Kotlin Design Pattern: 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 1:

// Step 3: Client code

fun main(args: Array<String>) {
    val context = Context()
    println(context.currentState)

    context.goWild()
    println(context.currentState)

    context.calmDown()
    print(context.currentState)
}

// Step 1: Define various state via "sealed class"

sealed class State {
    class RobertBruceBanner : State()
    class GreenHulk : State()
}


// Step 2: Create context class

class Context {
    private var state: State = State.RobertBruceBanner()

    fun goWild() {
        state = State.GreenHulk()
    }

    fun calmDown() {
        state = State.RobertBruceBanner()
    }

    val currentState: String
        get() = when(state){
            is State.RobertBruceBanner -> "I am calm."
            is State.GreenHulk -> "I am wild."
        }
}

/**
        prints

        I am calm.
        I am wild.
        I am calm.
        **/

Example 2:

My way of move depends on which ground I am on (a.k.a., my state):

• I walk on land;
• I swim in water;
• I skip on ice

So, behavior depends on state.

// Step 3: Client code

fun main(args: Array<String>) {
    val myContext = MyContext()

    myContext.state = MyState.StateOnLand()
    println(myContext)
    myContext.state.move()

    myContext.state = MyState.StateOnIce()
    println(myContext)
    myContext.state.move()

    myContext.state = MyState.StateInWater()
    println(myContext)
    myContext.state.move()
}

// Step 1: Define various states via sealed class

sealed class MyState{
    class StateOnLand: MyState(){
        override fun move() {
            println("I walk on land\n")
        }
    }

    class StateInWater: MyState(){
        override fun move(){
            println("I swim in water\n")
        }
    }

    class StateOnIce: MyState(){
        override fun move() {
            println("I skip on ice\n")
        }
    }
    abstract fun move()
}

// Step 2: Create a context class

class MyContext{
    var state: MyState = MyState.StateOnIce()
    override fun toString(): String {
        return state.javaClass.simpleName + ":"
    }
}

/**
        prints

        StateOnLand:
        I walk on land

        StateOnIce:
        I skip on ice

        StateInWater:
        I swim in water
        **/