THE WELL

1
THE WELL

• XOOGA

2
Design Goals

• Game Framework
• Easy to use
• Easily extensible
• Pluggable components

3
MVC

• Standard MVC design
• Viewer responsible for game selection through a
  drop down menu (can be provided dynamically by
  examining available xml files) and showing game
  play.
• Model takes care of the game execution and
  contains all the game state information.
• Controller provides Model with information about
  user input. User input can be recorded so that
  the game can be replayed with identical user
  actions.

4
Overall program execution

• MVC gets connected.
• User chooses a game from the available set of
  options.
• Game gets loaded from xml using a parser.
• Game state is initialized to first level and play
  begins.
• Model maintains knowledge of game state in a
  GameState object.
• Game runs in a loop until either the win or lose
  conditions get satisfied (or the player quits the
  game), at which point the appropriate action is
  taken (advance to next level, end playing, etc.)
• Viewer has a list of elements to be displayed
  that gets modified as needed.

5
Game Representation
6
Game and GameLevel Classes

• Stores all the information obtained from parser
  levels, game state, conditions, etc.
• Basically they are placeholders for information.
• The Model loads required game state information
  from these classes and uses it to play the game.

7
GameEntity Class

• Every element of the game is a GameEntity (no
  subclassing needed).
• Allows to plug in behavior, image, and any other
  properties.
• This allows for flexibility in creating new game
  elements.

8
GameEntity displaying

• A GameEntity implements Animatable, so it can be
  used in a Canvas, but drawing functionality is
  delegated to an Animatable that is an instance
  member of GameEntity.
• Through this the appearance of a GameEntity is
  easily replaceable with a completely different
  shape. For example an alien animation can turn
  into an explosion animation.

9
GameEntity properties

• A GameEntity can have any number of property
  objects ID, velocity, dimension, number of
  lives, point value, etc.
• This is done by using a map of Strings to
  Objects.
• This adds flexibility allowing different elements
  (bricks, balls, etc.) to have different
  properties.
• Entities can be queried about their properties
  and these properties can be modified externally,
  if necessary.
• Corresponds to characteristic tag in xml spec.

10
GameEntity behavior

• Behavior anything that an entity can do move,
  morph, disappear, shoot bullets (spawn new
  GameEntities), teleport, including special
  entities like timers ticking off seconds.
• Each GameEntity has a List of Behaviors that it
  exhibits.
• Behaviors can be exhibited in different ways
  always, every now and then, etc. This is
  supported by concrete implementations of the
  abstract Behavior class.
• More details on Behaviors later on.

11
GameEntity interaction

• The Model contains information about all the
  elements currently in the game (via a GameState
  object) and identifies collisions between them.
• Upon a collision, each involved GameEntity is
  messaged about colliding with the other entity.
• Each GameEntity has a mapping (determined by
  entity property correspondence) of other entities
  to reaction Behaviors for collisions with those
  entities.
• This way objects can behave differently based on
  what kind of other GameEntity they collide with
  (useful in Pac-Man)

12
Behavior Hierarchy diagram
13
Behavior Hierarchy and Events

• Behaviors represent any activity that an entity
  can perform.
• Concrete implementations control when those
  activities get performed (randomly,
  conditionally, continually, etc.).
• Behaviors can be added and removed from
  GameEntities. This can be used in Tetris, where
  the UserControlled behavior gets reasigned to new
  game pieces as necessary or in Breakout when a
  user collects an upgrade and gains new abilities
  (i.e. Behaviors).

14
Events

• The execution of a Behavior is carried out in
  terms of Event objects. Many Events might be
  needed to carry out a Behavior.
• Events are the unit actions the make up a
  behavior. For example, when a brick gets knocked
  out, the following events happen make ball
  bounce, delete brick from game, add points to
  score.
• Events are executed on a target, which is
  provided dynamically (for example, which brick
  should disappear) or predefined (there is only
  one score entity that needs to be updated)
  depending on the actual concrete Event used.

15
Condition Hierarchy

• Conditions return a boolean value based on the
  state of the game or other factors.
• Can have different kinds of conditions flags,
  timers that switch value after time runs out,
  counters, conditions based on object properties,
  etc.
• Conditions can be composed using logical
  operators and, or, not to create complex
  conditions.
• Conditions can be used to evaluate the state of
  the game, reaching game goals (all enemies die),
  etc.
• They can be used to enable behaviors. This
  mechanism can be used to provide rudimentary AI

16
Putting it together

• Components are structured to be independent of
  one another and provide flexibility in composing
  basic building blocks (behaviors, properties,
  shapes, conditions) to create a wide variety of
  game elements.
• This separation allows for easy expansion by
  creating additional building blocks.

17
Putting it together exampleImplementing a
Breakout paddle

• A paddle is a GameEntity (since its part of the
  game).
• Properties appearance (an Animatable), size,
  position
• Behaviors moveable by user
• Interactions comes in contact with a ball, the
  walls, and possibly power-ups. It has appropriate
  reaction Behaviors installed.
• Collision events gets hit by a ball nothing
  happens (the ball will know to bounce on its
  own), hits a wall movement behavior gets
  changed to disallow going past the wall,catches a
  power-up appropriate power-up behavior will
  modify the properties of the paddle (appearance)
  and its behavior (e.g. ability to shoot)
• Ball will have Behavior to know if it is out of
  bounds, and will modify the game state through
  appropriate Events.
• An extension like sounds can easily be added by
  adding extra Events to the appropriate Behaviors
  that would be responsible for playing the sounds.