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How can you implement the command pattern in game development?

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The command pattern is a design pattern that allows you to encapsulate actions as objects and execute them later, undo them, or store them. It is useful for game development because it can simplify the implementation of complex input systems, game logic, and replay features. In this article, you will learn how to implement the command pattern in game development using C# as an example language.

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1 What is a command?

A command is an object that represents an action or a request. It has a method that executes the action, and optionally, a method that undoes the action. A command can also have parameters that define the details of the action, such as the target, the direction, or the speed. A command can be created by a user input, a game event, or a script.

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Harsahib Singh

AI Enthusiast | VR/AR Game Developer | Fin Tech Advocate | Passionate about AGI & ASI | Leadership in Product/Program Management
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While commands often represent actions, they are not the same. An action is the actual execution of the task, while a command is an instruction or request for that action to happen. Different Types of Commands: Commands can be classified further based on their behavior: Simple commands: Perform a single action without requiring context. Parameterized commands: Take parameters to define the specifics of the action (e.g., target, direction). Compound commands: Combine multiple simpler commands into a single unit for complex actions.

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Adam Martin

CTO/VPEng | AI, Data, Cloud, Graphics/VR | C#/Java/Python/Typescript/C++
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Commands aren't generally useful in game development: games require much tighter coupling between UI, game-logic, rendering, and other systems (physics etc). Commands are useful when you split the game into a "client" and "local server" (where the server is actually running on the same machine - this is not a network server). Commands allow you to decouple the Client from the Server ... and later if you add networking/multiplayer, you can have a different implementation of Server that sits on a physical / AWS server, but the game-logic doesn't need rewriting. This also lets you write code once for both singplayer and multiplayer, instead of maintaining two codebases.

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Zainul Abideen Iftikhar

Offering Game Development Services | Unreal & Unity
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A command can also have parameters that define the details of the action, such as the target, the direction, or the speed. A command can be created by a user input, a game event, or a script.

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2 How to create a command interface?

To implement the command pattern, you need to define a common interface for all commands. This interface should have a method called Execute that performs the action, and optionally, a method called Undo that reverses the action. For example, in C#, you can create an interface like this:

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

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Zainul Abideen Iftikhar

Offering Game Development Services | Unreal & Unity
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The provided ICommand interface offers a concise foundation for implementing the Command Pattern in C#. Its clarity and adherence to the pattern's principles promote flexible and maintainable code.

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3 How to create concrete commands?

To create concrete commands, you need to implement the command interface and provide the logic for the action and the undo. For example, you can create a command that moves a game object by a certain distance:

public class MoveCommand : ICommand
{
    private GameObject gameObject;
    private Vector3 distance;
    private Vector3 previousPosition;
    public MoveCommand(GameObject gameObject, Vector3 distance)
    {
        this.gameObject = gameObject;
        this.distance = distance;
    }
    public void Execute()
    {
        previousPosition = gameObject.transform.position;
        gameObject.transform.position += distance;
    }
    public void Undo()
    {
        gameObject.transform.position = previousPosition;
    }
}

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Zainul Abideen Iftikhar

Offering Game Development Services | Unreal & Unity
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By extending this pattern, one can create commands for rotation, scaling, color changes, audio control, visibility toggles, and more. This flexibility enables the encapsulation of various actions, promoting code modularity and maintainability in applications or games.

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4 How to execute commands?

To execute commands, you need to create an invoker that holds a reference to the command and calls its Execute method. The invoker can be a controller, a manager, or a handler that receives the commands from the user input, the game event, or the script. For example, you can create a controller that executes commands based on keyboard input:

public class Controller
{
    private ICommand upCommand;
    private ICommand downCommand;
    private ICommand leftCommand;
    private ICommand rightCommand;
    public Controller(ICommand upCommand, ICommand downCommand, ICommand leftCommand, ICommand rightCommand)
    {
        this.upCommand = upCommand;
        this.downCommand = downCommand;
        this.leftCommand = leftCommand;
        this.rightCommand = rightCommand;
    }
    public void Update()
    {
        if (Input.GetKeyDown(KeyCode.UpArrow))
        {
            upCommand.Execute();
        }
        if (Input.GetKeyDown(KeyCode.DownArrow))
        {
            downCommand.Execute();
        }
        if (Input.GetKeyDown(KeyCode.LeftArrow))
        {
            leftCommand.Execute();
        }
        if (Input.GetKeyDown(KeyCode.RightArrow))
        {
            rightCommand.Execute();
        }
    }
}

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Zainul Abideen Iftikhar

Offering Game Development Services | Unreal & Unity
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The invoker can be a controller, a manager, or a handler that receives the commands from the user input, the game event, or the script. For example, you can create a controller that executes commands based on keyboard input:

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5 How to undo commands?

To undo commands, you need to store them in a stack and pop them when you want to revert the action. The stack can be a part of the invoker or a separate object that manages the history of commands. For example, you can create a command manager that keeps track of the executed commands and allows undoing them:

public class CommandManager
{
    private Stack<ICommand> commandStack;
    public CommandManager()
    {
        commandStack = new Stack<ICommand>();
    }
    public void ExecuteCommand(ICommand command)
    {
        command.Execute();
        commandStack.Push(command);
    }
    public void UndoCommand()
    {
        if (commandStack.Count > 0)
        {
            ICommand command = commandStack.Pop();
            command.Undo();
        }
    }
}

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6 How to use the command pattern for replay features?

The command pattern can also be used to implement replay features in games. You can record the commands that are executed during the gameplay and store them in a file or a database. Then, you can replay the commands by reading them and executing them in the same order. This way, you can recreate the exact state and behavior of the game at any point in time.

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Adam Martin

CTO/VPEng | AI, Data, Cloud, Graphics/VR | C#/Java/Python/Typescript/C++
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If you want to add replays, the Command pattern provides 1% of the solution, and isn't important. 90% of the effort is to remove all non-determinism in your game logic: you must replace all Random calls with seeded-randoms (so that when you re-run the game, the same random numbers are generated, in the same order, every time). Where you want the game to be different each time, you use a seeded-Random to generate the seed for a second seeded-Random. You hide the first seed from the players, but this ensures the game is still Replay-safe (everything is seeded and nothing is random), but has the illusion of appearing random

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7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Harsahib Singh

AI Enthusiast | VR/AR Game Developer | Fin Tech Advocate | Passionate about AGI & ASI | Leadership in Product/Program Management
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The command pattern offers various benefits: Undo/Redo: Easily reverse or replay actions. Queuing: Buffer commands for later execution, allowing for complex action sequences. Decoupling: Separates action logic from invokers and receivers, promoting modularity and maintainability.

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Adam Martin

CTO/VPEng | AI, Data, Cloud, Graphics/VR | C#/Java/Python/Typescript/C++
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The command pattern makes games more expensive and difficult to write - if you don't need it, avoid it. The reason we do it is for the large benefits in reduced code (you can share singleplayer and multiplayer code, instead of duplicating them), in debugging (you can test and debug multiplayer code locally, quicker), and in sharing/replays (it makes it easier to write deterministic code - but the determinism / removal of Random is the harder part here)

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How can you implement the command pattern in game development?

1

2

3

4

5

6

7

1 What is a command?

2 How to create a command interface?

3 How to create concrete commands?

4 How to execute commands?

5 How to undo commands?

6 How to use the command pattern for replay features?

7 Here’s what else to consider

Game Development

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How can you implement the command pattern in game development?

1

2

3

4

5

6

7

1 What is a command?

2 How to create a command interface?

3 How to create concrete commands?

4 How to execute commands?

5 How to undo commands?

6 How to use the command pattern for replay features?

7 Here’s what else to consider

Game Development

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