[Qgis-developer] Customized VectorLayer for ArcGIS REST API Connector

[Carmelo Schumacher]

Hi Devs

I am currently working on the "ArcGIS REST API Connector Plugin"[1] trying to find a way to use QgsVectorLayer. The webservice (callef "ArcGIS Feature Service") returns valid GeoJSON and the QgsVectorLayer's datasource url can point to the service that provides this GeoJSON (this is done in the current version of the plugin). The problem is that there exists no query to retrieve all features from an ArcGIS Feature Layer at once/in one service call (if there are more than 1000 features). 

So my approach is the following: The feature list is populated through multiple service calls and temporarily stored/cached as GeoJSON-File. The QgsVectorLayer's datasource url points to this file. The file is repopulated occasionally on request/event. Probably the best way to achieve this would be to write a CustomVectorDataProvider that inherits from VectorDataProvider and is injected into the VectorLayer. But I didn't find a way to do this with PyQGIS.  

I then tried to use a QgsPluginLayer as an adapter to a QgsVectorLayer. The CustomPluginLayer would hold a reference to an instance of VectorLayer and delegates all the functionality of a VectorLayer to this instance. This would allow the (re)population of the local GeoJSON file, when the layer is loaded or refreshed. But I didn't find a way to achieve this. 

1. What actions are possibly needed to (re)connect? 
2. Or are there methods/structures missing in the Python API wrapper to QGIS C++ API?
3. Is there a good overview/doc over the activities/instances involved when an VectorLayer is added to a map by calling QgsMapLayerRegistry.instance().addMapLayer(layer) or alternatively when a VectorLayer is created/added through iface.addVectorLayer? 

Any input/ideas would be very much appreciated. 

Kind regards
Carmelo Schumacher

[1] http://giswiki.hsr.ch/QGIS_ArcGIS_REST_API_Connector_Plugin <http://giswiki.hsr.ch/QGIS_ArcGIS_REST_API_Connector_Plugin>

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Carmelo Schumacher
Institut für Software (IFS)
Hochschule für Technik Rapperswil 
Oberseestrasse 10, 8640 Rapperswil, CH
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