[Qt-interest] size of a widget with layout before shown

[Frank Mertens]

Karol Krizka wrote:
> On Sat, May 30, 2009 at 8:25 PM, Frank Mertens <frank at cyblogic.de> wrote:
>> Frequently I face the same problem with Qt's layout system.
>> How to determine the size a widget would take if shown as top-level widget?
>> Let's say a QMenu? -- Need to have it placed relative to its size.
>>
>> I'm used to hack around this issue by show()/hide() and some forced
>> manual event processing.
>>
>> But is there no clean solution?
> I'm actually attempting to solve the same problem, so let me know if
> you come up with anything clean.
> 
> For now, I'm also using a show()/hide() trick. Basically, I start the
> widget as hidden. When I want the size, I do the following:
> setUpdatesEnabled(false);
> widget->show();
> widget->hide();
> setUpdatesEnabled(true);
> 
> When the widget is hidden, I use a spacer to reserve space for it.
> 

After fighting down the same path again yesterday I figured things improved
with Qt 4.5. I have a custom widget, which I use as popup menu and I'm
using QMenu for context menus. QMenu::sizeHint() now always delivers meaningful
sizes, before shown the first time. For my custom widget I had to do the
following for Qt 4.4:

	layout()->activate();
	QApplication::processEvents(QEventLoop::ExcludeUserInputEvents);
	QSize sz = sizeHint();

On Qt 4.5 I can safely comment out the second line. If commenting out the
first two lines I get only a meaningful sizeHint() if the widgets layout
has not been changed.