Intent

Track a complicated state process with a controller.

Problem

Many Controllers are very complicated state machines which involve convoluted state transitions as circumstances progress and in response to events. Animation is the canonical example – both time and user input effect the state transitions of animations, often with many special cases and subtle complications.

Solution

A Controller subclass is created which contains the list of all state variables. For example, and animation might have: currectFrame, currentAnim, lastFrameTime, etc.
The process virtual of the controller contains a switch on some primary state. For example:
void Animation::doProcess() {
    switch( animState ) {
        case RUNNING_STARTING:
        case RUNNING:
        case RUNNING_STOPPING:
        ...
Each state updates and checks for transition conditions. For example, RUNNIG may check to see if it is at the end of the cycle, if so, restart it. It might also check to see if the mouse button is still down, if not, change to RUNNING_STOPPING.
State machines can become very complicated and difficult to maintain using this technique. One alternative is to use function pointers and setjmp/longjmp.

Structure

None at this time.

Examples

See http://www.totempole.net/statemachines.html for a sample implementation of this technique.

Issues and Risks

None at this time.

Related Patterns

Contoller, State

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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