[Qgis-developer] General questions & multiple map canvases

Martin Dobias wonder.sk at gmail.com
Fri May 5 14:32:15 EDT 2006


Hi all,

I'd like to discuss here the concept of multiple map canvas support.
The reason of starting the discussion is that I think often it's not
very clear (at least for me) what part of QGIS application should be
responsible of what task. Parts of QGIS I'm currently talking about
are map layer registry, project, map canvas and legend. Their
cooperation affects also how they should be handled in other
applications. The questions for which I don't have clear answers
include:
- should be every new layer added to map layer registry?
- should be every layer added to registry added also to the legend in
gui application?
- similarly, when a layer is removed from legend, should it be also
removed from registry and deleted?

It's easy to answer these questions when we consider only single
legend and map canvas. However with more map canvases, it depends of
what do we expect from multiple canvas support:
- allow different projection for every map canvas or the same for all
canvases in project?
- the same layer properties (e.g. symbology, labeling) in every canvas
or allow different ones?
- how should plugins work with canvas? E.g. enabling scale bar plugin
will show scale bar in all canvases or in active canvas only?
- should one legend control all map canvases or let's have one legend
for every canvas?
- how to present it to the users? By this I mean whether do you plan
to have just "Add canvas" and "Remove canvas" buttons or something
more complicated with more features?

Answers to these questions will help me to prepare API correctly for
3rd party applications.


Martin



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