Kotlin Design Pattern: Observer/Listener

NOTE:
Before reading, you should know about 'Delegate Properties' in Kotlin.
For more information, please find:
http://kotlinlang.org/docs/reference/delegated-properties.html

Observer

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.

Setup:

• Observer
• Subject
• Client

Observer Example:

import kotlin.properties.Delegates

// Step 4: Client code

fun main(args: Array<String>) {
    val chemeleon = Chemeleon()

    chemeleon.observer = LogChemeleonColorObserver()
    chemeleon.color = "RED"
    chemeleon.color = "GREEN"
    chemeleon.color = "GREY"
}

// Step 1: Create an observer interface

interface ChemeleonColorObserver{
    fun onColorChange(newColor: String)
}

// Step 2: Implement concrete observer class

class LogChemeleonColorObserver: ChemeleonColorObserver{
    override fun onColorChange(newColor: String) {
        println("Look, the chemeleon changes to $newColor")
    }
}

// Step 3: Create subject class

class Chemeleon {
    var observer: ChemeleonColorObserver? = null

    var color: String by Delegates.observable(""){
        property, oldValue, newValue ->  observer?.onColorChange(newValue)
    }
}

/*
Prints:

Look, the chemeleon changes to RED
Look, the chemeleon changes to GREEN
Look, the chemeleon changes to GREY
 */

Listener Example

import kotlin.properties.Delegates

// Step 4: Client usage example

fun main(args: Array<String>){
    val textView = TextView()

    textView.listener = PrintingTextChangedListener()
    textView.text = "Kotlin"
    textView.text = "java"
}

// Step 1: Create a listener interface

interface TextChangedListener {
    fun onTextChanged(newText: String)
}

// Step 2: Implement a listener class

class PrintingTextChangedListener: TextChangedListener{
    override fun onTextChanged(newText: String) {
        println("The text has been changed to $newText")
    }
}

// Step 3: Create an subject class

class TextView {
    var listener: TextChangedListener? = null

    var text: String by Delegates.observable(""){
        _, _, new -> listener?.onTextChanged(new)
    }
}

/*
prints

The text has been changed to Kotlin
The text has been changed to java
 */

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