  Go4 developed at GSI is an analysis framework with a general purpose non
blocking GUI. Go4 is based on ROOT. The GUI is implemented in Qt using GSI's
QtROOT interface. Analysis and GUI run in separate tasks communicating through
asynchronous object channels. A Go4 analysis may use any ROOT features. It can
be organized in steps, which can be controlled from the GUI according to the
user specifications. Each step is composed of event objects, the event
processing, and event IO. Go4 composite event classes allow the construction of
arbitrary complex events hierarchically composed of objects. The IO of the
composite event objects to and from ROOT trees/branches is provided without
explicit programming. The Go4 tree viewer can browse arbitrary hierarchy levels
of composite events. The GUI provides hooks to attach user written GUIs. These
GUIs have access to all objects of the analysis, i.e. events for asynchronous
event display. Using the Qt designer the development of such GUIs is very
efficient. The Go4 fit package (API and GUI) is a powerful and extendable tool
to model and fit experimental data. The Go4 framework is especially useful for
on-line monitoring. The HADES experiment at GSI integrated the existing ROOT
based analysis into Go4 and is using it on-line with dedicated GUIs.
