[Qgis-developer] [Qgis-user] Problem with "add postgis layers" in qgis 1.80

[Jürgen E. Fischer]

Hi Etienne,

On Thu, 19. Jul 2012 at 15:17:42 -0300, Etienne Tourigny wrote:
> A more generic question - is there a way that we could implement a
> generic mechanism for stopping a processing that takes forever, such
> as
> 
> - Identify tool in a *very* large vector layer
> - unresponsive WMS server (when opened in the browser dock)

> I would like to be able to be able to cancel such actions, for example
> wit the Esc key. Currently when this happens I have to kill the qgis
> application which is rather extreme.

> Would this require running processes in a thread, or is there a way QT
> can listen for a specific event and cancel whatever is happening,
> returning to a "inactive" application?

Threads can help, if you can't avoid to block, but other things can be solved
by switching from iterative processing to event driven processing (with or
without threads).

That's the "normal" way Qt does things, but as soon we leave the framework and
turn to external APIs like GEOS, GDAL, libpq etc. the intended (or just the
easiest) way there is iterative.  And therefore lots of stuff in QGIS is also
iteratively.  There are even a few places where event driven Qt stuff is
wrapped in a polling loop with QCoreApplication::processEvents(), when we need
event driven Qt stuff within iterative loops.

Although we could probably move all that blocking stuff into threads, that emit
signals whenever a blocking iterative loop within the thread continues.

So curring this would require major changes - also to the QGIS API. Add
complexity and also introduce new pitfalls (eg. race conditions, locking...).

BTW the threading branch was/is addressing some of these issues.


Jürgen

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