Solved Dragging QListWidgetItem into QGraphicScene and create QGraphicsItem

[tobiSF]

Hey.

I do have an object in my software which I have declared a QMetaType so I can store it in a QVariant. Now I have a QListWidget that shows all instances of this object with its value stored inside the individual QListWidgetItems as UserType.

Module module("Module A");
QListWidget* list = new QListWidget();
[...]

QListWidgetItem* item = new QListWidgetItem(module.name());
item->setData(Qt::UserRole, QVariant::fromValue(module));
list->addItem(item);

Now I want to drag this item onto a QGraphicsScene and extract the QVariant contained in the QListWidgetItem inside the QGraphicsScene::dropEvent(...) and create a new custom QGraphicsItem that is a graphical representation of the Module there, as well.

I was reading in the Drag & Drop Framework documentation as well as the documentation for QGraphicsView but since the QListWidgetItem supports drag&drop by default, I was wondering if there isn't an easy way to get this done.

Any help is really appreciated!

[raven-worx]

@tobiSF
IIRC thats not cleanly possible with QListWidget.

the model needs to provide the drop-data and a mime-type you can listen to in the QGraphicsScene (see this). QListWidget / QStandardItemModel lacks such a possibility.
In the QGraphicsScene dropEvent handler you then can listen for your custom mime-type and read the drop-data (item-type, content,...) from the model.

[tobiSF]

@raven-worx, I was afraid this was the answer. Which is too bad, when seemingly everything you'd need is in place on one side of the drag&drop operation and you would only need to access that information on the receiving end.

[raven-worx]

@tobiSF
it's not that bad. All you have to do is just implement a new model class.

[tobiSF]

@raven-worx not bad in a hard/difficult way. More disappointing that there's no straightforward way. Thank you, anyway!

[VRonin]

I did not test it but it looks like everything is already in Qt, just not documented.

void dropEvent(QGraphicsSceneDragDropEvent *event){
	const QByteArray mimeData = event->mimeData()->data("application/x-qabstractitemmodeldatalist");
	QDataStream mimeReader(mimeData);
	int row,col;
	QMap<int,QVariant> modelData;
	for(;;){
		mimeReader.startTransaction();
		mimeReader >> row >> col >> modelData;
		if(!mimeReader.commitTransaction())
			break;
		const auto userRoleIter = modelData.constFind(Qt::UserRole);
		if(userRoleIter!=modelData.cend())
			doSomethingWithModule(userRoleIter->value<Module>());
	}
}

[tobiSF]

@VRonin said in Dragging QListWidgetItem into QGraphicScene and create QGraphicsItem:

const QByteArray mimeData = event->mimeData()->data("application/x-qabstractitemmodeldatalist");
QDataStream mimeReader(mimeData);
int row,col;
QMap<int,QVariant> modelData;
for(;;){
mimeReader.startTransaction();
mimeReader >> row >> col >> modelData;
if(!mimeReader.commitTransaction())
break;
const auto userRoleIter = modelData.constFind(Qt::UserRole);
if(userRoleIter!=modelData.cend())
doSomethingWithModule(userRoleIter->value<Module>());
}

@VRonin, thank you very much. This works like a charm and does exactly what I was looking for.

[raven-worx]

@tobiSF
@VRonin
that's what i meant it's not "cleanly possible". I knew that some internal data was in the mime-data, but i believed that you need a reference to the model afterall, thus "not cleanly". But thats not the case.

Sorry for not being more specific on this.

[tobiSF]

@raven-worx no worries, your help is appreciated and I think I understood you correctly.