The 'GA' in osgGA stands for 'GUI Abstraction'; the osgGA namespace provides facilities to
help developers write the glue to allow the osg to work with varying window systems.
As a cross-platform, window system-agnostic class library, the OpenSceneGraph
has no direct ties to any given windowing environment. Viewers, however, must at
some level interact with a window system - where Window system may refer to a windowing
API, e.g. GLUT, Qt, FLTK, MFC, ...
There is much commonality in the implementation of Viewers for varying windowing
environments. E.g. most Viewers will update a Camera position in response to a mouse
event, and may request that a timer be started as a result of a model being 'spun'.
The purpose of the osgGA namespace is to centralise the common areas of this
functionality. The viewer writer needs then only write a GUIEventAdapter, a
GUIActionAdapter, and assemble a collection of GUIEventHandlers
as appropriate for the viewer.
Events from the windowing environment are adpated, and then fed into the GUIEventHandlers.
The GUIEventHandlers analyse and take action, and make requests of the windowing
environemnt via the GUIActionAdapter. The viewer writer should then honour these
requests, translating them into calls to the windowing API.