        observable.observe { notified = true }
        observable.value = 2
        assertTrue(notified)
    }

    @Test
    fun `Observable should notify all observers`() {
        val observable = Observable(1)
        var notified1 = false
        var notified2 = false
        observable.observe { notified1 = true }
        observable.observe { notified2 = true }
        observable.value = 2
        assertTrue(notified1)
        assertTrue(notified2)
    }

    @Test
    fun `Observable should notify observers with new value`() {
        val observable = Observable(1)
        var value = 0
        observable.observe { value = it }
        observable.value = 2
        assertEquals(2, value)
    }
}