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# puzzlewidget.cpp Example File
draganddrop/puzzle/puzzlewidget.cpp

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     #include <QtGui>

     #include "puzzlewidget.h"

     PuzzleWidget::PuzzleWidget(QWidget *parent)
         : QWidget(parent)
     {
         setAcceptDrops(true);
         setMinimumSize(400, 400);
         setMaximumSize(400, 400);
     }

     void PuzzleWidget::clear()
     {
         pieceLocations.clear();
         piecePixmaps.clear();
         pieceRects.clear();
         highlightedRect = QRect();
         inPlace = 0;
         update();
     }

     void PuzzleWidget::dragEnterEvent(QDragEnterEvent *event)
     {
         if (event->mimeData()->hasFormat("image/x-puzzle-piece"))
             event->accept();
         else
             event->ignore();
     }

     void PuzzleWidget::dragLeaveEvent(QDragLeaveEvent *event)
     {
         QRect updateRect = highlightedRect;
         highlightedRect = QRect();
         update(updateRect);
         event->accept();
     }

     void PuzzleWidget::dragMoveEvent(QDragMoveEvent *event)
     {
         QRect updateRect = highlightedRect.unite(targetSquare(event->pos()));

         if (event->mimeData()->hasFormat("image/x-puzzle-piece")
             && findPiece(targetSquare(event->pos())) == -1) {

             highlightedRect = targetSquare(event->pos());
             event->setDropAction(Qt::MoveAction);
             event->accept();
         } else {
             highlightedRect = QRect();
             event->ignore();
         }

         update(updateRect);
     }

     void PuzzleWidget::dropEvent(QDropEvent *event)
     {
         if (event->mimeData()->hasFormat("image/x-puzzle-piece")
             && findPiece(targetSquare(event->pos())) == -1) {

             QByteArray pieceData = event->mimeData()->data("image/x-puzzle-piece");
             QDataStream dataStream(&pieceData, QIODevice::ReadOnly);
             QRect square = targetSquare(event->pos());
             QPixmap pixmap;
             QPoint location;
             dataStream >> pixmap >> location;

             pieceLocations.append(location);
             piecePixmaps.append(pixmap);
             pieceRects.append(square);

             highlightedRect = QRect();
             update(square);

             event->setDropAction(Qt::MoveAction);
             event->accept();

             if (location == QPoint(square.x()/80, square.y()/80)) {
                 inPlace++;
                 if (inPlace == 25)
                     emit puzzleCompleted();
             }
         } else {
             highlightedRect = QRect();
             event->ignore();
         }
     }

     int PuzzleWidget::findPiece(const QRect &pieceRect) const
     {
         for (int i = 0; i < pieceRects.size(); ++i) {
             if (pieceRect == pieceRects[i]) {
                 return i;
             }
         }
         return -1;
     }

     void PuzzleWidget::mousePressEvent(QMouseEvent *event)
     {
         QRect square = targetSquare(event->pos());
         int found = findPiece(square);

         if (found == -1)
             return;

         QPoint location = pieceLocations[found];
         QPixmap pixmap = piecePixmaps[found];
         pieceLocations.removeAt(found);
         piecePixmaps.removeAt(found);
         pieceRects.removeAt(found);

         if (location == QPoint(square.x()/80, square.y()/80))
             inPlace--;

         update(square);

         QByteArray itemData;
         QDataStream dataStream(&itemData, QIODevice::WriteOnly);

         dataStream << pixmap << location;

         QMimeData *mimeData = new QMimeData;
         mimeData->setData("image/x-puzzle-piece", itemData);

         QDrag *drag = new QDrag(this);
         drag->setMimeData(mimeData);
         drag->setHotSpot(event->pos() - square.topLeft());
         drag->setPixmap(pixmap);

         if (!(drag->exec(Qt::MoveAction) == Qt::MoveAction)) {
             pieceLocations.insert(found, location);
             piecePixmaps.insert(found, pixmap);
             pieceRects.insert(found, square);
             update(targetSquare(event->pos()));

             if (location == QPoint(square.x()/80, square.y()/80))
                 inPlace++;
         }
     }

     void PuzzleWidget::paintEvent(QPaintEvent *event)
     {
         QPainter painter;
         painter.begin(this);
         painter.fillRect(event->rect(), Qt::white);

         if (highlightedRect.isValid()) {
             painter.setBrush(QColor("#ffcccc"));
             painter.setPen(Qt::NoPen);
             painter.drawRect(highlightedRect.adjusted(0, 0, -1, -1));
         }

         for (int i = 0; i < pieceRects.size(); ++i) {
             painter.drawPixmap(pieceRects[i], piecePixmaps[i]);
         }
         painter.end();
     }

     const QRect PuzzleWidget::targetSquare(const QPoint &position) const
     {
         return QRect(position.x()/80 * 80, position.y()/80 * 80, 80, 80);
     }