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Thread: drag and drop QLabels

  1. 4th June 2009, 14:20 #1
    tonio
    tonio is offline Beginner
    Join Date
    Jun 2009
    Posts
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    Qt products
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    Default drag and drop QLabels

    Hello,

    I'm trying to understand a strange problem, if it is a problem, when doing a drag and drop over QLables in a QFrame.
    I'm using the example of Qt about the drag and drop "Draggable Icons".
    I modified just a little bit the code in the constructor DragWidget :: DragWidget(QWidget *parent) and in the
    void DragWidget::mousePressEvent(QMouseEvent *event) method.

    In the constructor I've put this 3 lines for each label:

    boatIcon->setObjectName(QString("BOAT"));

    carIcon->setObjectName(QString("CAR"));

    houseIcon->setObjectName(QString("HOUSE"));

    In the mouse handler method I've put this line:

    qDebug("child name %s", child->objectName().toLatin1().data());

    So as you see nothing special. But when I'm pressing with the mouse over the labels there is no name object, more precisely qDebug shows only the first time the name and after the objetName() returns empty string ....

    Here is the entire code of the modified example:

    dragwidget.h
    Qt Code: 
    1. #ifndef DRAGWIDGET_H
    2. #define DRAGWIDGET_H
    3.  
    4. #include <QFrame>
    5.  
    6. QT_BEGIN_NAMESPACE
    8. class QDropEvent;
    9. QT_END_NAMESPACE
    10.  
    11. //! [0]
    12. class DragWidget : public QFrame
    13. {
    14. public:
    15. DragWidget(QWidget *parent=0);
    16.  
    17. protected:
    18. void dragEnterEvent(QDragEnterEvent *event);
    19. void dragMoveEvent(QDragMoveEvent *event);
    20. void dropEvent(QDropEvent *event);
    21. void mousePressEvent(QMouseEvent *event);
    22. };
    23. //! [0]
    24.  
    25. #endif
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    dragwidget.cpp

    Qt Code: 
    1. #include <QtGui>
    2.  
    3. #include "dragwidget.h"
    4.  
    5. //! [0]
    6. DragWidget::DragWidget(QWidget *parent)
    7. : QFrame(parent)
    8. {
    9. setMinimumSize(200, 200);
    10. setFrameStyle(QFrame::Sunken | QFrame::StyledPanel);
    11. setAcceptDrops(true);
    12.  
    13. QLabel *boatIcon = new QLabel(this);
    14. boatIcon->setObjectName(QString("BOAT")); //<---- here
    15. boatIcon->setPixmap(QPixmap(":/images/boat.png"));
    16. boatIcon->move(20, 20);
    17. boatIcon->show();
    18. boatIcon->setAttribute(Qt::WA_DeleteOnClose);
    19.  
    20. QLabel *carIcon = new QLabel(this);
    21. carIcon->setObjectName(QString("CAR")); //<---- here
    22. carIcon->setPixmap(QPixmap(":/images/car.png"));
    23. carIcon->move(120, 20);
    24. carIcon->show();
    25. carIcon->setAttribute(Qt::WA_DeleteOnClose);
    26.  
    27. QLabel *houseIcon = new QLabel(this);
    28. houseIcon->setObjectName(QString("HOUSE")); //<---- here
    29. houseIcon->setPixmap(QPixmap(":/images/house.png"));
    30. houseIcon->move(20, 120);
    31. houseIcon->show();
    32. houseIcon->setAttribute(Qt::WA_DeleteOnClose);
    33. }
    34. //! [0]
    35.  
    36. void DragWidget::dragEnterEvent(QDragEnterEvent *event)
    37. {
    38. if (event->mimeData()->hasFormat("application/x-dnditemdata")) {
    39. if (event->source() == this) {
    40. event->setDropAction(Qt::MoveAction);
    41. event->accept();
    42. } else {
    43. event->acceptProposedAction();
    44. }
    45. } else {
    46. event->ignore();
    47. }
    48. }
    49.  
    50. void DragWidget::dragMoveEvent(QDragMoveEvent *event)
    51. {
    52. if (event->mimeData()->hasFormat("application/x-dnditemdata")) {
    53. if (event->source() == this) {
    54. event->setDropAction(Qt::MoveAction);
    55. event->accept();
    56. } else {
    57. event->acceptProposedAction();
    58. }
    59. } else {
    60. event->ignore();
    61. }
    62. }
    63.  
    64. void DragWidget::dropEvent(QDropEvent *event)
    65. {
    66. if (event->mimeData()->hasFormat("application/x-dnditemdata")) {
    67. QByteArray itemData = event->mimeData()->data("application/x-dnditemdata");
    68. QDataStream dataStream(&itemData, QIODevice::ReadOnly);
    69.  
    70. QPixmap pixmap;
    71. QPoint offset;
    72. dataStream >> pixmap >> offset;
    73.  
    74. QLabel *newIcon = new QLabel(this);
    75. newIcon->setPixmap(pixmap);
    76. newIcon->move(event->pos() - offset);
    77. newIcon->show();
    78. newIcon->setAttribute(Qt::WA_DeleteOnClose);
    79.  
    80. if (event->source() == this) {
    81. event->setDropAction(Qt::MoveAction);
    82. event->accept();
    83. } else {
    84. event->acceptProposedAction();
    85. }
    86. } else {
    87. event->ignore();
    88. }
    89. }
    90.  
    91. //! [1]
    92. void DragWidget::mousePressEvent(QMouseEvent *event)
    93. {
    94. QLabel *child = static_cast<QLabel*>(childAt(event->pos()));
    95. if (!child)
    96. return;
    97.  
    98. qDebug("child name %s", child->objectName().toLatin1().data());
    99.  
    100. QPixmap pixmap = *child->pixmap();
    101.  
    102. QByteArray itemData;
    103. QDataStream dataStream(&itemData, QIODevice::WriteOnly);
    104. dataStream << pixmap << QPoint(event->pos() - child->pos());
    105. //! [1]
    106.  
    107. //! [2]
    108. QMimeData *mimeData = new QMimeData;
    109. mimeData->setData("application/x-dnditemdata", itemData);
    110. //! [2]
    111.  
    112. //! [3]
    113. QDrag *drag = new QDrag(this);
    114. drag->setMimeData(mimeData);
    115. drag->setPixmap(pixmap);
    116. drag->setHotSpot(event->pos() - child->pos());
    117. //! [3]
    118.  
    119. QPixmap tempPixmap = pixmap;
    120. QPainter painter;
    121. painter.begin(&tempPixmap);
    122. painter.fillRect(pixmap.rect(), QColor(127, 127, 127, 127));
    123. painter.end();
    124.  
    125. child->setPixmap(tempPixmap);
    126.  
    127. if (drag->exec(Qt::CopyAction | Qt::MoveAction, Qt::CopyAction) == Qt::MoveAction)
    128. child->close();
    129. else {
    130. child->show();
    131. child->setPixmap(pixmap);
    132. }
    133. }
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    main.cpp

    Qt Code: 
    1. #include <QApplication>
    2. #include <QHBoxLayout>
    3. #include "dragwidget.h"
    4.  
    5. int main(int argc, char *argv[])
    6. {
    7. Q_INIT_RESOURCE(draggableicons);
    8.  
    9. QApplication app(argc, argv);
    10.  
    11. QWidget mainWidget;
    12. QHBoxLayout *horizontalLayout = new QHBoxLayout;
    13. horizontalLayout->addWidget(new DragWidget);
    14. horizontalLayout->addWidget(new DragWidget);
    15.  
    16. mainWidget.setLayout(horizontalLayout);
    17. mainWidget.setWindowTitle(QObject::tr("Draggable Icons"));
    18. mainWidget.show();
    19.  
    20. return app.exec();
    21. }
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    If someone has an explanation, I'll be happy to hear it

    Best regards,

    Anton

    PS. The images needed in this example are like attached files, you should put them in a "image" directory in the directory where the source files for this example are.
    Attached Images Attached Images
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