Hi people,

I'm using qt-creator to make a simple application with drag and drop system. The idea is understand the process.

I create a simple windows with two panels:


The idea is using drag and drop to change the icons from the panels.

mainwindow.h
Qt Code: 
  1. #ifndef MAINWINDOW_H
  2. #define MAINWINDOW_H
  3.  
  4. #include <QMainWindow>
  5.  
  6. namespace Ui {
  7.     class MainWindow;
  8. }
  9.  
  10. QT_BEGIN_NAMESPACE
  12. class QDropEvent;
  13. QT_END_NAMESPACE
  14.  
  15. class MainWindow : public QMainWindow {
  16.     Q_OBJECT
  17. public:
  18.     MainWindow(QWidget *parent = 0);
  19.     ~MainWindow();
  20.  
  21. protected:
  22.     void dragEnterEvent(QDragEnterEvent *event);
  23.     void dragMoveEvent(QDragMoveEvent *event);
  24.     void dropEvent(QDropEvent *event);
  25.     void mousePressEvent(QMouseEvent *event);
  26.  
  27. private:
  28.     Ui::MainWindow *ui;
  29. };
  30.  
  31. #endif // MAINWINDOW_H
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mainwindow.cpp
Qt Code: 
  1. #include "mainwindow.h"
  2. #include "ui_mainwindow.h"
  3.  
  4. MainWindow::MainWindow(QWidget *parent) :
  5.     QMainWindow(parent),
  6.     ui(new Ui::MainWindow)
  7. {
  8.     ui->setupUi(this);
  9. }
  10.  
  11. MainWindow::~MainWindow()
  12. {
  13.     delete ui;
  14. }
  15.  
  16. void MainWindow::dragEnterEvent(QDragEnterEvent *event)
  17. {
  18.     if (event->mimeData()->hasFormat("application/x-dnditemdata")) {
  19.         if (event->source() == this) {
  20.             event->setDropAction(Qt::MoveAction);
  21.             event->accept();
  22.         } else {
  23.             event->acceptProposedAction();
  24.         }
  25.     } else {
  26.         event->ignore();
  27.     }
  28. }
  29.  
  30. void MainWindow::dragMoveEvent(QDragMoveEvent *event)
  31. {
  32.     if (event->mimeData()->hasFormat("application/x-dnditemdata")) {
  33.         if (event->source() == this) {
  34.             event->setDropAction(Qt::MoveAction);
  35.             event->accept();
  36.         } else {
  37.             event->acceptProposedAction();
  38.         }
  39.     } else {
  40.         event->ignore();
  41.     }
  42. }
  43.  
  44. void MainWindow::dropEvent(QDropEvent *event)
  45. {
  46.     if (event->mimeData()->hasFormat("application/x-dnditemdata")) {
  47.         QByteArray itemData = event->mimeData()->data("application/x-dnditemdata");
  48.         QDataStream dataStream(&itemData, QIODevice::ReadOnly);
  49.  
  50.         QPixmap pixmap;
  51.         QPoint offset;
  52.         dataStream >> pixmap >> offset;
  53.  
  54.         QLabel *newIcon = new QLabel(this);
  55.         newIcon->setPixmap(pixmap);
  56.         newIcon->move(event->pos() - offset);
  57.         newIcon->show();
  58.         newIcon->setAttribute(Qt::WA_DeleteOnClose);
  59.  
  60.         if (event->source() == this) {
  61.             event->setDropAction(Qt::MoveAction);
  62.             event->accept();
  63.         } else {
  64.             event->acceptProposedAction();
  65.         }
  66.     } else {
  67.         event->ignore();
  68.     }
  69. }
  70.  
  71. void MainWindow::mousePressEvent(QMouseEvent *event)
  72. {
  73.     QLabel *child = static_cast<QLabel*>(childAt(event->pos()));
  74.     if (!child)
  75.         return;
  76.  
  77.     QPixmap pixmap = *child->pixmap();
  78.  
  79.     QByteArray itemData;
  80.     QDataStream dataStream(&itemData, QIODevice::WriteOnly);
  81.     dataStream << pixmap << QPoint(event->pos() - child->pos());
  82.  
  83.     QMimeData *mimeData = new QMimeData;
  84.     mimeData->setData("application/x-dnditemdata", itemData);
  85.  
  86.     QDrag *drag = new QDrag(this);
  87.     drag->setMimeData(mimeData);
  88.     drag->setPixmap(pixmap);
  89.     drag->setHotSpot(event->pos() - child->pos());
  90.  
  91.     QPixmap tempPixmap = pixmap;
  92.     QPainter painter;
  93.     painter.begin(&tempPixmap);
  94.     painter.fillRect(pixmap.rect(), QColor(127, 127, 127, 127));
  95.     painter.end();
  96.  
  97.     child->setPixmap(tempPixmap);
  98.  
  99.     if (drag->exec(Qt::CopyAction | Qt::MoveAction, Qt::CopyAction) == Qt::MoveAction)
  100.         child->close();
  101.     else {
  102.         child->show();
  103.         child->setPixmap(pixmap);
  104.     }
  105. }
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To me is ok, but I've got some errors:

Qt Code: 
  1. ../drag/mainwindow.cpp:: In member function ‘virtual void MainWindow::dragEnterEvent(QDragEnterEvent*)’:
  2. ../drag/mainwindow.cpp:18: error: invalid use of incomplete type ‘struct QDragEnterEvent’
  3. ..drag-build-desktop/../../qtsdk-2010.05/qt/include/QtGui/qwidget.h:88: error: forward declaration of ‘struct QDragEnterEvent’
  4. [...]
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Thanks for all help.

Junix