[QGIS-Developer] QGIS and Qt 6: A proposal

[Nyall Dawson]

On Mon, 6 Jul 2020 at 22:38, Greg Troxel <gdt at lexort.com> wrote:
>
> Bo Victor Thomsen <bo.victor.thomsen at gmail.com> writes:
>
> > 3. I like the idea, that you can build QGIS with both the QT ver 5.x
> >    and QT ver 6.x in a (limited) time period. That gives the plugin
> >    developer some peace of mind, having both the "old" and the "new"
> >    QGIS as a testing ground before plugin release
>
> I like this too, but given many packaging systems and many reasons, I
> see "limited" as being like a year and maybe more.  Basically it would
> be nice if qgis worked on both until very few people had a reason to use
> qt5, and I think that just means  sticking to the non-deprecated API in
> 5.15.

Actually I'd be perfectly fine if we start adding new features which
rely on functionality added in Qt 6, so long as these are
automatically disabled on the Qt 5 builds. We have precedent for this
in the past -- when QGIS 3D was introduced it was only available if
the Qt library version was sufficiently recent, and also when GeoPDF
export was added we only enabled it for GDAL 3 builds.

I think any kind of hard rule to say "no new functionality in qgis
unless it also works in Qt 5" is a bit dangerous, because it's
encouraging us to do downstream work when we should be working closely
with upstream projects (Qt, GDAL, etc). We've been grossly negligent
about this in the past, and only in recent times are starting to
reverse this mentality. If we delay and stick to Qt 5 for the next 2-3
years then we'll slide right back into ignoring upstream again.

Nyall


> A really big question is how packaging systems are going to deal with
> qt6.  With qt4->qt5, pkgsrc had qt4 and qt5 packages.   qt4 remains,
> although less and less uses it.   I am pretty sure some useful code that
> uses qt5 is going to take a really long time to build against qt6, and
> thus qt5 is likely to remain for several years.  This leads to needing
> to have qt6 packages, rather than updating qt5 to "qt5-6.0.0".
>
> I epxect Debian and Fedora are going to add qt6 that can be installed
> alongside qt5, rather than just updating the qt5 packages to qt6,
> because replacing qt5 with qt6 would lead to removing all qt5-using
> packages  that don't have a release that works with qt6.  Certainly I
> expect pkgsrc to head that way, and it would be interesting to hear from
> Debian, FC, FreeBSD ports, the various mac-oly packaging systems, etc.
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