Design Patterns In Kotlin ?? New article about testing: Unit Testing with Mockito 2 Project maintained by @dbacinski (Dariusz Baci?ski) Inspired by Design-Patterns-In-Swift by @nsmeme (Oktawian Chojnacki) Table of Contents Behavioral Patterns Observer / Listener Strategy Command State Chain of Responsibility Visitor Mediator Memento Creational Patterns Builder / Assembler Factory Method Singleton Abstract Factory Structural Patterns Adapter Decorator Facade Protection Proxy Composite Behavioral In software engineering, behavioral design patterns are design patterns that identify common communication patterns between objects and realize these patterns. By doing so, these patterns increase flexibility in carrying out this communication. Source: wikipedia.org Observer / Listener 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 { listeners.add( 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 Strategy 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 = " LOREM ipsum DOLOR sit amet " val lowerCasePrinter = Printer (lowerCaseFormatter) lowerCasePrinter.printString(inputString) val upperCasePrinter = Printer (upperCaseFormatter) upperCasePrinter.printString(inputString) val prefixPrinter = Printer { " Prefix: $it " } prefixPrinter.printString(inputString) Output lorem ipsum dolor sit amet LOREM IPSUM DOLOR SIT AMET Prefix: LOREM ipsum DOLOR sit amet Command 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 State 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 Chain of Responsibility 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 \" = \" dbacinski \"\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"="dbacinski" } Headers: ContentType: json Body: { "username"="dbacinski" } Visitor 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 Mediator 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 john = ChatUser (mediator, " John " ) mediator .addUser( ChatUser (mediator, " Alice " )) .addUser( ChatUser (mediator, " Bob " )) .addUser(john) john.send( " Hi everyone! " ) Output John: Sending Message= Hi everyone! Alice: Message received: Hi everyone! Bob: Message received: Hi everyone! Memento 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 Creational In software engineering, creational design patterns are design patterns that deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. The basic form of object creation could result in design problems or added complexity to the design. Creational design patterns solve this problem by somehow controlling this object creation. Source: wikipedia.org Builder / Assembler 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 // Let's assume that Dialog class is provided by external library. // We have only access to Dialog public interface which cannot 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 Factory Method 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() //Kotlin 1.0 could only be an inner class or object } object Spain : Country() //Kotlin 1.1 declared as top level class/ object in the same file class Greece ( val someProperty : String ) : Country() data class Canada ( val someProperty : String ) : Country() //Kotlin 1.1 data class extends other class // object Poland : 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 " ) } // try to add a new country Poland, it won't even compile without adding new branch to 'when' } 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 UK currency: No Currency Code Available Singleton 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 Abstract Factory 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 Structural In software engineering, structural design patterns are design patterns that ease the design by identifying a simple way to realize relationships between entities. Source: wikipedia.org Adapter 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 Decorator 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 modify 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 Facade 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 ( " dbacinski " ) userRepository.save(user) val resultUser = userRepository.findFirst() println ( " Found stored user: $resultUser " ) Ouput Reading data from file: /data/default.prefs Storing cached data: {USER_KEY=dbacinski} to file: /data/default.prefs Found stored user: User(login=dbacinski) Protection Proxy 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 " ) Ouput Incorrect password. Access denied! Password is correct: secret Reading file: readme.md Composite The composite pattern is used to compose zero-or-more similar objects so that they can be manipulated 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 ( 70 , " 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()) Ouput 600 Info Descriptions from: Gang of Four Design Patterns Reference Sheet
