Component based game engine design

Question

I have been looking at game engine design (specifically focused on 2d game engines, but also applicable to 3d games), and am interested in some information on how to go about it. I have heard that many engines are moving to a component based design nowadays rather than the traditional deep-object hierarchy.

Do you know of any good links with information on how these sorts of designs are often implemented? I have seen evolve your hierarchy, but I can't really find many more with detailed information (most of them just seem to say "use components rather than a hierarchy" but I have found that it takes a bit of effort to switch my thinking between the two models).

Any good links or information on this would be appreciated, and even books, although links and detailed answers here would be preferred.

Answer 1

Update 2013-01-07: If you want to see a good mix of component-based game engine with the (in my opinion) superior approach of reactive programming take a look at the V-Play engine. It very well integrates QTs QML property binding functionality.

We did some research on CBSE in games at our university and I collected some material over the years:

CBSE in games literature:

• Game Engine Architecture
• Game Programming Gems 4: A System for Managin Game Entities Game
• Game Programming Gems 5: Component Based Object Management
• Game Programming Gems 5: A Generic Component Library
• Game Programming Gems 6: Game Object Component System
• Object-Oriented Game Development
• Architektur des Kerns einer Game-Engine und Implementierung mit Java (german)

A very good and clean example of a component-based game-engine in C# is the Elephant game framework.

If you really want to know what components are read: Component-based Software Engineering! They define a component as:

A software component is a software element that conforms to a component model and can be independently deployed and composed without modification according to a composition standard.

A component model defines specific interaction and composition standards. A component model implementation is the dedicated set of executable software elements required to support the execution of components that conform to the model.

A software component infrastructure is a set of interacting software components designed to ensure that a software system or subsystem constructed using those components and interfaces will satisfy clearly defined performance specifications.

My opinions after 2 years of experience with CBSE in games thought are that object-oriented programming is simply a dead-end. Remember my warning as you watch your components become smaller and smaller, and more like functions packed in components with a lot of useless overhead. Use functional-reactive programming instead. Also take a look at my fresh blog post (which lead me to this question while writing it :)) about Why I switched from component-based game engine architecture to FRP.

CBSE in games papers:

• Component Based Game Development – A Solution to Escalating Costs and Expanding Deadlines?
• A Flexible And Expandable Architecture For Computer Games (404)
• A Software Architecture for Games
• A Generic Framework For Game Development (WebArchive)
• Smart Composition Of Game Objects Using Dependency Injection

CBSE in games web-links (sorted by relevancy):

• Component based objects Wiki (Empty wiki)
• Evolve Your Hierachy
• Game Object Structure: Inheritance vs. Aggregation
• A Data-Driven Game Object System (PDF)
• A Data-Driven Game Object System (PPT)
• Component-based prototyping tool for flash
• Theory and Practice of Game Object Component Architecture (404)
• Entity Systems are the Future of MMOs
• ogre3d.org forum: Component Based Objects
• gamedev.net: Outboard component-based entity system architecture
• gamedev.net: Entity System question
• Brainfold entity-system blog (WebArchive)

Answer 2

In this context components to me sound like isolated runtime portions of an engine that may execute concurrently with other components. If this is the motivation then you might want to look at the actor model and systems that make use of it.

Answer 3

Interesting artcle...

I've had a quick hunt around on google and found nothing, but you might want to check some of the comments - plenty of people seem to have had a go at implementing a simple component demo, you might want to take a look at some of theirs for inspiration:

http://www.unseen-academy.de/componentSystem.html
http://www.mcshaffry.com/GameCode/thread.php?threadid=732
http://www.codeplex.com/Wikipage?ProjectName=elephant

Also, the comments themselves seem to have a fairly in-depth discussion on how you might code up such a system.

Answer 4

There does seem to be a lack of information on the subject. I recently implemented this system, and I found a really good GDC Powerpoint that explained the details that are often left behind quite well. That document is here: Theory and Practice of Game Object Component Architecture

In addition to that Powerpoint, there are some good resources and various blogs. PurplePwny has a good discussion and links to some other resources. Ugly Baby Studios has a bit of a discussion around the idea of how components interact with each other. Good luck!

Answer 5

I researched and implemented this last semester for a game development course. Hopefully this sample code can point you in the right direction of how you might approach this.

class Entity {
public:
    Entity(const unsigned int id, const std::string& enttype);
    ~Entity();

    //Component Interface
    const Component* GetComponent(const std::string& family) const;
    void SetComponent(Component* newComp);
    void RemoveComponent(const std::string& family);
    void ClearComponents();

    //Property Interface
    bool HasProperty(const std::string& propName) const;
    template<class T> T& GetPropertyDataPtr(const std::string& propName);
    template<class T> const T& GetPropertyDataPtr(const std::string& propName) const;

    //Entity Interface
    const unsigned int GetID() const;
    void Update(float dt);

private:
    void RemoveProperty(const std::string& propName);
    void ClearProperties();
    template<class T> void AddProperty(const std::string& propName);
    template<class T> Property<T>* GetProperty(const std::string& propName);
    template<class T> const Property<T>* GetProperty(const std::string& propName) const;

    unsigned int m_Id;
    std::map<const string, IProperty*> m_Properties;
    std::map<const string, Component*> m_Components;
};

Components specify behavior and operate on properties. Properties are shared between all components by a reference and get updates for free. This means no large overhead for message passing. If there's any questions I'll try to answer as best I can.

Answer 6

It is open-source, and available at http://codeplex.com/elephant

Some one’s made a working example of the gpg6-code, you can find it here: http://www.unseen-academy.de/componentSystem.html

or here: http://www.mcshaffry.com/GameCode/thread.php?threadid=732

regards

Answer 7

While not a complete tutorial on the subject of game engine design, I have found that this page has some good detail and examples on use of the component architecture for games.

Answer 8

I am currently researching this exact topic in the many (MANY) threads at GameDev.net and found the following two solutions to be good candidates on what I will develop for my game:

• Critique my component-based entity system
• Outboard component-based entity system architecture -> Lord_Evil's proposal