[Development] QML preprocessor

Albert Astals Cid albert.astals.cid at kdab.com
Mon Mar 18 09:11:38 CET 2019

El diumenge, 17 de març de 2019, a les 13:34:09 CET, Alberto Mardegan va escriure:
> Hi there!
>   When developing a QML module for the greater public one wants to
> provide a source package that can be built on multiple Qt versions.
> However, QML's lack of a preprocessor makes this rather cumbersome.
> Suppose that my module needs to provide an element like this:
> ====
> import QtQuick 2.10
> Item {
>     ...
>     Flickable {
>         boundsMovement: Flickable.StopAtBounds
>         ...
>     }
> }
> ====
> If I wanted to make this available to users of Qt 5.9 and older, I'd
> need to ship another file, instead, importing QtQuick 2.9 and removing
> that "boundsMovement" property.
> Keeping two or more copies of the same file and installing the proper
> one depending on the Qt version is certainly doable, but it carries a
> considerable maintenance cost.
> I've personally decided to keep a single copy of each QML file but use
> different git branches; however, this also has its maintenance costs and
> makes things harder for people using the project, in that they need to
> checkout the right branch of the project, otherwise it won't work on
> their machines.
> Has this problem been recognized and discussed before? 

We have fixed this problem in some places by using a Loader that changes the source on loading error

MyItem.qml code (which is what users of the code are supposed to use would be)
	Loader { 
		source: "MyItemForQuick210.qml" 
		onStatusChanged: if (loader.status == Loader.Error) source = "MyItemForQuick26.qml"

Not real code so may not "compile", but you should get the idea.

It's not awesome but for our use-case it worked.


> Would a QML
> preprocessor be a viable option?
> I understand that this is not a problem for Qt itself, and it's a minor
> problem for app developers; however, developing a portable QML module
> depending on QtQuick is nearly impossible due to this, unless one agrees
> to settle on a certain old version of Qt and renounce to all the goodies
> that were added in later versions.
> Ciao,
>   Alberto
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Albert Astals Cid | albert.astals.cid at kdab.com | Software Engineer
Klarälvdalens Datakonsult AB, a KDAB Group company
Tel: Sweden (HQ) +46-563-540090, USA +1-866-777-KDAB(5322)
KDAB - The Qt, C++ and OpenGL Experts

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