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

[Bo Victor Thomsen]

Hi Nyall -

I totally agree with you on the subject of exposing *new* QT6-only 
functionality in QGIS. Just that the existing *common* features doesn't 
change with different qt versions. And the QT6-only features is marked 
as such in the documentation.

-- 
Med venlig hilsen / Kind regards

Bo Victor Thomsen

Den 07-07-2020 kl. 00:45 skrev 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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