[Qgis-developer] QgsMapLayerRegistry (and QgsMapCanvas) problem

[Martin Dobias]

Hi Carson,

On Fri, Oct 17, 2008 at 4:39 PM, Carson Farmer <carson.farmer at gmail.com> wrote:
> Hello gang,
>
> According to the API, QgsMapLayerRegistry.instance().mapLayers() should
> return (in the case of python) a dict with the following format:
> { layer0 : QgsMapLayer, layer1: QgsMapLayer }
> However, what I appear to be getting is slightly different. Instead of
> QgsMapLayers, I get QgsVectorLayers and QgsRasterLayers. This also occurs
> when trying to get a QgsMapLayer from the map canvas. Is this intended, and

this is correct behaviour. Python doesn't support casting in a way
that c++ does. I.e. one you have a QgsMapLayer reference, you can't
cast it to QgsVectorLayer (although it would be a correct cast). So
SIP has a mechanism (which PyQGIS uses) that returns a subclass
instead of parent class. You can take a look at qgsmaplayer.sip in
python/core dir to get an idea.

> if so then this creates a problem:
>
> QgsMapLayer.type() returns (as of 9490-ish) either VectorLayer (0) or
> RasterLayer (1) [ QgsMapLayer.LayerType ]
> QgsVectorLayer,type() returns either Point (0), Line (1), Polygon (2), or
> UnknownGeometry (3) [ QGis.geometryType ]
> QgsRasterLayer.type() returns (as far as I can tell) the same as
> QgsMapLayer.type(), so: RasterLayer (1)
>
> [...]
>
> I see three solutions to this problem:
> a) I'm doing this all wrong, and hopefully someone will tell me this,
> b) My favoured solution: Since QgsVectorLayer inherits QgsMapLayer,
> QgsVectorLayer.type() should be changed to QgsVectorLayer.vectorType()
> c) All of these enumerators should return something other than numbers?

b) is correct! type() should be really renamed back to vectorType()
because current behaviour is not acceptable.

Bye
Martin