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
        **/