This document provides code examples for enabling drag and drop functionality in Qt applications. It demonstrates how to allow dragging files onto a window, initiate dragging from a QListWidget, and add drag and drop to a QTableWidget. It also discusses subclassing QMimeData to support custom drag data types and handling the clipboard by subclassing QMimeData and setting it on the clipboard.

1. Ch9.Drag and Drop
Browny
23, May, 2011

2. Outline
• Enabling Drag and Drop
• Supporting Custom Drag Types
• Clipboard Handling

3. Drag file onto Window (1/4)
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MainWindow();
protected:
void dragEnterEvent(QDragEnterEvent *event);
void dropEvent(QDropEvent *event);
private:
bool readFile(const QString &fileName);
QTextEdit *textEdit;
};

4. Drag file onto Window (2/4)
MainWindow::MainWindow()
{
textEdit = new QTextEdit;
setCentralWidget(textEdit);
textEdit->setAcceptDrops(false);
setAcceptDrops(true);
setWindowTitle(tr("Text Editor"));
}
QTextEdit
setAcceptDrops(false)
setAcceptDrops(true)
MainWindow

5. Drag file onto Window (3/4)
void MainWindow::dragEnterEvent(QDragEnterEvent *event)
{
if (event->mimeData()->hasFormat("text/plain"))
event->acceptProposedAction();
}
Standard MIME types are defined by the Internet Assigned
Numbers Authority (IANA). They consist of a type and a subtype
separated by a slash.
The official list of MIME types is available at http://www.iana.org/
assignments/media-types/

6. Drag file onto Window (4/4)
void MainWindow::dropEvent(QDropEvent *event)
{
QList<QUrl> urls = event->mimeData()->urls();
if (urls.isEmpty())
return;
QString fileName = urls.first().toLocalFile();
if (fileName.isEmpty())
return;
if (readFile(fileName))
setWindowTitle(tr("%1 - %2").arg(fileName)
.arg(tr("Drag File")));
}

7. Initiate a Drag and Accept a Drop
(1/4)
• Create a QListWidget subclass that
supports drag and drop

8. Initiate a Drag and Accept a Drop
(2/4)
class ProjectListWidget : public QListWidget
{
Q_OBJECT
public:
ProjectListWidget(QWidget *parent = 0);
protected:
void mousePressEvent(QMouseEvent *event);
void mouseMoveEvent(QMouseEvent *event);
void dragEnterEvent(QDragEnterEvent *event);
void dragMoveEvent(QDragMoveEvent *event);
void dropEvent(QDropEvent *event);
private:
void performDrag(); QWidget
5
QPoint startPos;
};

9. Initiate a Drag and Accept a Drop
(3/4)
ProjectListWidget::ProjectListWidget(QWidget *parent)
: QListWidget(parent)
{
setAcceptDrops(true);
}
void ProjectListWidget::mousePressEvent(QMouseEvent *event)
{
if (event->button() == Qt::LeftButton)
startPos = event->pos();
QListWidget::mousePressEvent(event);
}
void ProjectListWidget::mouseMoveEvent(QMouseEvent *event)
{
if (event->buttons() & Qt::LeftButton) {
int distance = (event->pos() - startPos).manhattanLength();
if (distance >= QApplication::startDragDistance())
performDrag();
}
QListWidget::mouseMoveEvent(event);
}

10. Initiate a Drag and Accept a Drop
(4/4)
void ProjectListWidget::performDrag()
{
QListWidgetItem *item = currentItem();
if (item) {
QMimeData *mimeData = new QMimeData;
mimeData->setText(item->text());
QDrag *drag = new QDrag(this);
drag->setMimeData(mimeData); QDrag
drag->setPixmap(QPixmap(":/images/person.png"));
if (drag->exec(Qt::MoveAction) == Qt::MoveAction)
delete item;
} QDrag::exec()
}

11. void ProjectListWidget::dragEnterEvent(QDragEnterEvent *event)
{
ProjectListWidget
ProjectListWidget *source =
qobject_cast<ProjectListWidget *>(event->source());
if (source && source != this) {
event->setDropAction(Qt::MoveAction);
event->accept();
}
}
void ProjectListWidget::dropEvent(QDropEvent *event)
{
ProjectListWidget *source =
qobject_cast<ProjectListWidget *>(event->source());
if (source && source != this) {
addItem(event->mimeData()->text());
event->setDropAction(Qt::MoveAction);
event->accept();
}
}

12. Drag custom data (1/2)
1. Provide arbitrary data as a QByteArray using
QMimeData::setData() and extract it later
using QMimeData::data()
2. Subclass QMimeData and re-implement
formats() and retrieveData() to handle our
custom data types
3. For drag and drop operations within a
single application, we can subclass
QMimeData and store the data using any
data structure we want

13. Drag custom data (2/2)
• Drawbacks of Method 1
‣ Need
to
convert
our
data
structure
to
a
QByteArray
even
if
the
drag
is
not
ul1mately
accepted
‣ Providing
several
MIME
types
to
interact
nicely
with
a
wide
range
of
applica=ons,
we
need
to
store
the
data
several
1mes
‣ If
the
data
is
large,
this
can
slow
down
the
applica1on
needlessly

14. Add drag and drop capabilities to a
QTableWidget (1/3)
• Method 1
void MyTableWidget::mouseMoveEvent(QMouseEvent *event)
{
if (event->buttons() & Qt::LeftButton) {
int distance = (event->pos() - startPos).manhattanLength();
if (distance >= QApplication::startDragDistance())
performDrag();
}
QTableWidget::mouseMoveEvent(event);
}

15. Add drag and drop capabilities to a
QTableWidget (2/3)
void MyTableWidget::performDrag()
{
QString plainText = selectionAsPlainText(); Chap4 (p.87)
if (plainText.isEmpty())
return;
QMimeData *mimeData = new QMimeData;
mimeData->setText(plainText);
mimeData->setHtml(toHtml(plainText));
mimeData->setData("text/csv", toCsv(plainText).toUtf8());
QDrag *drag = new QDrag(this);
drag->setMimeData(mimeData);
if (drag->exec(Qt::CopyAction | Qt::MoveAction) == Qt::MoveAction)
deleteSelection();
}

16. Add drag and drop capabilities to a
QTableWidget (3/3)
void MyTableWidget::dropEvent(QDropEvent *event)
{
if (event->mimeData()->hasFormat("text/csv")) {
QByteArray csvData = event->mimeData()->data("text/csv");
QString csvText = QString::fromUtf8(csvData);
...
event->acceptProposedAction();
} else if (event->mimeData()->hasFormat("text/plain")) {
QString plainText = event->mimeData()->text();
...
event->acceptProposedAction();
}
}
QTableWidget Html OK

17. Subclass QMimeData (1/3)
class TableMimeData : public QMimeData
{
Q_OBJECT
public:
TableMimeData(const QTableWidget *tableWidget,
const QTableWidgetSelectionRange &range);
const QTableWidget *tableWidget() const { return myTableWidget; }
QTableWidgetSelectionRange range() const { return myRange; }
QStringList formats() const;
protected:
QVariant retrieveData(const QString &format,
QVariant::Type preferredType) const;
private:
static QString toHtml(const QString &plainText);
static QString toCsv(const QString &plainText);
QString text(int row, int column) const;
QString rangeAsPlainText() const;
const QTableWidget *myTableWidget; ,
QTableWidgetSelectionRange myRange;
QStringList myFormats; QTableWidget ,
};

18. Subclass QMimeData (2/3)
TableMimeData::TableMimeData(const QTableWidget *tableWidget,
const QTableWidgetSelectionRange &range) {
myTableWidget = tableWidget;
myRange = range;
myFormats << "text/csv" << "text/html" << "text/plain";
}
QStringList TableMimeData::formats() const {
return myFormats;
}
QVariant TableMimeData::retrieveData(const QString &format,
QVariant::Type preferredType) const {
if (format == "text/plain")
return rangeAsPlainText();
else if (format == "text/csv")
return toCsv(rangeAsPlainText());
else if (format == "text/html") {
return toHtml(rangeAsPlainText());
else
return QMimeData::retrieveData(format, preferredType);
}

19. Subclass QMimeData (3/3)
void MyTableWidget::dropEvent(QDropEvent *event)
{
const TableMimeData *tableData =
qobject_cast<const TableMimeData *>(event->mimeData());
if (tableData) {
const QTableWidget *otherTable = tableData->tableWidget();
QTableWidgetSelectionRange otherRange = tableData->range();
...
event->acceptProposedAction();
} else if (event->mimeData()->hasFormat("text/csv")) {
QByteArray csvData = event->mimeData()->data("text/csv");
QString csvText = QString::fromUtf8(csvData);
...
we can directly access the table
}
QTableWidget::mouseMoveEvent(event); data instead of going through
QMimeData's API
}

20. Clipboard Handling
• Access clipboard: QApplication::clipboard()
• Built-in functionality might not be sufficient
(not just text or an image)
‣ Subclass
QMimeData
and
re-­‐implement
a
few
virtual
func=ons
‣ Reuse
the
QMimeData
subclass
and
put
it
on
the
clipboard
using
the
setMimeData()
func=on.
To
retrieve
the
data,
we
can
call
mimeData()
on
the
clipboard
• Clipboard's contents change
‣ QClipboard::dataChanged()
signal

21. Thank you :)