Object Oriented Game Framework Design

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Object Oriented Game Framework Design

• Riz Verghese Joj

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Frameworks What are they?

• Semi complete applications
• Complete applications can be developed by
  inheriting from and instantiating parameterized
  framework components.
• Provide domain-specific functionality
• Business, telecom, databases, games.
• Exhibit inversion of control at run-time
• Determines which objects and methods to invoke in
  response to events.

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Benefits

• The most profoundly elegant framework will
  never be reused unless the cost of understanding
  it and then using its abstractions is lower than
  the programmers perceived cost of writing then
  from scratch.
• - Booch, Dr. Dobbs Journal, 1994.

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Design Patterns

• Each pattern describes a problem which occurs
  over and over again in our environment and then
  describes the core of the solution to that
  problem, in such a way that you can use the
  solution a million times over, without ever doing
  it the same way twice.
• - Christopher Alexander, A Pattern Language, 1977.

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Design Patterns what are they?

• A Solution to a problem in context
• Provide experience reuse instead of code reuse.
• Hundreds of patterns exists!
• Vocabulary Facilitates communication among
  developers
• Lets you think at an architecture level, not
  implementation level.
• Eg Factory, Observer, Composite, Singleton

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Frameworks and Design Patterns
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An example game framework

• Classic arcade style games
• 2D tile based games
• Basic elementary collision
• Supports dynamic class loading

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Another Game
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The Design

• Resource Manager
• Resource Image
• Resource Sound
• Resource Sprite
• Game board
• Tile Indexer
• Tile manager
• Tiles
• Immovable Tile
• Movable Tile

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Patterns

• Resource Manager
• Factory
• Reads from an xml file
• Dynamic creation of classes
• Associate images and sounds with a tile
• Tile Manager
• Observer
• Tiles created subscribes to the Tile Manager
• Tile Manager tells the board where the tiles are
• Tiles have to request movement
• from the Tile Manager.

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Game specific patterns

• Patterns arent limited to the framework
• Game specific code can have lots of patterns
• The Snake game
• Especially uses the composite pattern
• Framework objects mostly follow the singleton
  pattern where as game specific objects do not.

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How does the framework work?

• Resource manager loads the tiles in memory from
  information in the XML file.
• Level loader prepares an instance of the board,
  tiles and placement of the tiles on the board.
• Game specific class maps input to tile action
  (usually movement)
• Tile asks board if it can move to the desired
  place
• Board lets tile move or not
• Property of the tile determine what happens when
  two tiles occupy the same space
• Collide, Consume, Cancel.

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Implementation specifics

• Two threads
• The first allows the board to redraw/refresh
• The second allows every tile to think
• The think method allows the tiles to
• Keep moving
• Keep falling
• Know what to do on collision
• Tells other tiles what to do

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More specifics

• Tiles can have relations
• The observer/observable relationship
• Eg Acno game Door disappears when key is eaten
• Key tells door to disappear
• Eg Snake gets longer when apple is eaten
• Apple tells snake body to spawn

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Advantages

• Allows you to step back and generalize
• More time spent on the framework
• Isnt worth it for just one game
• Allows you to spend time on the game (content)
  not the implementation
• Saves a LOT of time for one particular game
• Clean implementation
• The framework code is generally hidden
• Game specific code is small (from reusing
  framework components)

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Disadvantages

• Games are limited to the framework
• Cant build all games
• Features Limited to framework provision and
  functionality
• Sometimes overcome through ugly
  workarounds/tricks/hacks

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References

• The development of a Game Playing Framework using
  Interface-based programming, Mark Cohen, Issue
  12.1, ACM Student Magazine, Fall 2005.
• Object-oriented Game Development, Julian Gold,
  ISBN 032117660X, Addison Wesley, 2004.
• Introducing Object Oriented Frameworks, Taligent
  white paper.
• Designing Reusable Classes, R. Johnson and B.
  Foote, Journal of Object-Oriented Programming,
  1(2)22--35, June/July 1988
• Design patterns Abstraction and reuse of
  object-oriented design, E. Gamma, R. Helm, R.
  Johnson, and J. Vlissides, In European Conference
  on Object-Oriented Programming Proceedings
  (ECOOP'93), volume 707 of Lecture Notes in
  Computer Science. Springer-Verlag, July 1993.

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The end
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