[Qgis-developer] Vector buffer commitChanges (was Interaction between QGis and ArcGis)

[Martin Dobias]

Hi Radim

On Sat, Apr 20, 2013 at 8:39 AM, Radim Blazek <radim.blazek at gmail.com> wrote:
> On Fri, Apr 19, 2013 at 12:07 PM, JVerholle <julien.verholle at eaurmc.fr> wrote:
>> Hi all,
>>
>> If I want to open the same shapefile in both softwares at the same time, the
>> file is locked in ArcGis (even if the edition mode isn't used in QGis).
>
> QGIS opens layers (files) in update mode ("r+") if possible (file
> permission + OGR driver support). It may (but also may not) be a
> problem also for #6448 (slow shp over network) I am currently
> struggling with. In general, I think that it is bad to open files
> always in update mode even if in most cases they are not going to be
> edited.

Agreed.


> Currently there are no QgsVectorDataProvider::startEditing() and
> commitChanges() which may also be a problem for database providers
> because QgsVectorLayerEditBuffer::commitChanges() calls more provider
> methods changing
> data (deleteAttributes, addAttributes, deleteFeatures,
> addFeatures...).  The commitChanges() should do everything in one
> transaction IMO.
>
> My proposal is to:
> 1) Add QgsVectorDataProvider::startEditing() and
> QgsVectorDataProvider::commitChanges()
> 2) In OGR provider try to open layer in update mode only to get
> capabilities (get info if it can be modified when the provider is
> constructed) but then to reopen in read only mode.
> 3) Call QgsVectorDataProvider::startEditing() from
> QgsVectorLayer::startEditing() to be sure that the layer is still
> editable (permissions could change or it was opened by another
> application for editing since the layer was opened) and to reopen
> files in update mode (files based) or to start transaction (DB based).
> 4) Call QgsVectorDataProvider::commitChanges() from
> QgsVectorLayer::commitChanges()
> 4) In QgsOgrProvider::startEditing() reopen the layer in update mode
> 5) In QgsOgrProvider::commitChanges() reopen the layer in read only mode

When would be startEditing() called - when user starts editing in GUI
or when user is going to commit the changes, just before issuing
provider editing functions?
Maybe beginTransaction() and endTransaction() or commitTransaction()
would be better names? It would be practical to have also another
method to detect whether the provider is in editing mode.

What about legacy code that directly makes changes to
QgsVectorDataProvider? Will it need to wrap the editing functions with
these two additional calls? It would be good if that would not be
necessary and the provider would start and stop the transaction
automatically if not in editing mode.

Regarding 2) is it necessary to open layer in update mode in order to
get editing capabilities?


> We don't really have to implement startEditing() and commitChanges()
> in all providers for 2.0, but it should be in the vector API.

Agreed.

Regards
Martin