Qt5 -> KDE5?

Andreas Hartmetz ahartmetz at gmail.com
Mon May 9 17:41:34 BST 2011

On Monday 09 May 2011 18:03:18 Olivier Goffart wrote:
> Hi,
> With Qt5 around the corner[1], I think it is time to start thinking about
> KDE5
> Raw summary:
>  - Qt5 is planed to be released in about one year from now if everything
> goes well.
>  - It should be mostly source compatible with Qt4, and is just an
> opportunity to break binary compatibility.
>  - QWidget just stay for compatibility. All focus is put on QML. Do not
> expect new development on QWidgets from Nokia.
>  - The "OpenGouvernance" should finaly come into light, meaning we (as
> KDE), can contribute easier to Qt.
> I guess it make sens, as Qt breaks binary compatibility, to do the same in
> kdelibs.
> Does that mean "KDE 5"? or "KDE SC 5"? Not necessarily.
> We can break binary compatibility, and change the .so version of our
> library without changing the major version of KDE itself.
> But I think it would anyway still be a smart move to do it.
> And I think this is a perfect opportunity to get some KDE class in Qt as we
> planed. [2]
> Some item we might want to think about:
>  - Do we want "KDE 5" to be a big change, or just a small increment?
>  - Do we want to focus on QML, or stay with QWidget?
I think this one is both obvious and difficult: Everything else being equal QML 
because it is, for all we know now, more "future compatible".

There are many open questions:
- Can we migrate QWidget-based code in some way?
- Which QWidget-based code is considered important? I'm thinking of
  KXmlGuiWindow and such things here.
- Will QML do what we need? Layouts, custom painting and event handling, one
  language for everything (we probably won't get that one).
- Can QML work on older hardware? (see Maksim's message)
- Can QML look and act "native" on the desktop? I don't care if it looks
  different, but it should be suitable for the given screen dimensions and
  input devices.
- Can we realistically pull off a minimal migration in time for the planned
   release date?
- What will the users think? -> We should not lose too many user-visible

I think it's not looking good for mostly switching to QML and releasing KDE5 
shortly after Qt5. Even when QML's own issues have been fixed there will be a 
huge amount of work for KDE, part of which can only happen once QML is there.
Maybe we should simply stay at KDE4/Qt4 for a while and release KDE5 for Qt5 
when it's done, but no later than in ~2 years if possible.
For developers releasing KDE5 with QT5 and later KDE6 with Qt5 and more QML 
seems tempting, but users won't like it.

>  - Shall we try drive Qt5 based on KDE5's need?
>  - Do we have more visions for what KDE5 should or should not be?
> I guess there is as many opinions as people involved :-)
> Many things to think about, and that can be discussed further, and decided
> in Platform11 [3] (I will be there)
> But in my opinion, if there is something we should learn from the KDE4
> transition, it is not to be too ambitious.

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