Kommand

As described in Wikipedia, the command pattern ..

is a behavioral design pattern in which an object is used to encapsulate all information needed to perform an action or trigger an event at a later time.

In a nutshell, the Kommand library implements the command pattern in Kotlin, using Kotlin's coroutines to execute suspending command requests.

What problems does the Command pattern solve

• decoupling of the invoker of a request from a particular request
• configuring an object that invokes a request with a request

What solutions does the Command patern describe

• defines a separate command object that encapsulate a request
• invoker class delegates a request to a command object instead of implementing a particular request directly

This allows us to have a class that is not coupled to a particular request and is independent of how the request is carried out. The execution of a command, on the other hand, is done in a suspending function that may return immediately or suspend until the request is completes once it is triggered.

Kommand's API

interface Command {
    suspend fun execute(): Any
    suspend fun undo(): Any

    interface Invoker {
        fun undoCount(): Int
        fun redoCount(): Int

        fun undoPeek(): Command
        fun redoPeek(): Command

        suspend fun execute(cmd: Command): Any
        suspend fun undo(): Any
        suspend fun redo(): Any
        suspend fun cancel(): Any
    }
}

Class diagram of Kommand

Usage

private val cmdInvoker = CommandInvokerImpl()

...

    showBusy(true)
    cmdInvoker.execute(ChangeBackgroundColor(vg_root, newColor = getRandomColor(), oldColor = vg_root.backgroundColor))
    showBusy(false)
    
...

    showBusy(true)
    try {
        cmdInvoker.undo()
    } catch (e: NothingToUndoException) {
        Toast.makeText(this, "Nothing to undo", Toast.LENGTH_SHORT).show()
    }
    showBusy(false)
    
...

class ChangeBackgroundColor(private val view: View,
                            private val newColor: Int,
                            private val oldColor: Int) : Command {
    override suspend fun execute() {
        // Simulate some expensive work by delaying the actual execution of the command
        delay((1000..2000).random())
        view.backgroundColor = newColor
    }

    override suspend fun undo() {
        // Simulate some expensive work by delaying the actual execution of the command
        delay((1000..2000).random())
        view.backgroundColor = oldColor
    }
}

For a complete demonstration, please refer to the sample app and the unit tests in the library project.

Demonstration

• See the unit tests in the library project
• See the sample app

Note

Please note that the execute() and undo() return Any, according to the Command interface. Therefore, you can return the output of the request/command back to the caller. However, you would need to type cast from Any to your known object. Perhaps we can now benefit from (Kotlin contracts)[https://kotlinlang.org/docs/reference/whatsnew13.html] to make a smart cast, but I haven't looked into it yet in depth.

Example:

val result = cmdInvoker.execute(MyCommand(arg1,arg2..))
doSomethingWith(result as? MyOutputObject)

To do

• Documentation