QGraphicsView scroll fade out

[Cruz]

Hello!

With the QGraphicsView framework I want to implement a scroll fade out feature. That means that I drag the scene with the mouse just like in the ScrollHandDrag mode, but when I release the button it shouldn't stop abruptly, but keep scrolling and slow down to come to a stop. I would love to build upon the existing ScrollHandDrag mode, because it does its job really well. It scrolls smoothly and stops at the scene border. I have no idea how to trigger it programmatically though.

Question one: Is there a way I can reuse the ScrollHandDrag mode programmatically?

Alright, so I thought I'll just override the mouse events and e.g. use translate() in the mouseMoveEvent like so.

void GraphicsViewWidget::mouseMoveEvent(QMouseEvent *event)
{
    QPoint mouse = event->pos();
    QPoint mouseDiff = mouse - lastMouse;
    lastMouse = mouse;
    translate(mouseDiff.x(), mouseDiff.y());
    update();
}

void GraphicsViewWidget::mouseMoveEvent(QMouseEvent *event)
{
    QPoint mouse = event->pos();
    QPoint mouseDiff = mouse - lastMouse;
    lastMouse = mouse;
    translate(mouseDiff.x(), mouseDiff.y());
    update();
}

To copy to clipboard, switch view to plain text mode 

Nope, it doesn't do much. The scene barely moves by a pixel. Even if I use it with hard coded values, say translate(100,100) the scene does not move. Apparently, setting any QTransform with setTransform() seems to have no effect.

Question two: Why doesn't any custom view transformation seems to work for me?


When I replace translate() with scroll(), then the scene does move, but it's very jittery. And also it doesn't stop at the scene border.
To avoid the jitter, I could remember the transform when the mouse was clicked and then calculate a new transform that is translated by the entire length of what the mouse has moved since the click was placed. But I don't see anything like a "scrollTo()" interface to go with that approach.

Question three: How do I do it right?

I don't want the scroll bars at all btw. I just keep them hidden and hope that they don't interfere.

[Cruz]

Ya, it was the scrollbars that interfered. After setting

setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
verticalScrollBar()->disconnect();
horizontalScrollBar()->disconnect();

setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
verticalScrollBar()->disconnect();
horizontalScrollBar()->disconnect();

To copy to clipboard, switch view to plain text mode 

in the constructor, now I can scroll the view properly when dragging it with the mouse. This is the reimplementation of the ScrollHandDragMode:

void GraphicsViewWidget::mousePressEvent(QMouseEvent *event)
{
	setCursor(Qt::ClosedHandCursor);
}
 
void GraphicsViewWidget::mouseMoveEvent(QMouseEvent *event)
{
	QPoint mouse = event->pos();
	QPoint mouseDiff = (mouse - lastMouse);
	lastMouse = mouse;
	QPointF mappedMouseDiff = mapToScene(mouseDiff) - mapToScene(0,0);
 
	if (event->buttons() & (Qt::LeftButton | Qt::RightButton | Qt::MiddleButton))
	{
		translate(mappedMouseDiff.x(), mappedMouseDiff.y());
	}
}
 
void GraphicsViewWidget::mouseReleaseEvent(QMouseEvent *event)
{
	unsetCursor();
}

void GraphicsViewWidget::mousePressEvent(QMouseEvent *event)
{
	setCursor(Qt::ClosedHandCursor);
}

void GraphicsViewWidget::mouseMoveEvent(QMouseEvent *event)
{
	QPoint mouse = event->pos();
	QPoint mouseDiff = (mouse - lastMouse);
	lastMouse = mouse;
	QPointF mappedMouseDiff = mapToScene(mouseDiff) - mapToScene(0,0);

	if (event->buttons() & (Qt::LeftButton | Qt::RightButton | Qt::MiddleButton))
	{
		translate(mappedMouseDiff.x(), mappedMouseDiff.y());
	}
}

void GraphicsViewWidget::mouseReleaseEvent(QMouseEvent *event)
{
	unsetCursor();
}

To copy to clipboard, switch view to plain text mode 

It scrolls properly no matter how the scene is scaled. It doesn't include the scroll fade out feature yet, it will remain my secret.