Submitted by Zachary Booth Simpson
on 12/5/2000

2000 - Zachary Booth Simpson. Copied with permission from If you find any of this work useful, please sign Zack's guest book:


Update a Model's state based on circumstance.


Controllers implement the rules of a game. They determine how objects behave given a circumstance, and isolate these rules from the objects themselves. This makes both the controllers and models more reusable and maintainable.


Controllers relate to Models and Views as follows:
  • Models are read-writeable by Controllers.
  • Controllers are created and destroyed by Models, but are otherwise invisible.
  • Views are invisible to Controllers and vice-versa.
  • Controllers are often associated with only one model instance. For example: animation, AI, pathfinding. In these cases the controller instance is usually created and destroyed synchronously with the associated model.

    Some Controllers inherently have more than one associated Model. For example: multi-body physics, target tracking (heat seeking missiles, etc). These controllers often maintain Model references which must be notified / garbage collected when the referenced object dies. This is called the "model death problem". The creation and deletion of these multi-owner controllers is usually done by some primary owner.

    Controllers are often implemented as "processes" in a mini cooperative multi-tasking kernel. (See Mini-kernel) but may also be implemented as "hard wired updates" in the main loop, especially for large multi-model controllers like physics.

    Some simple Controllers are stateless. For example, a homing missile controller may just compute the direction to the target and apply force as necessary. Most controllers, however, are state-aware. For example, an animation tracks progress through the animation and changes states accordingly; e.g. if (frame > 10) state = STOP_WALKING.

    State-aware controllers often become significantly complicated with large switch statements. (See State Machine Controller.)


    Not available at this time.

    Issues and Risks

    None at this time.

    Related Patterns

    Mini-kernels aggregate Controllers, giving each controller some time to work.

    Controllers modify Model's states.

    Views may translate Model state with an Appearance Map.

    Complicated state-aware controllers may use a Conroller State Machine.

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    Date this article was posted to 6/19/2001
    (Note that this date does not necessarily correspond to the date the article was written)

    See Also:
    Design Patterns

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