[Qgis-developer] guestimating plug-in dev time?

[Rémi Cura]

I think a more global solution is possible.
After all when you load a postgis layer in qgis, if somebody else changes
it, the view is not refreshed, which is annoying (and sometimes dangerous
because you may edit old data).

It would be easy to ad this (which would also work for my case) using the
LISTEN / NOTIFY feature of postgres.

Basically when selecting a table as a new qgis layer, you automatically add
a triger to it.
The trigger will be fired at any change (create/delete/update).
The trigger would simply send a message to qgis via NOTIFY.
example : "NOTIFY qgis_change_XXXX ;", where XXXX is a unique id.
Of course qgis would be listening, which is very simple in python
(import psycopg2;
conn = psycopg.connect(my_database_string)
conn.autocommit(1)
curs = conn.cursor()
curs.execute("LISTEN qgis_change_XXXX;"))

Of course one would need a mechanism to remove this trigger when not
necessary (for instance, name the channel "qgis_change_*time_stamp*",
delete channel too old automatically, and change channel periodically (20
in for instance)).

I don't know if from a security point of view it would be possibe tough
(would require the permission to create a trigger).
I don't know if typical qgis user have the right to create trigger on
database they use.

I use a lot of different version of QGIS and I can guarantee at least a
dozen crash a day with version 1.8 to 2.3,
plus pending transaction with postgis whenever the crash occurs.
I'm installing 2.6 to see if postgis layers are less fragile right now ^^

Cheers,
Rémi-C

2015-01-19 11:29 GMT+01:00 Martin Dobias <wonder.sk at gmail.com>:

> I see - indeed QGIS would need to do quite some database inspection to
> understand that layer A and layer B are related. This should be however
> also easy to address in a custom plugin.
>
> It is surprising to me that you say PostGIS layers are fragile in QGIS -
> they are very commonly used by lots of people and we haven't heard about
> crashes in the issue tracker. If you do experience crashes please consider
> filing reports on hub.qgis.org with instructions on how to replicate them.
>
> Regards
> Martin
>
>
> On Mon, Jan 19, 2015 at 5:18 PM, Rémi Cura <remi.cura at gmail.com> wrote:
>
>> Thanks Martin,
>> one day to several days feel like a very reasonnable order of magnitude.
>>
>> QGis is not refreshing the view because when editing layer A the postgis
>> data base automatically also edit layer B.
>> I don't think qgis has a mechanism of tracking dbchanges (it would be
>> possible tough).
>> Morevover PostGIS layer are still "fragile" in qgis (crashes often).
>>
>> Thanks for the help,
>> Cheers
>> Rémi-C
>>
>> 2015-01-19 10:56 GMT+01:00 Martin Dobias <wonder.sk at gmail.com>:
>>
>>> Hi Remi
>>>
>>> Such plugin should be quite easy to write - I would assume up to one day
>>> of work for an experienced pyqgis developer. For a newcomer it may take few
>>> more days to get familiar with the API, tools, environment and so on.
>>>
>>> For the refresh of rendering after geometry editing - that should work
>>> out of the box and I would consider it a bug if it does not.
>>>
>>> Regards
>>> Martin
>>>
>>>
>>> On Mon, Jan 19, 2015 at 3:16 PM, Rémi Cura <remi.cura at gmail.com> wrote:
>>>
>>>> Hello dear list.
>>>> I'm usually active on the postgis or postgres list,
>>>> but today I would like to ask a question regarding dev time.
>>>>
>>>> In your opinion,
>>>> How much time would be needed to write a plugin for multiple versions
>>>> of qgis (2.2 to current 2.6) doing 3 simple things :
>>>>  - auto commit the edition of geometry when in edition mode (no need to
>>>> click on save  changes each time)
>>>>  - force refresh of rendering after edition of a geometry
>>>>  - every camera changes (zoom or position), send a sql query containing
>>>> user name and geometry of view (a rectangle in wkt for instance).
>>>>
>>>> I can estimate this at my poor qgis-dev level, but I would like to have
>>>> the timing for an expert qgis dev.
>>>>
>>>> Many thanks,
>>>>
>>>> Remi-C
>>>>
>>>> _______________________________________________
>>>> Qgis-developer mailing list
>>>> Qgis-developer at lists.osgeo.org
>>>> http://lists.osgeo.org/mailman/listinfo/qgis-developer
>>>>
>>>
>>>
>>
>
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