[Qgis-developer] Field calculator does attribute changes silently

Marco Hugentobler marco.hugentobler at sourcepole.ch
Tue Jan 7 00:34:09 PST 2014

Hi Matthias

I remember there was a performance consideration leading to the 
emitSignal parameter in changeAttributeValue. But I don't know if it is 
still needed, seems to be only used from field calculator, offline 
editing and a label tool.
So +1 for your proposal.


On 06.01.2014 14:01, Matthias Kuhn wrote:
> Hi,
> Currently,when the field calculator does changes to attributes 
> (modification, not adding a new column) it blocks signals of the 
> vector layer while doing this. This leads e.g. to bug #9268 [1], that 
> the attribute table is not updating properly. The fix would be very 
> easy (i.e. remove the blockSignals() call) but I wanted to ask first, 
> if there is a reason not to do this.
> The main reason why I am afraid is, that apart from the blockSignals() 
> call, there is also the paramter emitSignal specified as false in the 
> call to vectorlayer->changeAttributeValue(). This parameter is 
> currently not working, so I suppose that the blockSignals() call was 
> introduced as a workaround.
> However, there was a good reason, to block the signals explained in 
> issue #7071 [2], fixed by Jürgen, that at the time when we only 
> supported one iterator per layer, emitting this signal would break the 
> update process and produce only partially updated results. This is 
> fortunately no longer the case, so this reason can be considered 
> obsolete.
> Another reason which I could imagine (but I have no proof of) is that 
> for big layers, emitting the signal for each feature can possibly take 
> a long time. However, if this is actually the case we should rather 
> think about batching the signal or fix unperformant consumer slots of 
> this signal.
> Be aware, that at the current state, there is no possibility, neither 
> for plugins nor in QGIS core, to get reliable push-notifications when 
> an attribute changes.
> So my proposal is:
>   * Remove the blockSignal calls and let attribute changes always be 
> promoted
>   * If there are problems introduced, lets fix the problems instead of 
> working around them at the cost of unreliability
>   * Don't fix the emitSignal parameter for 
> QgsVectorLayer::changeAttribute and deprecate it for future removal.
> If there is no objection, I would vote to make this changes before the 
> QGIS 2.2 freeze
> Best
> Matthias
> [1] http://hub.qgis.org/issues/9268
> [2] http://hub.qgis.org/issues/7071
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Dr. Marco Hugentobler
Sourcepole -  Linux & Open Source Solutions
Weberstrasse 5, CH-8004 Zürich, Switzerland
marco.hugentobler at sourcepole.ch http://www.sourcepole.ch
Technical Advisor QGIS Project Steering Committee

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