[Qgis-developer] Layer-level metadata

[Barry Rowlingson]

2009/2/3 Florian El Ahdab <felahdab at gmail.com>:

> Typical example: I want to develop a plugin to implement S52 representation
> of S57 maps. To do that, I must identify the layer type (as in S57 layer
> type). Of course I get the layer type when opening the datasource from OGR
> (the ogrsublayersdialog now displays this name). But once this has been
> done, unless I get down to the QgsOgrProvider object, I have no way to
> retrieve this information simply. Moreover, I can imagine designing my own
> layers that I would like to render as S57 layers of such or such type and
> therefore requiring some additionnal data that OGR wouldn't give me in that
> case...

 You can do this, depending on how much control you have of the layer
loading process...

 Python lets you add attributes to just about any object. So if you
create a new menu option 'Load My Special Layer' which lets the user
choose a shapefile or other data source, you can create a
QgsVectorLayer and then set something on it. Load a vector layer and
then try this in the python console: l = iface.mapCanvas().layer(0)
followed by l.fnord = "hello world". The .fnord attribute is now set,
and other python code can check for presence of this. Any decent
object-oriented programmer would throw up at this point at the way
you've violated their object...

 So you can subclass QgsVectorLayer to keep them happy. You do (something like):

class SpecialLayer(QgsVectorLayer):
   def __init__(self,file):
      QgsVectorLayer.__init__(self,file)
   def setFnord(self,value):
       self.fnord=value

 BUT (and it's a big but) none of this gets saved in project files.
You could write a writeXML method on your layer which could save the
metadata, but when it is read back Qgis will just create a plain
QgsVectorLayer and throw away your extra information.

 It would be nice to have a userMetadata object saved and restored
with layers which would be some key-value structure accessible to
Python and C++. At the moment such a thing doesn't exist.

 A possibility is to save it in the project settings (see QgsProject)
which is separate from the layers. I'm thinking about this for the
thing I'm working on.

Barry