Hands-On Design Patterns with Kotlin

Undoing commands

While not directly related, one of the advantages of the Command design pattern is the ability to undo commands. What if we wanted to support such a functionality?

Undoing is usually very tricky, because it involves one of the following:

• Returning to the previous state (impossible if there's more than one client, requires a lot of memory)
• Computing deltas (tricky to implement)
• Defining opposite operations (not always possible)

In our case, the opposite of the command move from (0,0) to (0, 20) would be move from wherever you're now to (0,0). This could be achieved by storing a pair of commands:

private val commands = mutableListOf<Pair<Command, Command>>()

You can also add pairs of commands:

fun appendMove(x: Int, y: Int) ...