How to drag and drop a specific QPixmap into a QGraphicsView? - c++

Question

On a subclassed QListWidget I have several items. Every QListWidget item (e.g. "ROS Init", "Images" etc) that is shown below is associated with a specific icon.
The problem I have is that I am trying to drag and drop the specific icon corresponding to that QListWidget item, but nothing happens.
Below the function responsible for the dragging:
void ListView::startDrag(Qt::DropActions supportedActions)
{
QMap<int, QString> icons;
icons.insert(IT_RosInit, "ROS Init");
icons.insert(IT_Images, "Images");
icons.insert(IT_Path, "Path");
icons.insert(IT_RosShutDown, "ROS Shutdown");
if (supportedActions & Qt::CopyAction)
{
const QList<QListWidgetItem *> &m_items(selectedItems());
if (m_items.isEmpty())
return;
QPixmap pixmapLaser("/home/images/laserscan.png");
QPixmap pixmapPCloud2("/home/images/pcloud2.png");
// etc ...
QStringList iconImages;
for(int i = 0; i < icons.count(); ++i)
{
for (const QString &tableType : iconImages) {
if (tableType == "ROS Init")
{
auto *data = mimeData(m_items);
auto *drag = new QDrag(this);
drag->setPixmap(pixmapLaser);
drag->setMimeData(data);
drag->setHotSpot(pixmapLaser.rect().center());
drag->exec(Qt::CopyAction);
}
else if(tableType == "Images")
{
auto *data2 = mimeData(m_items);
auto *drag2 = new QDrag(this);
drag2->setPixmap(pixmapPCloud2);
drag2->setMimeData(data2);
drag2->setHotSpot(pixmapPCloud2.rect().center());
drag2->exec(Qt::CopyAction);
}
}
}
}
else
{
QListWidget::startDrag(supportedActions);
}
}
After subclassing the QListWidget I just reimplemented the usual drag and drop function. All other function are working properly but the startDrag and in fact as I try to drag the proper QPixmap, I actually see nothing being dragged.
I consulted this source, useful, and also this other source which was useful but it didn't reimplement the startDrag but instead dropEvent which for me is not a problem because that is working well.
I also consulted this source and this other source but that also didn't help fixing the problem.
Thanks for shedding light on this matter for solving the problem

Answer 1

Solution
I would approach this problem in the following way:
Set the ItemIsDragEnabled flag of QListWidgetItem to enable the item for dragging:
item->setFlags(Qt::ItemIsEnabled | Qt::ItemIsSelectable | Qt::ItemIsDragEnabled);
Set the desired data for each item:
item->setData(Qt::UserRole, type);
where type is one of the enumerated values IT_RosInit, IT_Images, etc.
Enable the drag functionality of the ListWidget:
setDragEnabled(true);
setDragDropMode(QAbstractItemView::DragOnly);
Use the item settings to setup the QDrag object, created in startDrag.
Example
Here is an example I have prepared for you to demonstrate how the proposed solution could be implemented:
MainWindow.cpp
MainWindow::MainWindow(QWidget *parent) :
QWidget(parent)
{
auto *l = new QVBoxLayout(this);
auto *list = new ListView(this);
list->addItem(createItem(":/pix/images/laserscan.png", tr("RosInit"), IT_RosInit));
list->addItem(createItem(":/pix/images/icons/pcloud2.png", tr("Images"), IT_Images));
list->addItem(createItem(":/pix/images/icons/some_icon.png", tr("Path"), IT_Path));
list->addItem(createItem(":/pix/images/icons/another_icon.png", tr("RosShutDown"), IT_RosShutDown));
l->addWidget(list);
resize(300, 400);
setWindowTitle("IconDrag");
}
QListWidgetItem *MainWindow::createItem(const QString &pm, const QString &text, int type)
{
auto *item = new QListWidgetItem(QIcon(QPixmap(pm)), text);
item->setFlags(Qt::ItemIsEnabled | Qt::ItemIsSelectable | Qt::ItemIsDragEnabled);
item->setData(Qt::UserRole, type);
return item;
}
ListView.cpp
ListView::ListView(QWidget *parent) :
QListWidget(parent)
{
setDragEnabled(true);
setDragDropMode(QAbstractItemView::DragOnly);
setSelectionBehavior(QAbstractItemView::SelectRows);
setSelectionMode(QAbstractItemView::SingleSelection);
}
void ListView::startDrag(Qt::DropActions supportedActions)
{
if (supportedActions & Qt::CopyAction) {
const QList<QListWidgetItem *> &items(selectedItems());
if (items.isEmpty())
return;
const QPixmap &pm(items.first()->icon().pixmap(64));
auto *item = items.first();
auto *mimeData = new QMimeData();
auto *drag = new QDrag(this);
mimeData->setData("text/plain", item->data(Qt::UserRole).toByteArray());
drag->setPixmap(pm);
drag->setMimeData(mimeData);
drag->setHotSpot(pm.rect().center());
drag->exec(Qt::CopyAction);
}
}