qt5base-lts/examples/widgets/draganddrop/draggableicons/dragwidget.cpp

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#include <QtWidgets>

#include "dragwidget.h"

//! [0]
DragWidget::DragWidget(QWidget *parent)
    : QFrame(parent)
{
    setMinimumSize(200, 200);
    setFrameStyle(QFrame::Sunken | QFrame::StyledPanel);
    setAcceptDrops(true);

    QLabel *boatIcon = new QLabel(this);
    boatIcon->setPixmap(QPixmap(":/images/boat.png"));
    boatIcon->move(10, 10);
    boatIcon->show();
    boatIcon->setAttribute(Qt::WA_DeleteOnClose);

    QLabel *carIcon = new QLabel(this);
    carIcon->setPixmap(QPixmap(":/images/car.png"));
    carIcon->move(100, 10);
    carIcon->show();
    carIcon->setAttribute(Qt::WA_DeleteOnClose);

    QLabel *houseIcon = new QLabel(this);
    houseIcon->setPixmap(QPixmap(":/images/house.png"));
    houseIcon->move(10, 80);
    houseIcon->show();
    houseIcon->setAttribute(Qt::WA_DeleteOnClose);
}
//! [0]

void DragWidget::dragEnterEvent(QDragEnterEvent *event)
{
    if (event->mimeData()->hasFormat("application/x-dnditemdata")) {
        if (event->source() == this) {
            event->setDropAction(Qt::MoveAction);
            event->accept();
        } else {
            event->acceptProposedAction();
        }
    } else {
        event->ignore();
    }
}

void DragWidget::dragMoveEvent(QDragMoveEvent *event)
{
    if (event->mimeData()->hasFormat("application/x-dnditemdata")) {
        if (event->source() == this) {
            event->setDropAction(Qt::MoveAction);
            event->accept();
        } else {
            event->acceptProposedAction();
        }
    } else {
        event->ignore();
    }
}

void DragWidget::dropEvent(QDropEvent *event)
{
    if (event->mimeData()->hasFormat("application/x-dnditemdata")) {
        QByteArray itemData = event->mimeData()->data("application/x-dnditemdata");
        QDataStream dataStream(&itemData, QIODevice::ReadOnly);
        
        QPixmap pixmap;
        QPoint offset;
        dataStream >> pixmap >> offset;

        QLabel *newIcon = new QLabel(this);
        newIcon->setPixmap(pixmap);
        newIcon->move(event->pos() - offset);
        newIcon->show();
        newIcon->setAttribute(Qt::WA_DeleteOnClose);

        if (event->source() == this) {
            event->setDropAction(Qt::MoveAction);
            event->accept();
        } else {
            event->acceptProposedAction();
        }
    } else {
        event->ignore();
    }
}

//! [1]
void DragWidget::mousePressEvent(QMouseEvent *event)
{
    QLabel *child = static_cast<QLabel*>(childAt(event->pos()));
    if (!child)
        return;

    QPixmap pixmap = *child->pixmap();

    QByteArray itemData;
    QDataStream dataStream(&itemData, QIODevice::WriteOnly);
    dataStream << pixmap << QPoint(event->pos() - child->pos());
//! [1]

//! [2]
    QMimeData *mimeData = new QMimeData;
    mimeData->setData("application/x-dnditemdata", itemData);
//! [2]
        
//! [3]
    QDrag *drag = new QDrag(this);
    drag->setMimeData(mimeData);
    drag->setPixmap(pixmap);
    drag->setHotSpot(event->pos() - child->pos());
//! [3]

    QPixmap tempPixmap = pixmap;
    QPainter painter;
    painter.begin(&tempPixmap);
    painter.fillRect(pixmap.rect(), QColor(127, 127, 127, 127));
    painter.end();

    child->setPixmap(tempPixmap);

    if (drag->exec(Qt::CopyAction | Qt::MoveAction, Qt::CopyAction) == Qt::MoveAction)
        child->close();
    else {
        child->show();
        child->setPixmap(pixmap);
    }
}