[Qgis-developer] Labeling: data-defined fields shifted after new vector API?

[Martin Dobias]

Hi Larry

On Wed, Feb 6, 2013 at 4:25 AM, Larry Shaffer <larrys at dakotacarto.com> wrote:
> Is mFields[i].originIndex (where mFields is a QgsFields vector) the same
> index used for the keys in QgsFieldMap?

No, it's not the same thing. "originIndex" is a mapping from layer's
attribute indices to provider's indices. That's needed when one or
more fields have been removed in layer's editing mode (and also used
for identification of joined attributes). For example, if you have a
layer with attributes attr1, attr2, attr3 and you decide to delete
attr2, the index of attr3 will change from 2 to 1, but the originIndex
will stay the same. Before new API merge, if an attribute was deleted
in editing mode, indices would not change (because holes were
allowed).


> In other words, will this new method, QgsFields::toQgsFieldMap(), function
> correctly [0]? If so, I can use it to attempt to auto-convert
> QgsFieldMap-index-based data defined properties over to field-name-based
> properties, without having think about reintroducing any QgsFieldMap
> functionality at the provider level.

That wouldn't help, because the numbering of fields has been changed
on provider level (e.g. a provider used to return indices 0,1,3, now
it returns 0,1,2), so I am afraid there's no simple way to establish
mapping of attribute indices from old API providers to new API
providers (because some have changed, others haven't).

Regards
Martin