Implementing Hard Callbacks

Implementing hard callbacks is a bit harder than enhancing the hoc language, but offers utmost flexibility in handling the data. You should know something about the C++ programming language, see [str93]. Hard callbacks behave much like the callback mechanism from the X-window system. The idea is not to actively process all data, but to ''get called'' by the main program as a sort of interrupt routine.

Every hard callback is a implemented as a C++ class, derived from a baseclass HardCallback. This baseclass provides the framework, in which callback's functions must be written. Study HardCallback.h and HardCallback.cc for details. Each callback ''lives'' in association with YODA's Eventstream class. This class is the central device for reading, writing and dispatching containerevents and subevents. See Eventstream.h and Eventstream.cc for details.

Hard callbacks can access data of and add subevents to the incoming eventstream. The user must specify the following:

• the virtual function init() which is called at any time a variable managed by the broker utility has changed.
• the virtual function analyze(), called for every accepted event and does the actual data analysis.
• the virtual function finish(). This function is called whenever YODA is going offline.
• the subevents to be selected or accessed (same meaning as in the hoc language.
• the subevents which are created by the hard callback and should be added to the eventstream after the analysis.