[Development] Notes from the QWidget session

[Alan Alpert]

On Tue, 26 Jun 2012 12:56:06 ext Harri Porten wrote:
> Here are my notes from the QWidgets session on June 22th, 2012, 11:30
> o'clock:
> 
> http://qt-project.org/groups/qt-contributors-summit-2012/wiki/QtWidgets
> 
> ,,,
>
> Help with migration from Qt 3/4 to 5
> ====================================
>   - Desktop QML components (using QStyle)
>   - Wrap QWidgets in QML container element

That's not going to be quite as easy as the inverse. While I think QML needs 
to be embeddable inside a QWidget scene for 5.0, there are a couple problems 
with the inverse so it will probably be a while until that works. See 
examples/declarative/cppextensions/qwidgets (Qt 4.x) for an example, it 
requires a QGraphicsProxyWidget equivalent (QGPW was almost 2k LoC, a lot more 
complex than the 103 line QWindowContainer WIP) and it requires a custom class 
to expose widget properties to QML. So even a complex container element is not 
enough, you also have to make every widget QML compatible (not actually that 
hard, most of them use Q_PROPERTY already).

>   - Educate developers about UI/backend separation
>   - Converter of .ui files to QML?

That's a good idea, and actually pretty close to useful already. All you 
really need to do is to register all the QWidget types into a QtWidgets 
import, they're already QObjects and I think all designable attributes are 
already marked as properties. Write a plugin that's all qmlRegisterType lines, 
and a utility that turns XML into QML (trivial, and I think I still have one 
lying around from ye olde days ;) ) and it should be pretty simple.

Mind you, there's a big difference between QML and .ui2 - this is one reason 
why there's a QtQuick module instead of just using the Qt Widgets in QML. 
Without signals on the properties, bindings won't work. Also without a default 
children property, the hierarchy doesn't build up naturally (or at all, the 
plugin of just qmlRegisterTypes might only work for single widget scenes). 
Neither the property set nor the property implementation is geared towards 
animation in widgets, so I doubt you'd get good results from trying to animate 
it using QML animations (worth a test though). All are fixable, but would 
require significant work on the widget set. All these issues are also reaching 
beyond features that are provided by .ui files, so go past the issue of porting 
from Qt 4.

I thought I had an ancient example which exposed QWidget in 4.6, but I can't 
find it. For the reasons described, it wasn't very useful anyway. The key point 
is that you can have any tree of QObjects described in QML, and it would be 
trivial to create a wrapper that places the root item as the central widget in 
a QMainWindow, as opposed to the current wrapper which places the root item 
into a QQuickItem scene.

--
Alan Alpert