Get sizeHint of a QLayout, doesn't update after adding items

[mortoray]

It appears that QLayout doesn't update sizeHint() any time a new item is added. Instead it seems like it differs it to some future time. I realize this by adding some items, checking sizeHint of the layout and seeing that it doesn't change.

I need a way to get the sizeHint immediately after adding several items to the layout. This is part of a system where a user selects several widgets and then the new window size is the preferred size of the layout. The user can however adjust the size later so I can't just forcibly use the size policy system.

Any ideas on how to get the proper sizeHint from QLayout immediately after adding several items? Calling "update" on it doesn't appear to do it.

[wysota]

Try calling QLayout::activate().

[mortoray]

I've tried that and it doesn't change the situation.

I've also checked now that as I add the elements the sizeHint on all the individual elements appears to be correct.

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Added after 14 minutes:

Tracking through the QT source the best I can say is that it is a defect in the QWidgetItemV2 class. The underlying widget has the correct sizeHint, but this LayoutItem wrapper is reporting 0,0 for max/min/hint sizes.

Now I have to look for a workaround.

[wysota]

Does it return true or false?

[mortoray]

Didn't check. Wouldn't matter. If it did return "false" I'd still be stuck in the same position: not knowing how to get the size.

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Added after 6 minutes:

This simple code shows that sizeHint doesn't work on the items if the target widget is already visible. I'll go file this at nokia.

#include <QtGui>
 
int main( int argc, char ** argv )
{
	QApplication app( argc, argv );
 
	QWidget * win = new QWidget();
	win->show(); //without this here it works as expected
 
	QBoxLayout * layout = new QHBoxLayout( win );
	QPushButton * button = new QPushButton( win );
	button->setText( "Sample" );
	layout->addWidget( button );
 
	qDebug() << button->sizeHint() << layout->itemAt( 0 )->sizeHint() << layout->sizeHint();
}

#include <QtGui>

int main( int argc, char ** argv )
{
	QApplication app( argc, argv );

	QWidget * win = new QWidget();
	win->show(); //without this here it works as expected
	
	QBoxLayout * layout = new QHBoxLayout( win );
	QPushButton * button = new QPushButton( win );
	button->setText( "Sample" );
	layout->addWidget( button );

	qDebug() << button->sizeHint() << layout->itemAt( 0 )->sizeHint() << layout->sizeHint();
}

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---

Added after 15 minutes:

QT Defect:
http://bugreports.qt.nokia.com/browse/QTBUG-15832

[wysota]

Didn't check. Wouldn't matter. If it did return "false" I'd still be stuck in the same position: not knowing how to get the size.

It does matter.

This simple code shows that sizeHint doesn't work on the items if the target widget is already visible. I'll go file this at nokia.

It's not a bug, the behaviour is correct. Your report will be rejected.

The real "bug" is in your way of thinking because you "think" you need the value immediately whereas in reality you don't. You only need it when the layout repositions the items it controls which is much later than "immediately". Stop thinking synchronously, start thinking asynchronously - it's not easy but it's worth it.

[mortoray]

It's not a bug, the behaviour is correct. Your report will be rejected.

I have provided the exact reason why I need this value immediately. I wish to automatically size my parent widget when the user changes the items that are present in it.

Because I have a workaround there is no reason not to believe this isn't just a defect in the QT code.

[wysota]

I have provided the exact reason why I need this value immediately.

Well, I can say that I need 2+2 to be 5 but it doesn't make it a bug that 2+2 is really 4.

I wish to automatically size my parent widget when the user changes the items that are present in it.

This does not require immediate knowledge of the layout size. You only need that when the Qt decides (by processing the events from the event loop) to show or hide those items which is waaaay later than when you put those items in the layout.

Because I have a workaround there is no reason not to believe this isn't just a defect in the QT code.

I can have a workaround for the 2+2=4 "issue" which doesn't make it a defect in C++ specification.

[mortoray]

This does not require immediate knowledge of the layout size. You only need that when the Qt decides (by processing the events from the event loop) to show or hide those items which is waaaay later than when you put those items in the layout.

Please provide a solution on how to do as I wish without requiring the size immediately. After the user changes the items in a display (Widgets in a BoxLayout) the parent widget should be resized to the preferred size of the layout (sizeHint under normal circumstances). The user may, after this point, adjust the size as they see fit.

[wysota]

Please provide a solution on how to do as I wish without requiring the size immediately.

I'm here to help you find solutions not to provide ready solutions for you. In extremly interesting cases I do that but that's certainly not one of them.

To help you understand why what you are trying to do doesn't work and shouldn't work let's make a simple theoretical experiment. Suppose you have a widget containing a bunch of children that sums to a total sizeHint() of 200x200. Now you add another widget (with sizeHint of 200x100) to a vertical layout controling the main widget. But the new child remains hidden - should the sizeHint of the layout immediately become 200x300?

After the user changes the items in a display (Widgets in a BoxLayout) the parent widget should be resized to the preferred size of the layout (sizeHint under normal circumstances). The user may, after this point, adjust the size as they see fit.

So you always want to force the sizeHint() regardless of the current size of the widget, yes? Hmm... strange but ok. There are a couple of ways to do it or at least a couple of things to look into. The first thing I would try would be to set the size constraint (Minimum) on the layout. If that's not what you want then you can for example subclass the layout class to provide a signal you can connect to whenever the layout is recalculated (this is most probably done in QLayoutItem::setGeometry() but you have to check it out in the source code). Then connect to the signal and resize the main widget. Instead of emitting a signal you can also resize the widget directly from within the layout effectively simulating what size constraints do.

You may also get away with simply reimplementing resizeEvent() of the main widget.