[Qgis-developer] General questions & multiple map canvases

[Tim Sutton]

Hi Martin

On 5/5/06, Martin Dobias <wonder.sk at gmail.com> wrote:
> 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.

Right we need to iron these things out nicely still....

> 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?

Yes but we should change maplayer so that it only stores the 'from'
part of the SRS so that a layer can be used in multiple canvases which
have different canvas SRS.

I do wonder if we would not be better having a formalised
QgsMapLayerCollection with iterator capability (and doing away with
the map layer registry singleton). The QgsMapLayerCollection would
store all the handles to map layers for a given canvas. Layers could
be shared among canvases by virtue of being pointers. With reference
counting the layer can be destroyed when last ref to it is lost.
However from the UI point of view the user would need to understand
that layers shared between canvases are linked and that changes to
symbology in on canvas will affect all its representations in other
canvaes.

> - should be every layer added to registry added also to the legend in
> gui application?

I would like to see a situation where we can do:

QgsLegend->setLayersCollection(QgsMapCanvas->layersCollection())

Where the QgsMapCanvas->layersCollection() would return a pointer to
the QgsMapLayersCollection so that the legend would automatically sync
with canvas and verca vica. It certainly would be useful to be able to
hide individual layers form the legend in bespoke apps (e.g. where you
wish to have an immuteable backdrop layer). This could probably be
best achieved with a QgsMapLayer->hideFromLegend(true) type of call.

> - similarly, when a layer is removed from legend, should it be also
> removed from registry and deleted?

We should ref count it. But see comments above, I think moving to a
MapLayersCollection may be a better  bet.

>
> 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?

Each canvas should be able to specify its own canvas SRS

> - the same layer properties (e.g. symbology, labeling) in every canvas
> or allow different ones?

I think initially we should share symbology across shared instances of
a layer. We need to asess how much overhead having duplicates of the
same data source as separate QgsMapLayer objects with different
symbologies versus splitting symbology furter away from map layer so
that multiple symbologies can be assigend to the same qgsmaplayer
instance.

> - how should plugins work with canvas? E.g. enabling scale bar plugin
> will show scale bar in all canvases or in active canvas only?

Good point. They should be assigned on a per canvas basis I think. We
should adapt the plugin api to cater for this and in most cases the
plugin writer himself should dictate whether the plugins scope is
global or to a specific canvas instance.

> - should one legend control all map canvases or let's have one legend
> for every canvas?


Each canvas should have its own legend instance. We should swap out th
elegend control with which ever canvase is currently in focus.

> - 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?

Yes simple add and remove canvas buttons, and this must be exposed to
plugin api. Also plugin api should be able to addTab, removeTab where
the tab can be used by plugin writer for other non canvas stuff e.g.
displaying a form.

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

Go Martin Go! :-) These are my thoughts anyway, other people may have
different ideas / better ideas.

Regards

Tim

>
>
> Martin
> _______________________________________________
> Qgis-developer mailing list
> Qgis-developer at lists.qgis.org
> http://lists.qgis.org/cgi-bin/mailman/listinfo/qgis-developer
>


--
Tim Sutton

Visit http://qgis.org for a great Open Source GIS
Skype: timlinux
MSN: tim_bdworld at msn.com
Yahoo: tim_bdworld at yahoo.com
Jabber: timlinux
Irc: timlinux on #qgis at freenode.net