Tag: Design Patterns

Hello Readers, CoolMonkTechie heartily welcomes you in this article (Is Awesome Design Patterns Valuable In Kotlin?).

In this article, we will learn about why design patterns are valuable and frequently used in Kotlin. When we are new in programming languages, we don’t know which design patterns we should use with it and how to implement them. Design Patterns determine certain factors to differentiate between a good code and a bad code in Kotlin. This may be the code structure or the comments used or the variable names or something else. Being able to use a relevant design pattern is a prerequisite to creating functional, high-quality, and secure applications in Android with use of Kotlin. 

So every developer should follow Design Patterns while writing the Kotlin code of an Android application.

A famous quote about learning is :

” One learns from books and example only that certain things can be done. Actual learning requires that you do those things. “

So Let’s begin.

Design Patterns: What they are and why know them ?

A software design pattern is a solution to a particular problem we might face when designing an app’s architecture. But unlike out-of-the-box services or open-source libraries, we can’t paste a design pattern into our application because it isn’t a piece of code. Rather, it’s a general concept for how to solve a problem. A design pattern is a template that tells us how to write code, but it’s up to us to fit our code to this template.

Design patterns bring several benefits:

• Tested solutions. We don’t need to waste time and reinvent the wheel trying to solve a particular software development problem, as design patterns already provide the best solution and tell us how to implement it.
• Code unification. Design patterns provide us with typical solutions that have tested for drawbacks and bugs, helping us make fewer mistakes when designing our app architecture.
• Common vocabulary. Instead of providing in-depth explanations of how to solve this or that software development problem, we can say what design pattern we used and other developers will immediately understand what solutions we implemented.

Design Patterns: What is it ?

” A Design Pattern is a general, reusable solution to a commonly occurring problem within a given context. “

So, Design Patterns are a pattern solution that follows to solve a particular feature. These are the best practices that any programmer can use to build an application.

We use Design Patterns that makes our code easier to understand and more reusable in Android.

Design Patterns: Patterns Types In Kotlin

Before we describe the most common architecture patterns in Android Kotlin, we should first learn the three types of software design patterns and how they differ:

• Creational Design Patterns
• Structural Design Patterns
• Behavioral Design Patterns

1. Creational Design Patterns

Creational software design patterns deal with object creation mechanisms, which increase flexibility and reuse of existing code. They try to instantiate objects in a manner suitable for the particular situation. 

This Pattern is used to create some object without showing the logic or the steps that involves in creating the object. So, every time we want an object, we need not instantiate the object by using the new operator. So, this makes creating an object easier and can be easily created again and again.

Here are several creational design patterns:

• Builder Pattern
• Singleton Pattern
• Factory Method Pattern
• Abstract Factory

2. Structural Design Patterns

Structural design patterns aim to simplify the design by finding a simple way of realizing relationships between classes and objects. These patterns explain how to assemble objects and classes into larger structures while keeping these structures flexible and efficient.

In this Design Pattern, we concern about the structure of the code. Here, we follow some particular structural pattern that will help in understanding the code and the working of code just by looking at the structure of the code. These are some structural architecture patterns:

• Adapter Pattern
• Facade Pattern
• Decorator Pattern
• Composite Pattern
• Protection Proxy Pattern

3. Behavioral Design Patterns

Behaviour design patterns identify common communication patterns between entities and implement these patterns. This Patterns mainly tells how the objects of the classes will communicate with each other. These patterns help us in understanding the code in a better way because by viewing the code we can identify the pattern and then we can understand the code in a better way.

• Observer / Listener Pattern
• Command Pattern
• Strategy Pattern
• State Pattern
• Chain of Responsibility Pattern
• Visitor Pattern
• Mediator Pattern
• Memento Pattern

Most Frequently Used Design Patterns In Kotlin

We’re going to provide only the essential information about each software design pattern–namely, how it works from the technical point of view and when it should be applied. We’ll also give an illustrative example in the Kotlin programming language.

1. Creational: Builder Pattern

The builder pattern is used to create complex objects with constituent parts that must be created in the same order or using a specific algorithm. An external class controls the construction algorithm.

Example

For example, Let’s assume that external library provides Dialog class, and we have only accessed to Dialog Public interface, which can not be changed.

class Dialog {

    fun showTitle() = println("showing title")

    fun setTitle(text: String) = println("setting title text $text")

    fun setTitleColor(color: String) = println("setting title color $color")

    fun showMessage() = println("showing message")

    fun setMessage(text: String) = println("setting message $text")

    fun setMessageColor(color: String) = println("setting message color $color")

    fun showImage(bitmapBytes: ByteArray) = println("showing image with size ${bitmapBytes.size}")

    fun show() = println("showing dialog $this")
}

//Builder:
class DialogBuilder() {
    constructor(init: DialogBuilder.() -> Unit) : this() {
        init()
    }

    private var titleHolder: TextView? = null
    private var messageHolder: TextView? = null
    private var imageHolder: File? = null

    fun title(init: TextView.() -> Unit) {
        titleHolder = TextView().apply { init() }
    }

    fun message(init: TextView.() -> Unit) {
        messageHolder = TextView().apply { init() }
    }

    fun image(init: () -> File) {
        imageHolder = init()
    }

    fun build(): Dialog {
        val dialog = Dialog()

        titleHolder?.apply {
            dialog.setTitle(text)
            dialog.setTitleColor(color)
            dialog.showTitle()
        }

        messageHolder?.apply {
            dialog.setMessage(text)
            dialog.setMessageColor(color)
            dialog.showMessage()
        }

        imageHolder?.apply {
            dialog.showImage(readBytes())
        }

        return dialog
    }

    class TextView {
        var text: String = ""
        var color: String = "#00000"
    }
}

Usage

//Function that creates dialog builder and builds Dialog
fun dialog(init: DialogBuilder.() -> Unit): Dialog {
    return DialogBuilder(init).build()
}

val dialog: Dialog = dialog {
	title {
    	text = "Dialog Title"
    }
    message {
        text = "Dialog Message"
        color = "#333333"
    }
    image {
        File.createTempFile("image", "jpg")
    }
}

dialog.show()

Output

setting title text Dialog Title
setting title color #00000
showing title
setting message Dialog Message
setting message color #333333
showing message
showing image with size 0
showing dialog Dialog@5f184fc6

AlertDialog Example

One of the common examples of Builder pattern that we all use in our daily life is that of AlertDialog. In AleartDialog, we call only the required methods like:

AlertDialog.Builder(this)
    .setTitle("This is a title")
    .setMessage("This is some message")
    .show()

2. Creational: Singleton Pattern

The singleton pattern ensures that only one object of a particular class is ever created. All further references to objects of the singleton class refer to the same underlying instance. There are very few applications, do not overuse this pattern!

Example

object PrinterDriver {
    init {
        println("Initializing with object: $this")
    }

    fun print() = println("Printing with object: $this")
}

Usage

println("Start")
PrinterDriver.print()
PrinterDriver.print()

Output

Start
Initializing with object: PrinterDriver@6ff3c5b5
Printing with object: PrinterDriver@6ff3c5b5
Printing with object: PrinterDriver@6ff3c5b5

3. Creational: Factory Method Pattern

The factory pattern is used to replace class constructors, abstracting the process of object generation so that the type of the object instantiated can be determined at run-time.

Example

sealed class Country {
    object USA : Country()
}

object Spain : Country() 
class Greece(val someProperty: String) : Country()
data class Canada(val someProperty: String) : Country() 

class Currency(
    val code: String
)

object CurrencyFactory {

    fun currencyForCountry(country: Country): Currency =
        when (country) {
            is Greece -> Currency("EUR")
            is Spain -> Currency("EUR")
            is Country.USA -> Currency("USD")
            is Canada -> Currency("CAD")
        }  
}

Usage

val greeceCurrency = CurrencyFactory.currencyForCountry(Greece("")).code
println("Greece currency: $greeceCurrency")

val usaCurrency = CurrencyFactory.currencyForCountry(Country.USA).code
println("USA currency: $usaCurrency")

assertThat(greeceCurrency).isEqualTo("EUR")
assertThat(usaCurrency).isEqualTo("USD")

Output

Greece currency: EUR
US currency: USD

4. Creational: Abstract Factory Pattern

The abstract factory pattern is used to provide a client with a set of related or dependant objects. The “family” of objects created by the factory are determined at run-time.

Example

interface Plant

class OrangePlant : Plant

class ApplePlant : Plant

abstract class PlantFactory {
    abstract fun makePlant(): Plant

    companion object {
        inline fun <reified T : Plant> createFactory(): PlantFactory = when (T::class) {
            OrangePlant::class -> OrangeFactory()
            ApplePlant::class  -> AppleFactory()
            else               -> throw IllegalArgumentException()
        }
    }
}

class AppleFactory : PlantFactory() {
    override fun makePlant(): Plant = ApplePlant()
}

class OrangeFactory : PlantFactory() {
    override fun makePlant(): Plant = OrangePlant()
}

Usage

val plantFactory = PlantFactory.createFactory<OrangePlant>()
val plant = plantFactory.makePlant()
println("Created plant: $plant")

Output

Created plant: OrangePlant@4f023edb

5. Structural: Adapter Pattern

The adapter pattern is used to provide a link between two otherwise incompatible types by wrapping the “adaptee” with a class that supports the interface required by the client.

Example

interface Temperature {
    var temperature: Double
}

class CelsiusTemperature(override var temperature: Double) : Temperature

class FahrenheitTemperature(var celsiusTemperature: CelsiusTemperature) : Temperature {

    override var temperature: Double
        get() = convertCelsiusToFahrenheit(celsiusTemperature.temperature)
        set(temperatureInF) {
            celsiusTemperature.temperature = convertFahrenheitToCelsius(temperatureInF)
        }

    private fun convertFahrenheitToCelsius(f: Double): Double = (f - 32) * 5 / 9

    private fun convertCelsiusToFahrenheit(c: Double): Double = (c * 9 / 5) + 32
}

Usage

val celsiusTemperature = CelsiusTemperature(0.0)
val fahrenheitTemperature = FahrenheitTemperature(celsiusTemperature)

celsiusTemperature.temperature = 36.6
println("${celsiusTemperature.temperature} C -> ${fahrenheitTemperature.temperature} F")

fahrenheitTemperature.temperature = 100.0
println("${fahrenheitTemperature.temperature} F -> ${celsiusTemperature.temperature} C")

Output

36.6 C -> 97.88000000000001 F
100.0 F -> 37.77777777777778 C

6. Structural: Facade Pattern

The facade pattern is used to define a simplified interface to a more complex subsystem.

Example

class ComplexSystemStore(val filePath: String) {

    init {
        println("Reading data from file: $filePath")
    }

    val store = HashMap<String, String>()

    fun store(key: String, payload: String) {
        store.put(key, payload)
    }

    fun read(key: String): String = store[key] ?: ""

    fun commit() = println("Storing cached data: $store to file: $filePath")
}

data class User(val login: String)

//Facade:
class UserRepository {
    val systemPreferences = ComplexSystemStore("/data/default.prefs")

    fun save(user: User) {
        systemPreferences.store("USER_KEY", user.login)
        systemPreferences.commit()
    }

    fun findFirst(): User = User(systemPreferences.read("USER_KEY"))
}

Usage

val userRepository = UserRepository()
val user = User("coolmonktechie")
userRepository.save(user)
val resultUser = userRepository.findFirst()
println("Found stored user: $resultUser")

Output

Reading data from file: /data/default.prefs
Storing cached data: {USER_KEY=coolmonktechie} to file: /data/default.prefs
Found stored user: User(login=coolmonktechie)

7. Structural: Decorator Pattern

The decorator pattern is used to extend or alter the functionality of objects at run-time by wrapping them in an object of a decorator class. This provides a flexible alternative to using inheritance to change behaviour.

Example

interface CoffeeMachine {
    fun makeSmallCoffee()
    fun makeLargeCoffee()
}

class NormalCoffeeMachine : CoffeeMachine {
    override fun makeSmallCoffee() = println("Normal: Making small coffee")

    override fun makeLargeCoffee() = println("Normal: Making large coffee")
}

//Decorator:
class EnhancedCoffeeMachine(val coffeeMachine: CoffeeMachine) : CoffeeMachine by coffeeMachine {

    // overriding behaviour
    override fun makeLargeCoffee() {
        println("Enhanced: Making large coffee")
        coffeeMachine.makeLargeCoffee()
    }

    // extended behaviour
    fun makeCoffeeWithMilk() {
        println("Enhanced: Making coffee with milk")
        coffeeMachine.makeSmallCoffee()
        println("Enhanced: Adding milk")
    }
}

Usage

val normalMachine = NormalCoffeeMachine()
    val enhancedMachine = EnhancedCoffeeMachine(normalMachine)

    // non-overridden behaviour
    enhancedMachine.makeSmallCoffee()
    // overriding behaviour
    enhancedMachine.makeLargeCoffee()
    // extended behaviour
    enhancedMachine.makeCoffeeWithMilk()

Output

Normal: Making small coffee

Enhanced: Making large coffee
Normal: Making large coffee

Enhanced: Making coffee with milk
Normal: Making small coffee
Enhanced: Adding milk

8. Structural: Composite Pattern

The composite pattern is used to compose zero-or-more similar objects so it can manipulate them as one object.

Example

open class Equipment(private var price: Int, private var name: String) {
    open fun getPrice(): Int = price
}


/*
[composite]
*/

open class Composite(name: String) : Equipment(0, name) {
    val equipments = ArrayList<Equipment>()

    fun add(equipment: Equipment) {
        this.equipments.add(equipment)
    }

    override fun getPrice(): Int {
        return equipments.map { it.getPrice() }.sum()
    }
}


/*
 leafs
*/

class Cabbinet : Composite("cabbinet")
class FloppyDisk : Equipment(80, "Floppy Disk")
class HardDrive : Equipment(250, "Hard Drive")
class Memory : Equipment(280, "Memory")

Usage

var cabbinet = Cabbinet()
cabbinet.add(FloppyDisk())
cabbinet.add(HardDrive())
cabbinet.add(Memory())
println(cabbinet.getPrice())

Output

610

9. Structural: Protection Proxy Pattern

The proxy pattern is used to provide a surrogate or placeholder object, which references an underlying object. Protection proxy is restricting access.

Example

interface File {
    fun read(name: String)
}

class NormalFile : File {
    override fun read(name: String) = println("Reading file: $name")
}

//Proxy:
class SecuredFile : File {
    val normalFile = NormalFile()
    var password: String = ""

    override fun read(name: String) {
        if (password == "secret") {
            println("Password is correct: $password")
            normalFile.read(name)
        } else {
            println("Incorrect password. Access denied!")
        }
    }
}

Usage

val securedFile = SecuredFile()
securedFile.read("readme.md")

securedFile.password = "secret"
securedFile.read("readme.md")

Output

Incorrect password. Access denied!
Password is correct: secret
Reading file: readme.md

10. Behavioral: Observer / Listener Pattern

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.

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 = "old name"
    text = "new name"
}

Output

Text is changed <empty> -> old name
Text is changed old name -> new name

11. Behavioral: Command Pattern

The command pattern is used to express a request, including the call to be made and all of its required parameters, in a command object. The command may then be executed immediately or held for later use.

Example

interface OrderCommand {
    fun execute()
}

class OrderAddCommand(val id: Long) : OrderCommand {
    override fun execute() = println("Adding order with id: $id")
}

class OrderPayCommand(val id: Long) : OrderCommand {
    override fun execute() = println("Paying for order with id: $id")
}

class CommandProcessor {

    private val queue = ArrayList<OrderCommand>()

    fun addToQueue(orderCommand: OrderCommand): CommandProcessor =
        apply {
            queue.add(orderCommand)
        }

    fun processCommands(): CommandProcessor =
        apply {
            queue.forEach { it.execute() }
            queue.clear()
        }
}

Usage

CommandProcessor()
    .addToQueue(OrderAddCommand(1L))
    .addToQueue(OrderAddCommand(2L))
    .addToQueue(OrderPayCommand(2L))
    .addToQueue(OrderPayCommand(1L))
    .processCommands()

Output

Adding order with id: 1
Adding order with id: 2
Paying for order with id: 2
Paying for order with id: 1

12. Behavioral: Strategy Pattern

The strategy pattern is used to create an interchangeable family of algorithms from which the required process is chosen at run-time.

Example

class Printer(private val stringFormatterStrategy: (String) -> String) {

    fun printString(string: String) {
        println(stringFormatterStrategy(string))
    }
}

val lowerCaseFormatter: (String) -> String = { it.toLowerCase() }
val upperCaseFormatter = { it: String -> it.toUpperCase() }

Usage

val inputString = "OLD name NEW name "

val lowerCasePrinter = Printer(lowerCaseFormatter)
lowerCasePrinter.printString(inputString)

val upperCasePrinter = Printer(upperCaseFormatter)
upperCasePrinter.printString(inputString)

val prefixPrinter = Printer { "Prefix: $it" }
prefixPrinter.printString(inputString)

Output

old name new name
OLD NAME NEW NAME
Prefix: OLD name NEW name

13. Behavioral: State Pattern

The state pattern is used to alter the behaviour of an object as its internal state changes. The pattern allows the class for an object to apparently change at run-time.

Example

sealed class AuthorizationState

object Unauthorized : AuthorizationState()

class Authorized(val userName: String) : AuthorizationState()

class AuthorizationPresenter {

    private var state: AuthorizationState = Unauthorized

    val isAuthorized: Boolean
        get() = when (state) {
            is Authorized -> true
            is Unauthorized -> false
        }

    val userName: String
        get() {
            val state = this.state //val enables smart casting of state
            return when (state) {
                is Authorized -> state.userName
                is Unauthorized -> "Unknown"
            }
        }

    fun loginUser(userName: String) {
        state = Authorized(userName)
    }

    fun logoutUser() {
        state = Unauthorized
    }

    override fun toString() = "User '$userName' is logged in: $isAuthorized"
}

Usage

val authorizationPresenter = AuthorizationPresenter()

authorizationPresenter.loginUser("admin")
println(authorizationPresenter)

authorizationPresenter.logoutUser()
println(authorizationPresenter)

Output

User 'admin' is logged in: true
User 'Unknown' is logged in: false

14. Behavioral: Chain of Responsibility Pattern

The chain of responsibility pattern is used to process varied requests, each of which may be dealt with by a different handler.

Example

interface HeadersChain {
    fun addHeader(inputHeader: String): String
}

class AuthenticationHeader(val token: String?, var next: HeadersChain? = null) : HeadersChain {

    override fun addHeader(inputHeader: String): String {
        token ?: throw IllegalStateException("Token should be not null")
        return inputHeader + "Authorization: Bearer $token\n"
            .let { next?.addHeader(it) ?: it }
    }
}

class ContentTypeHeader(val contentType: String, var next: HeadersChain? = null) : HeadersChain {

    override fun addHeader(inputHeader: String): String =
        inputHeader + "ContentType: $contentType\n"
            .let { next?.addHeader(it) ?: it }
}

class BodyPayload(val body: String, var next: HeadersChain? = null) : HeadersChain {

    override fun addHeader(inputHeader: String): String =
        inputHeader + "$body"
            .let { next?.addHeader(it) ?: it }
}

Usage

//create chain elements
val authenticationHeader = AuthenticationHeader("123456")
val contentTypeHeader = ContentTypeHeader("json")
val messageBody = BodyPayload("Body:\n{\n\"username\"=\"coolmonktechie\"\n}")

//construct chain
authenticationHeader.next = contentTypeHeader
contentTypeHeader.next = messageBody

//execute chain
val messageWithAuthentication =
    authenticationHeader.addHeader("Headers with Authentication:\n")
println(messageWithAuthentication)

val messageWithoutAuth =
    contentTypeHeader.addHeader("Headers:\n")
println(messageWithoutAuth)

Output

Headers with Authentication:
Authorization: Bearer 123456
ContentType: json
Body:
{
"username"="coolmonktechie"
}

Headers:
ContentType: json
Body:
{
"username"="coolmonktechie"
}

15. Behavioral: Visitor Pattern

The visitor pattern is used to separate a relatively complex set of structured data classes from the functionality that may be performed upon the data that they hold.

Example

interface ReportVisitable {
    fun <R> accept(visitor: ReportVisitor<R>): R
}

class FixedPriceContract(val costPerYear: Long) : ReportVisitable {
    override fun <R> accept(visitor: ReportVisitor<R>): R = visitor.visit(this)
}

class TimeAndMaterialsContract(val costPerHour: Long, val hours: Long) : ReportVisitable {
    override fun <R> accept(visitor: ReportVisitor<R>): R = visitor.visit(this)
}

class SupportContract(val costPerMonth: Long) : ReportVisitable {
    override fun <R> accept(visitor: ReportVisitor<R>): R = visitor.visit(this)
}

interface ReportVisitor<out R> {

    fun visit(contract: FixedPriceContract): R
    fun visit(contract: TimeAndMaterialsContract): R
    fun visit(contract: SupportContract): R
}

class MonthlyCostReportVisitor : ReportVisitor<Long> {

    override fun visit(contract: FixedPriceContract): Long =
        contract.costPerYear / 12

    override fun visit(contract: TimeAndMaterialsContract): Long =
        contract.costPerHour * contract.hours

    override fun visit(contract: SupportContract): Long =
        contract.costPerMonth
}

class YearlyReportVisitor : ReportVisitor<Long> {

    override fun visit(contract: FixedPriceContract): Long =
        contract.costPerYear

    override fun visit(contract: TimeAndMaterialsContract): Long =
        contract.costPerHour * contract.hours

    override fun visit(contract: SupportContract): Long =
        contract.costPerMonth * 12
}

Usage

val projectAlpha = FixedPriceContract(costPerYear = 10000)
val projectGamma = TimeAndMaterialsContract(hours = 150, costPerHour = 10)
val projectBeta = SupportContract(costPerMonth = 500)
val projectKappa = TimeAndMaterialsContract(hours = 50, costPerHour = 50)

val projects = arrayOf(projectAlpha, projectBeta, projectGamma, projectKappa)

val monthlyCostReportVisitor = MonthlyCostReportVisitor()

val monthlyCost = projects.map { it.accept(monthlyCostReportVisitor) }.sum()
println("Monthly cost: $monthlyCost")
assertThat(monthlyCost).isEqualTo(5333)

val yearlyReportVisitor = YearlyReportVisitor()
val yearlyCost = projects.map { it.accept(yearlyReportVisitor) }.sum()
println("Yearly cost: $yearlyCost")
assertThat(yearlyCost).isEqualTo(20000)

Output

Monthly cost: 5333
Yearly cost: 20000

16. Behavioral: Mediator Pattern

Mediator design pattern is used to provide a centralized communication medium between different objects in a system. This pattern is very helpful in an enterprise application where multiple objects are interacting with each other.

Example

class ChatUser(private val mediator: ChatMediator, val name: String) {
    fun send(msg: String) {
        println("$name: Sending Message= $msg")
        mediator.sendMessage(msg, this)
    }

    fun receive(msg: String) {
        println("$name: Message received: $msg")
    }
}

class ChatMediator {

    private val users: MutableList<ChatUser> = ArrayList()

    fun sendMessage(msg: String, user: ChatUser) {
        users
            .filter { it != user }
            .forEach {
                it.receive(msg)
            }
    }

    fun addUser(user: ChatUser): ChatMediator =
        apply { users.add(user) }

}

Usage

val mediator = ChatMediator()
val user1 = ChatUser(mediator, "User1")

mediator
    .addUser(ChatUser(mediator, "User2"))
    .addUser(ChatUser(mediator, "User3"))
    .addUser(user1)
user1.send("Hello everyone!")

Output

User1: Sending Message= Hello everyone!
User2: Message received: Hello everyone!
user3: Message received: Hello everyone!

17. Behavioral: Memento Pattern

The memento pattern is a software design pattern that provides the ability to restore an object to its previous state (undo via rollback).

Example

data class Memento(val state: String)

class Originator(var state: String) {

    fun createMemento(): Memento {
        return Memento(state)
    }

    fun restore(memento: Memento) {
        state = memento.state
    }
}

class CareTaker {
    private val mementoList = ArrayList<Memento>()

    fun saveState(state: Memento) {
        mementoList.add(state)
    }

    fun restore(index: Int): Memento {
        return mementoList[index]
    }
}

Usage

val originator = Originator("initial state")
val careTaker = CareTaker()
careTaker.saveState(originator.createMemento())

originator.state = "State #1"
originator.state = "State #2"
careTaker.saveState(originator.createMemento())

originator.state = "State #3"
println("Current State: " + originator.state)
assertThat(originator.state).isEqualTo("State #3")

originator.restore(careTaker.restore(1))
println("Second saved state: " + originator.state)
assertThat(originator.state).isEqualTo("State #2")

originator.restore(careTaker.restore(0))
println("First saved state: " + originator.state)

Output

Current State: State #3
Second saved state: State #2
First saved state: initial state

That’s all about in this article.

Related Other Articles / Posts

• iOS – Why Are Architecture Patterns Important In iOS Application ?
• iOS – Is Awesome Design Pattern Valuable In Swift ?
• iOS – Is SOLID Valuable Design Principles Applicable For Swift ?

Conclusion

In this article, we understood about why design patterns are valuable and most frequently used in Kotlin. This article shows the most frequently used design patterns in Kotlin with an authentic example.

Thanks for reading! I hope you enjoyed and learned about Valuable Design Patterns concepts in Kotlin. Reading is one thing, but the only way to master it is to do it yourself.

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