[Qgis-developer] Patch for screen updates during long rendering operations

[Marco Hugentobler]

Hi qgis devs,

Some time ago I sent a proof-of-concept patch that restored screen updates 
during rendering. 

Now I generated an improved patch that also restores screen updates but it is 
done with less code changes (so simpler and closer to svn trunk code). 

Screen updates during rendering operations can be turned on by entering a 
value != 0 in Settings -> Options -> Rendering -> Update Display after 
rendering xxx features. The basic principle is that layers may emit the 
signal 'screenUpdateRequested()' such that the current rendering status shown 
in the qgis map window.

Also, the patch uses two Mutexes. One to protect the main rendering loop such 
that only one thread may render at one time. And the second to handle the 
problems when resizing the map window. The problem with resizing is that many 
refresh requests may arrive in short time. The solution implemented in the 
patch is that there is a waiting list where rejected threads leave their 
sizes. Then the mapcanvas considers only the last entry (which is the most 
important one) as soon as it is not busy with rendering any more. Therefore 
resizing should now also work properly for WMS layers.

On my todo list (but not implemented yet) is the possibility to stop the 
rendering. I plan to do this by introducing a QgsRenderingContext object, as 
suggested by Martin in an earlier Mail.

Please test the patch and report your experiences on the list.

Cheers,
Marco

-- 
Dr. Marco Hugentobler
Institute of Cartography
ETH Zurich
Technical Advisor QGIS Project Steering Committee
marco.hugentobler at karto.baug.ethz.ch
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