In this article, we will learn how to use and implement the observer pattern in Kotlin with an example.

The observer pattern is used to allow an object to publish changes to its state. Other objects subscribe to be immediately notified of any changes.

Observer / Listener Example

Example

interface TextChangedListener {

    fun onTextChanged(oldText: String, newText: String)
}

class PrintingTextChangedListener : TextChangedListener {
    
    private var text = ""
    
    override fun onTextChanged(oldText: String, newText: String) {
        text = "Text is changed: $oldText -> $newText"
    }
}

class TextView {

    val listeners = mutableListOf<TextChangedListener>()

    var text: String by Delegates.observable("<empty>") { _, old, new ->
        listeners.forEach { it.onTextChanged(old, new) }
    }
}

Usage

val textView = TextView().apply {
    listener = PrintingTextChangedListener()
}

with(textView) {
    text = "Lorem ipsum"
    text = "dolor sit amet"
}

Output

Text is changed <empty> -> Lorem ipsum
Text is changed Lorem ipsum -> dolor sit amet

Complete Example

Let's use JUnit to test this pattern:

import org.assertj.core.api.Assertions.assertThat
import org.junit.jupiter.api.Test
import kotlin.properties.Delegates

interface TextChangedListener {

    fun onTextChanged(oldText: String, newText: String)
}

class PrintingTextChangedListener: TextChangedListener {

    var text = ""

    override fun onTextChanged(oldText: String, newText: String) {
        text = "Text is changed: $oldText -> $newText"
    }
}

class TextView {

    val listeners = mutableListOf < TextChangedListener > ()

    var text: String by Delegates.observable("<empty>") {
        _,
        old,
        new - >
        listeners.forEach {
            it.onTextChanged(old, new)
        }
    }
}

class ListenerTest {

    @Test
    fun Listener() {
        val listener = PrintingTextChangedListener()

        val textView = TextView().apply {
            listeners.add(listener)
        }

        with(textView) {
            text = "Lorem ipsum"
            text = "dolor sit amet"
        }

        assertThat(listener.text).isEqualTo("Text is changed: Lorem ipsum -> dolor sit amet")
    }
}