[QGIS-trac] [Quantum GIS] #1974: Enable render caching where possible

[Quantum GIS]

#1974: Enable render caching where possible
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   Reporter:  timlinux           |              Owner:  timlinux     
       Type:  patch              |             Status:  new          
   Priority:  minor: annoyance   |          Milestone:  Version 1.4.0
  Component:  MapCanvas          |            Version:  HEAD         
   Keywords:  cache performance  |   Platform_version:               
   Platform:  All                |           Must_fix:  No           
Status_info:  0                  |  
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 User story 1:

 Joe is reordering layers, each of which points back to a postgis database
 with thousands of records. Each time he drops a layer to a new position,
 he waits a long time while each layer redraws, even though the content of
 the layers is the same, just being rendered in a different order.

 User story 2:

 Jane is changing symbology in a layer. When she clicks apply, she waits a
 long time while all other layers redraw too.

 User story 3:

 Pete is hiding and showing layers. Each time he hides or shows a layer, he
 waits a long time while the layers redraw.


 ---

 The solution to this is to provide some kind of caching mechanism. The
 attached patch is my first draft that addresses the above scenarios and
 makes QGIS much more responsive when engaged in the activities described.

 Please note the patch is a work in progress and there are some edge cases
 where it does not yet behave well. In order to activate caching, tick the
 checkbox for this in the rendering tab of the options panel (its disabled
 by default).

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Ticket URL: <https://trac.osgeo.org/qgis/ticket/1974>
Quantum GIS <http://qgis.org>
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