Delayed Encoding Example

SourceWidget Class Implementation

Let's start our code tour with a look at the startDrag() slot.

 void SourceWidget::startDrag()
 {
     mimeData = new MimeData;

     connect(mimeData, SIGNAL(dataRequested(const QString &)),
             this, SLOT(createData(const QString &)), Qt::DirectConnection);

     QDrag *drag = new QDrag(this);
     drag->setMimeData(mimeData);
     drag->setPixmap(QPixmap(":/images/drag.png"));

     drag->exec(Qt::CopyAction);
 }

We emit dataRequested() from MimeData when the operation has found a valid drop target.

We gallop along to createData():

 void SourceWidget::createData(const QString &mimeType)
 {
     if (mimeType != "image/png")
         return;

     QImage image(imageLabel->size(), QImage::Format_RGB32);
     QPainter painter;
     painter.begin(&image);
     imageLabel->renderer()->render(&painter);
     painter.end();

     QByteArray data;
     QBuffer buffer(&data);
     buffer.open(QIODevice::WriteOnly);
     image.save(&buffer, "PNG");
     buffer.close();

     mimeData->setData("image/png", data);
 }

Fortunately, Qt provides QSvgRenderer, which can render the SVG image to any QPaintDevice. Also, QImage has no problems saving to the PNG format.

Finally, we can give the data to MimeData.

 class MimeData : public QMimeData
 {
     Q_OBJECT

 public:
     MimeData();
     QStringList formats() const;

 signals:
     void dataRequested(const QString &mimeType) const;

 protected:
     QVariant retrieveData(const QString &mimetype, QVariant::Type type) const;
 };

 QStringList MimeData::formats() const
 {
     return QMimeData::formats() << "image/png";
 }

 QVariant MimeData::retrieveData(const QString &mimeType, QVariant::Type type)
          const
 {
     emit dataRequested(mimeType);

     return QMimeData::retrieveData(mimeType, type);
 }