Somblog

Design Pattern / Behavioral

[Design Pattern] Command Pattern

A pattern that turns requests into objects and handles execution, undo, and queueing flexibly.

1 min read Read in Korean
  • Design Pattern
  • Behavioral
Contents

One-line pattern summary

A pattern that turns requests into objects and handles execution, undo, and queueing flexibly.

Typical Unity use cases

  • When implementing input replay or macros.
  • When Undo / Redo support is needed.

Parts (roles)

  • Command
  • Invoker
  • Receiver

Unity example (C#)

The code below is a simplified Unity example based on the scenario described above.

using System.Collections.Generic;

public interface ICommand
{
    void Execute();
    void Undo();
}

public sealed class MoveUnitCommand : ICommand
{
    private readonly Unit controlledUnit;
    private readonly int deltaX;

    public MoveUnitCommand(Unit controlledUnit, int deltaX)
    {
        this.controlledUnit = controlledUnit;
        this.deltaX = deltaX;
    }

    public void Execute() => controlledUnit.X += deltaX;
    public void Undo() => controlledUnit.X -= deltaX;
}

public sealed class CommandHistory
{
    private readonly Stack<ICommand> executedCommands = new();

    public void ExecuteCommand(ICommand command)
    {
        command.Execute();
        executedCommands.Push(command);
    }
}

Advantages

  • Behavior is separated into smaller units, which reduces the impact of changes.
  • Adding or swapping rules is relatively safe.

Things to watch out for

  • As the number of objects and indirect calls increases, the flow can become harder to follow.
  • Ordering bugs should be pinned down with tests.

Interaction diagram

This shows the flow where requests are encapsulated as objects so execution, queueing, and undo can be separated.

flowchart LR

  input["Player Input"]
  factory["Command Factory"]
  invoker["Invoker"]
  command["Concrete Command"]
  receiver["Receiver"]
  undo_stack["Undo Stack"]

  input --> factory
  factory --> command
  command -->|submit| invoker
  invoker -->|execute| receiver
  invoker -->|push| undo_stack
  undo_stack -->|undo| receiver

Comments