Kotlin Design Pattern: 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.

Set up:

• Command Interface: Define the interface of various commands
• Concrete Command: Define behaviors of different commands
• Command Processor: Accept and execute commands
• Client Code

Example 1:

// Step 4: Client code

fun main(args: Array<String>) {
    CommandProcessor()
            .addOrder(OrderCoffee())
            .addOrder(OrderTea())
            .executeOrders()
}

// Step 1: Command Interface
interface Order{
    fun order()
    fun serve()
}

// Step 2: Concrete order classes

class OrderCoffee: Order{
    override fun order() {
        println("Please give me a cup of coffee.")
    }

    override fun serve() {
        println("Here is you coffee.")
    }
}

class OrderTea: Order{
    override fun order() {
        println("Please give me a cup of tea.")
    }

    override fun serve() {
        println("Here is your tea.")
    }
}

// Step 3: Invoker (Command Processor)

class CommandProcessor{

    val orders = ArrayList<Order>()

    fun addOrder(order: Order): CommandProcessor = apply{
        orders.add(order)
    }

    fun executeOrders(): CommandProcessor = apply{
        for (order in orders) {
            order.order()
            order.serve()
        }
    }
}

/**
        prints

        Please give me a cup of coffee.
        Here is you coffee.
        Please give me a cup of tea.
        Here is your tea.
 **/

Example 2:

// Step 4: Client code

fun main(args: Array<String>) {
    with(DoggyBrain()){
        receive(Sit())
        receive(StandUp())
        receive(RollOver())
        execute()
    }
}

// Step 1: Command interface

interface DogCommands{
    fun action()
}

// Step 2: Concrete commands

class Sit: DogCommands{
    override fun action() {
        println("Good doggy, sit put!")
    }
}

class StandUp: DogCommands{
    override fun action() {
        println("Good doggy, stand up!")
    }
}

class RollOver: DogCommands{
    override fun action() {
        println("Good doggy, roll over!")
    }
}

// Step 3: CommandProcessor

class DoggyBrain{
    val commands = ArrayList<DogCommands>()

    fun receive(command: DogCommands){
        commands.add(command)
        println(command.javaClass.simpleName + " in queue...")
    }

    fun execute(){
        for (command in commands)
            command.action()
    }

}

/**
        prints

        Sit in queue...
        StandUp in queue...
        RollOver in queue...
        Good doggy, sit put!
        Good doggy, stand up!
        Good doggy, roll over!
 **/

```