QGraphicsView scrolling problem with 4.3.0

[hb]

To work around the bug described in this post, I installed Qt 4.3.0 now. The bug seems fixed indeed, however, I am now experiencing another problem that was not present with Qt 4.2.3.

I derived a class from QGraphicsView, and did a custom implementation for scrolling when the user clicks on the background. That implementation worked flawlessly with Qt 4.2.3, but in Qt 4.3.0, it only works if I set at least one of the scrollbar policies not to ...AlwaysOff.

I attached a minimal self-containing compilable example below. I'd have said it's a bug introduced in 4.3.0, but in contrast to the original code, that example doesn't work as expected even if compiled against 4.2.3...

I am puzzled. Any help would be greatly appreciated.

#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsTextItem>
#include <QGraphicsScene>
#include <QMouseEvent>
#include <QScrollBar>
 
class MyWidget : public QGraphicsView
{
public:
	MyWidget(QGraphicsScene *scene, QWidget *parent = 0);
 
protected:
	virtual void mousePressEvent(QMouseEvent *event);
	virtual void mouseReleaseEvent(QMouseEvent *event);
	virtual void mouseMoveEvent(QMouseEvent *event);
 
private:
	void storeMouseEvent(QMouseEvent *event);
	bool scrolling;
	QMouseEvent lastMouseEvent;
};
 
MyWidget::MyWidget(QGraphicsScene *scene, QWidget *parent)
		: QGraphicsView(scene, parent), scrolling(false),
		lastMouseEvent(QEvent::None,QPoint(),Qt::NoButton,Qt::NoButton,Qt::NoModifier)
{
	// If at least one of these has the policy ..AlwaysOn, scrolling works
	// (in both directions)!
	setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
	setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
}
 
void MyWidget::storeMouseEvent(QMouseEvent *event)
{
	lastMouseEvent = QMouseEvent(event->type(), event->pos(),
	                             event->globalPos(),
	                             event->button(),event->buttons(),event->modifiers());
}
 
void MyWidget::mousePressEvent(QMouseEvent *event)
{
	// just handle left button presses on the background
	QGraphicsItem *item = itemAt(event->pos());
 
	if(!item) {
		if((event->button() == Qt::LeftButton)) {
			scrolling = true;
			storeMouseEvent(event);
			event->accept();
			return;
		}
	}
	event->ignore();
	QGraphicsView::mousePressEvent(event);
}
 
void MyWidget::mouseMoveEvent(QMouseEvent *event)
{
	// perform scrolling if scrolling is active
	if(scrolling) {
		QPoint delta = event->globalPos() - lastMouseEvent.globalPos();
		QScrollBar *hBar = horizontalScrollBar();
		QScrollBar *vBar = verticalScrollBar();
		hBar->setValue(hBar->value() + (isRightToLeft() ? delta.x() : -delta.x()));
		vBar->setValue(vBar->value() - delta.y());
		storeMouseEvent(event);
		event->accept();
		return;
	}
	event->ignore();
	QGraphicsView::mouseMoveEvent(event);
}
 
void MyWidget::mouseReleaseEvent(QMouseEvent *event)
{
	// end scrolling mode if it was active
	if((event->button() == Qt::LeftButton) && (scrolling == true)) {
		scrolling = false;
		storeMouseEvent(event);
		event->accept();
		return;
	}
	event->ignore();
	QGraphicsView::mouseReleaseEvent(event);
}
 
int main(int argc, char *argv[])
{
	QApplication app(argc, argv);
	QGraphicsScene scene;
	MyWidget widget(&scene);
	QGraphicsTextItem item("hello");
	scene.addItem(&item);
	widget.show();
	scene.setSceneRect(-1000,-1000,2000,2000);
	return app.exec();
}

#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsTextItem>
#include <QGraphicsScene>
#include <QMouseEvent>
#include <QScrollBar>

class MyWidget : public QGraphicsView
{
public:
	MyWidget(QGraphicsScene *scene, QWidget *parent = 0);

protected:
	virtual void mousePressEvent(QMouseEvent *event);
	virtual void mouseReleaseEvent(QMouseEvent *event);
	virtual void mouseMoveEvent(QMouseEvent *event);

private:
	void storeMouseEvent(QMouseEvent *event);
	bool scrolling;
	QMouseEvent lastMouseEvent;
};

MyWidget::MyWidget(QGraphicsScene *scene, QWidget *parent)
		: QGraphicsView(scene, parent), scrolling(false),
		lastMouseEvent(QEvent::None,QPoint(),Qt::NoButton,Qt::NoButton,Qt::NoModifier)
{
	// If at least one of these has the policy ..AlwaysOn, scrolling works
	// (in both directions)!
	setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
	setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
}

void MyWidget::storeMouseEvent(QMouseEvent *event)
{
	lastMouseEvent = QMouseEvent(event->type(), event->pos(),
	                             event->globalPos(),
	                             event->button(),event->buttons(),event->modifiers());
}

void MyWidget::mousePressEvent(QMouseEvent *event)
{
	// just handle left button presses on the background
	QGraphicsItem *item = itemAt(event->pos());

	if(!item) {
		if((event->button() == Qt::LeftButton)) {
			scrolling = true;
			storeMouseEvent(event);
			event->accept();
			return;
		}
	}
	event->ignore();
	QGraphicsView::mousePressEvent(event);
}

void MyWidget::mouseMoveEvent(QMouseEvent *event)
{
	// perform scrolling if scrolling is active
	if(scrolling) {
		QPoint delta = event->globalPos() - lastMouseEvent.globalPos();
		QScrollBar *hBar = horizontalScrollBar();
		QScrollBar *vBar = verticalScrollBar();
		hBar->setValue(hBar->value() + (isRightToLeft() ? delta.x() : -delta.x()));
		vBar->setValue(vBar->value() - delta.y());
		storeMouseEvent(event);
		event->accept();
		return;
	}
	event->ignore();
	QGraphicsView::mouseMoveEvent(event);
}

void MyWidget::mouseReleaseEvent(QMouseEvent *event)
{
	// end scrolling mode if it was active
	if((event->button() == Qt::LeftButton) && (scrolling == true)) {
		scrolling = false;
		storeMouseEvent(event);
		event->accept();
		return;
	}
	event->ignore();
	QGraphicsView::mouseReleaseEvent(event);
}

int main(int argc, char *argv[])
{
	QApplication app(argc, argv);
	QGraphicsScene scene;
	MyWidget widget(&scene);
	QGraphicsTextItem item("hello");
	scene.addItem(&item);
	widget.show();
	scene.setSceneRect(-1000,-1000,2000,2000);
	return app.exec();
}

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[spud]

Hi,
I reported this problem to the Qt Task tracker some time ago. It got the issue nr #167439 but I can't seem to find it anymore. Anyway the trolls answered:

Thank you for your bug-report.

> QGraphicsView ScrollBarPolicy behaviour changed with Qt 4.3
>
> What I did:
> Created a QGraphicsView and I set both scrollbarPolicies to
> ScrollBarAlwaysOff and the DragMode to ScrollHandDrag.
>
> What I expected to see:
> Up until Qt 4.2 I could use the hand or mouse wheel to scroll.

Happy days anyway. This bug is fixed in the latest snapshot, and will be
a part of the 4.3.1 release.

[jpn]

Try QAbstractScrollArea::scrollContentsBy():

scrollContentsBy((isRightToLeft() ? -delta.x() : delta.x()), delta.y());

scrollContentsBy((isRightToLeft() ? -delta.x() : delta.x()), delta.y());

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[hb]

Thanks spud. I'll wait for 4.3.1 to come out then, hoping that it will indeed fix this problem.

[hb]

Try QAbstractScrollArea::scrollContentsBy():

I was thinking about that function as well, but this quote from the docs made me not use it:

Calling this function in order to scroll programmatically is an error, use the scroll bars instead (e.g. by calling QScrollBar::setValue() directly).

[jpn]

I was thinking about that function as well, but this quote from the docs made me not use it:

Good notice. Anyway, I'm not exactly sure if it really causes any problems in this case since scroll bar policies are set to Qt::ScrollBarAlwaysOff. One would have to explore the sources a bit to find out..

[hb]

Good notice. Anyway, I'm not exactly sure if it really causes any problems in this case since scroll bar policies are set to Qt::ScrollBarAlwaysOff. One would have to explore the sources a bit to find out..

True, but I'd rather not go for that. Even if it works in this version, you can never know about future versions if that use-case is so directly discouraged in the docs. Thanks anyways

[Elgot]

Thanks spud. I'll wait for 4.3.1 to come out then, hoping that it will indeed fix this problem.

Is it just me, or is this broken in 4.3.1 too?

[Bitto]

Hm... the regression with scrollbars was fixed in 4.3.1, and there were no outstanding QGV bugs at the time 4.3.1 was released.