[SOLVED] QScrollArea: Vertical scroll only.

ultim8

Found a solution! simply look for resize events for the scrollAreaWithContents widgets and set the scrollArea's minimum width to the scrollAreaWithContents minimumSizeHint().with():

@VerticalScrollArea::VerticalScrollArea(QWidget *parent)
: QScrollArea(parent)
{
setWidgetResizable(true);
setFrameStyle(QFrame::NoFrame);
setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
setVerticalScrollBarPolicy(Qt::ScrollBarAsNeeded);
m_scrollAreaWidgetContents = new QWidget(this);
m_scrollAreaWidgetContents->setSizePolicy(QSizePolicy::Preferred, QSizePolicy::Preferred);
QVBoxLayout *baseLayout = new QVBoxLayout(m_scrollAreaWidgetContents);
setWidget(m_scrollAreaWidgetContents);
m_scrollAreaWidgetContents->installEventFilter(this);
}

bool VerticalScrollArea::eventFilter(QObject *o, QEvent *e)
{
if(o == m_scrollAreaWidgetContents && e->type() == QEvent::Resize)
setMinimumWidth(m_scrollAreaWidgetContents->minimumSizeHint().width() + verticalScrollBar()->width());

return false;
}@

ostlerc

Thanks for solving this problem! However, unless your minimum size changes on resize events you will only need to set this once after a show event.

galinette

Thanks for the solution!

By the way, this is close to be a bug. A QScrollArea with horizontal bars switched off should set its minimum width by considering the minimum width of the contents widget, no?

mstone1959

Hello,

I'm fairly new to Qt and I would like to know how I implement the VerticalScrollArea class to correct the problem?

ultim8

[quote author="mstone1959" date="1389726822"]Hello,

I'm fairly new to Qt and I would like to know how I implement the VerticalScrollArea class to correct the problem?[/quote]

You will want to create a new class "VerticalScrollArea" derived off of QScrollArea.

header:
@#ifndef VERTICALSCROLLAREA_H
#define VERTICALSCROLLAREA_H

#include <QScrollArea>

class VerticalScrollArea : public QScrollArea
{
Q_OBJECT
public:
explicit VerticalScrollArea(QWidget *parent = 0);

virtual bool eventFilter(QObject *o, QEvent *e);

};

#endif // VERTICALSCROLLAREA_H@

Implementation:
@#include <QEvent>
#include <QScrollBar>

#include "verticalscrollarea.h"

VerticalScrollArea::VerticalScrollArea(QWidget *parent) :
QScrollArea(parent)
{
setWidgetResizable(true);
setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
setVerticalScrollBarPolicy(Qt::ScrollBarAsNeeded);
}

bool VerticalScrollArea::eventFilter(QObject *o, QEvent *e)
{
// This works because QScrollArea::setWidget installs an eventFilter on the widget
if(o && o == widget() && e->type() == QEvent::Resize)
setMinimumWidth(widget()->minimumSizeHint().width() + verticalScrollBar()->width());

return QScrollArea::eventFilter(o, e);
}
@

trusktr

[quote author="galinette" date="1360135272"]Thanks for the solution!

By the way, this is close to be a bug. A QScrollArea with horizontal bars switched off should set its minimum width by considering the minimum width of the contents widget, no?[/quote]

I don't think so. I think setting the horizontal scroll bar to off should simply not show a horizontal scroll bar (but that doesn't mean the area can't be scrolled, e.g. with touch and drag). Changing the width of the scroll area should be a separate step if the developer so desires.

Jacques L.

Although old, this topic saved me time. So here are my two cents here. I change the constructor to this to support one-finger scrolling on Android (didn't try for iOS yet):

CVScrollArea::CVScrollArea(QWidget *parent)
   : QScrollArea(parent)
{
   setWidgetResizable(true);
   setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);

// One-finger swipe/scroll while default in Qt is 3-fingers!
   QScroller::grabGesture(this->viewport(), QScroller::LeftMouseButtonGesture);

// No scroll bar for Android (and probably iOS)
#ifndef Q_OS_ANDROID
      this->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
#else
      this->setVerticalScrollBarPolicy(Qt::ScrollBarAsNeeded);
#endif
}

viteo

Hello from 2022.
Used all my google-fu to find this answer. Thank you!
I confirm that there is still no standard QVerticalScroll, you have to implement it yourself with overriding eventFilter.

p.s.
...but the eventFilter() get a lot of calls from everythin (mouse, activation, resize etc)
I suggest to override resizeEvent(). Here is my approach:

HEADER:

protected:
    void resizeEvent(QResizeEvent *);

SOURCE:

void ClassName::resizeEvent(QResizeEvent* event)
{
    setMinimumWidth(widget()->minimumSizeHint().width() + verticalScrollBar()->width());
    QScrollArea::resizeEvent(event);
}

Vaquita Tim

Every couple of years this comes up...

Certainly setMinimumWidth() is key - it needs to fit the size of the scrolled widget().

In my derived class I actually connected a slot to the horizontal scroll bar, which changes if the widget() changes width

connect( horizontalScrollBar(), &QAbstractSlider::rangeChanged, this, &CVerticalScrollArea::ResetMinWidth );

then I calculate the minumum width bearing in mind the frame width. I do the same for the minimumSizeHint too (although maybe this is not required).

QSize CVerticalScrollArea::minimumSizeHint() const
{
	QSize MinSize( QScrollArea::minimumSizeHint() );
	const QWidget * pWidget( widget() );
	if( pWidget )
	{
		const int FrameWidth( 2 * frameWidth() );
		const int TotalWidth( pWidget->minimumSizeHint().rwidth() + FrameWidth );
		if( TotalWidth > MinSize.width() )
			MinSize.setWidth( TotalWidth );
	}
	return MinSize;
}

Steve King

Thank you for the tip;
I'm using Python, so I rewrote like this, and it works:

class VerticalScrollArea(QtWidgets.QScrollArea):

def __init__(self):
    super().__init__()
    self.setWidgetResizable(True)
    self.setHorizontalScrollBarPolicy(Qt.ScrollBarAlwaysOff)
    self.setVerticalScrollBarPolicy(Qt.ScrollBarAsNeeded)    

def eventFilter(self, object, event):
    # This works because QScrollArea::setWidget installs an eventFilter on the widget
    if object is not None and object == self and event.type() == QEvent.Resize:
        self.setMinimumWidth(self.minimumSizeHint().width() + self.verticalScrollBar.width())

    return super().eventFilter(object, event)