Intent

Update a Model's state based on circumstance.

Problem

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.

Solution

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.)

Structure

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 GameDev.net: 6/19/2001
(Note that this date does not necessarily correspond to the date the article was written)

See Also:
Design Patterns