Moving of QGraphicsItem

[Erlendhg]

Hi.

I am making a timeline widget based on Qt's amazing Graphics View framework.
Now as you an see on the picture attached, I have three lines where I am putting QGraphicsItems.

Now, I have made them movable with the ItemIsMovable flag, but I only want each item to be movable on the X axis (from left to right). Currently, you can move them in every direction.

What is the best way to achive this?

Many thanks,
Erlend Graff

[marcel]

The best way is to loose the ItemIsMovable flag and implement your own item moving functionality ( using the item mouse events in conjunction with scene coordinates ).

Then, all you have to do is restrict the Y position of the item and alow only X to be modified.

Regards

[Bitto]

There's a better way. In your item, reimplement the itemChange() virtual function. Handle the ItemPositionChange case to fixate one of your axises. You can keep QGraphicsItem's default move implementation, and still only allow moving along one axis.

QVariant MyItem::itemChange(GraphicsItemChange change, const QVariant &value)
{
    if (change == ItemPositionChange)
        return QPointF(pos().x(), value.toPointF().y());
    return QGraphicsItem::ItemChange(change, value);
}

QVariant MyItem::itemChange(GraphicsItemChange change, const QVariant &value)
{
    if (change == ItemPositionChange)
        return QPointF(pos().x(), value.toPointF().y());
    return QGraphicsItem::ItemChange(change, value);
}

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Neat? ;-)

[marcel]

Neat? ;-)

Indeed it is

[Erlendhg]

Wow.
That was a great solution. Got to keep so much as possible of Qt's fantastic native implementation.

Thanks

[Erlendhg]

Hi.
Now I am here again to annoy you with another question.

This is what I want to achieve:
When I move an item in my timeline widget, I want to have a tooltip showing the current position where the item starts, and where it stops.

This, I first wrote like this:

void eventItem::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
	QGraphicsItem::mouseMoveEvent(event);
	QToolTip::showText(event->screenPos(), QString("This item goes from position %1 to %2").arg(startOfItem).arg(startOfItem+lengthOfItem));
}

void eventItem::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
	QGraphicsItem::mouseMoveEvent(event);
	QToolTip::showText(event->screenPos(), QString("This item goes from position %1 to %2").arg(startOfItem).arg(startOfItem+lengthOfItem));
}

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where startOfItem and lengthOfItem is variables in the items class, telling the start and length of the item. These are updated in itemChange(), therefore the call to QGraphicsItem::mouseMoveEvent(event); before the tooltip.

But earlier, when I moved my items around, the program could suddenly freeze, and nothing would then happen.

This, I guess, may have been a problem that I have fixed somehow, 'cause later on now, I can't re-create it. (I am still quite n00b in Qt :P)

But my real question is:
Is there a better way to acheive this?
If possible, I would like to see the tooltip less "flashy", if you understand what I mean.

Many thanks,
Erlend Graff

[marcel]

Why a tooltip? Why not use a "floating" widget that you custom-draw( you could make it look better than a tooltip - transparent, etc ).

To keep the widget glued to the graphics item you can call QWidget::move, setGeometry, etc. You have the mouse position, anyway.

It really should be easy - just fill it, draw some text and set transparency.

regards

[Bitto]

Have you seen this blog? http://labs.trolltech.com/blogs/2006...raphics-items/

Like Marcel says, you probably want to do something like what this example shows. When you move the cursor over certain parts of the item, your "tooltips" could hover up just like the circular indicators show (in this example it's not even a separate item). If you want it to be resolution independent, you could also set QGraphicsItem::ItemIgnoresTransformations on the tooltip-ish item.

[Erlendhg]

Sorry, I didn't get that.
But I think I might have made me a bit unclear.
What I want, is that when I click and drag items in my timeline, a tooltip will always show the current start and stop position of the item, until the mouse button is released.

That is what my code does, except every time I move the mouse (while dragging an item), the tooltip is redrawn, and this makes it "flashy".

I want to make it as simple as possible, so I thought a tooltip would not be too hard, but it seems to be more difficult than expected.

[Erlendhg]

Hi again.
I am here with another problem.
I have stripped my timeline code, to show you the problem.

Attached is the code and a video (very crappy) trying to explain what happens.

When I add an item (click outside the QGraphicsView, and click "Q"), slides it to the beginning of the view, resizes it down quite a bit, and deletes it (with "W"), it doesn't disappear, but is resized up.

I can't understand why this happens, so I hope you guys can help me solve it.

(I'm pretty sure it must be something I have done wrong, because the process of the program also uses very much CPU!)

If it is relevant: I am using Qt 4.2.2

Many thanks,
Erlend Graff

[spud]

I can't reproduce your bug, it works fine for me. However, you have forgotten to delete the eventItems after you remove them. This might be the source of your problem.

Try

void EventView::removeItem(eventItem *item)
{
	int index = getIndexFromItem(item);
 
	eventScene->removeItem(eventList[index]);
	delete eventList.takeAt(index);
}

void EventView::removeItem(eventItem *item)
{
	int index = getIndexFromItem(item);

	eventScene->removeItem(eventList[index]);
	delete eventList.takeAt(index);
}

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[Matthew.J]

There's a better way. In your item, reimplement the itemChange() virtual function. Handle the ItemPositionChange case to fixate one of your axises. You can keep QGraphicsItem's default move implementation, and still only allow moving along one axis.

QVariant MyItem::itemChange(GraphicsItemChange change, const QVariant &value)
{
    if (change == ItemPositionChange)
        return QPointF(pos().x(), value.toPointF().y());
    return QGraphicsItem::ItemChange(change, value);
}

QVariant MyItem::itemChange(GraphicsItemChange change, const QVariant &value)
{
    if (change == ItemPositionChange)
        return QPointF(pos().x(), value.toPointF().y());
    return QGraphicsItem::ItemChange(change, value);
}

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Neat? ;-)

Awesome!!!!