[C++ gui programming with qt4] chap9

Ch9.Drag and Drop
Browny
23, May, 2011

Outline

• Enabling Drag and Drop
• Supporting Custom Drag Types
• Clipboard Handling

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;
};

MainWindow::MainWindow()
{
textEdit = new QTextEdit;
setCentralWidget(textEdit);

textEdit->setAcceptDrops(false);
setAcceptDrops(true);

setWindowTitle(tr("Text Editor"));
}

QTextEdit
setAcceptDrops(false)
setAcceptDrops(true)
MainWindow

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/


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")));
}

Initiate a Drag and Accept a Drop
(1/4)

• Create a QListWidget subclass that
supports drag and 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;
};

(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);
}

(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()
}

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();
}
}

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

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

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);
}

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();
}

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);
...
} else if (event->mimeData()->hasFormat("text/plain")) {
QString plainText = event->mimeData()->text();
...
}
}
QTableWidget Html OK

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 ,
};

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);
}

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();
...
} 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
}

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

[C++ gui programming with qt4] chap9

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