Kotlin state pattern

An example of a state pattern in Kotlin.

Table of contents for design patterns in Kotlin

The state pattern is a behavioral software design pattern that allows an object to alter its behavior when its internal state changes. This pattern is close to the concept of finite-state machines. The state pattern can be interpreted as a strategy pattern, which is able to switch a strategy through invocations of methods defined in the pattern’s interface. Wikipedia

Example:

AuthorizationState

sealed class AuthorizationState

object Unauthorized : AuthorizationState()

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

AuthorizationModel

class AuthorizationModel {

    private var state: AuthorizationState = Unauthorized

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

    val userName: String
        get() = this.state.let {
            when (it) {
                is Authorized -> it.userName
                is Unauthorized -> "Unknown"
            }
        }

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

    fun logout() {
        state = Unauthorized
    }
}

Usage:

fun main() {
    val model = AuthorizationModel()
    println("${model.isAuthorized} ${model.userName}")
    model.login("admin")
    println("${model.isAuthorized} ${model.userName}")
}

Result

false Unknown
true admin

kotlin pattern