Controllers are the plug-in type responsible for directing all animation in 3ds Max. The section uses the Position constraint controller as a running example. The files that are relevant for this discussion are:

• \MaxSDK\Samples\Controllers\Position_cnstrnt.cpp The source code for the constraint.
• \MaxSDK\Samples\Controllers\ctrl.cpp DLL implementation of some controllers
• \MaxSDK\Samples\Controllers\ctrl.h Header file for class control. The relevant line of code here is:
• \MaxSDK\Include\istdplug.h Interfaces into some of the standard plug-ins (including many of the controllers) that ship with Max.
• \MaxSDK\Include\plugapi.h This file contains class ID's for plug-in classes.

Controllers derive from the Control class. The primary methods that must be implemented by a plug-in are:

Control::GetValue() - Implementations usually cache the value and validity interval from the previous GetValue() call. An implementation is responsible to make sure that the validity interval gets updated correctly.

Control::SetValue() - The implementation is responsible for sending update notifications and needs to make sure any cached values get correctly cleared.

The class Control is derived from class Animatable. A controller will need to implement methods from this class, as the default implementations are not valid.