[QGIS-Developer] Refresh Layer at interval functionality: data or repaint?

[Tim Sutton]

Hi Richard

> On 17 Apr 2020, at 08:05, Richard Duivenvoorde <rdmailings at duif.net> wrote:
> 
> Hi,
> 
> on the User-mailing list:
> 
> https://lists.osgeo.org/pipermail/qgis-user/2020-April/045348.html
> 
> there was a question about the 'Refresh Layer at interval' checkbox in
> the Properties/Rendering tab.
> 
> I was wondering what the idea was behind it, as apparently it is only
> repainting a layer?

Hmm no it should be doing a complete redraw,

> While F5 (menu 'View/Refresh') is actually refreshing the data to (shown
> by retrieving a geojson layer from a mini service).
> 
> Is this by design? Or could it be an issue?

I think it is an issue. The original intent was to support applications like fleet tracking where you have the layer redraw every few seconds to update the current position of your fleet vehicles.

> 
> To me a real refresh checkbox (a la F5) would be useful to users.
> As this opens the use-case to for example hand an geojson-url to a
> vector layer which could retrieve a 'live data' containing the latest
> position of cars, or the latest measurements of a set of measuring
> points etc etc.
> And to be honest I do not see so much the value of repainting the canvas
> (other then when you want moving features created by an expression or so).
> And IF the repainting is valuable for a certain use-case: fine.
> Then: what about adding a second checkbox then there...?
> 
> Anybody has an idea about this?


Well the value of the refresh interval option is only there if the underlying data has changed, so maybe there was a regression. I wasn’t clear about the second check box you mentioned, but if the layer is not reloading it sounds like a bug. I think we mainly tested with a PG provided when Nyall added it. Note there is also some not so exposed magic by Nyall under the hood to optimise label redrawing so that the labels don’t ‘flash’ everytime the layer updates. I think you can only access it via python at the moment though given all the new temporal stuff it might be nice to surface that logic to the UI some time Nyall?

Regards

Tim


> 
> Regards,
> 
> Richard Duivenvoorde
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Tim Sutton

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