[Paraview] useful plugin: how to do it, really?

[B.W.H. van Beest]

Dear Paraview people,

I am investigating the option to use Paraview as the base platform for a
simulation project that we are currently working on.
Paraview's visualisation capabilities, as well as the (alleged)
extensibility (by writing plugins) suggest that this is the case indeed.

In order to get a better feeling of this, I want to try a small example
showing me how this all works.
However, I get lost in the documentation and code tree. Browsing the web
for information on writing a paraview plugin typically refer to
Paraview's web pages, but
I must admit that the are not really helpful, despite the amount of
words devoted to the topic.
I wonder if there is somebody who can help me, or point me to useful
information.

As a sample problem, this is what I would like to do:

1. Suppose I have created, interactively, a box, a sphere and a cone,
using the corresponding entries in the "Sources" menu.

2. Now I would like to *programmatically* (i.e. in  a c++ plugin) loop
over the sources (here: the box, sphere and cone) and for each of the
elements
get hold of the parameters that were used to define these sources (i.e.
the box dimensions, center, radii etc.). With these parameters I can
then do other things to build my own extension, but those details do not
matter here.

I have looked into the examples, but I really do not recognise in any of
them what I'm supposed to do.
Maybe it is there, but I find the code tree of such a complexity that I
have problems in finding what I need.

Your help is greatly appreciated.

Kind Regards,
Bertwim