<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<!-- delayedencoding.qdoc -->
<head>
<title>Qt 4.6: Delayed Encoding Example</title>
<link href="classic.css" rel="stylesheet" type="text/css" />
</head>
<body>
<table border="0" cellpadding="0" cellspacing="0" width="100%">
<tr>
<td align="left" valign="top" width="32"><a href="http://qt.nokia.com/"><img src="images/qt-logo.png" align="left" border="0" /></a></td>
<td width="1"> </td><td class="postheader" valign="center"><a href="index.html"><font color="#004faf">Home</font></a> · <a href="classes.html"><font color="#004faf">All Classes</font></a> · <a href="functions.html"><font color="#004faf">All Functions</font></a> · <a href="overviews.html"><font color="#004faf">Overviews</font></a></td></tr></table><h1 class="title">Delayed Encoding Example<br /><span class="subtitle"></span>
</h1>
<p>Files:</p>
<ul>
<li><a href="draganddrop-delayedencoding-mimedata-cpp.html">draganddrop/delayedencoding/mimedata.cpp</a></li>
<li><a href="draganddrop-delayedencoding-mimedata-h.html">draganddrop/delayedencoding/mimedata.h</a></li>
<li><a href="draganddrop-delayedencoding-sourcewidget-cpp.html">draganddrop/delayedencoding/sourcewidget.cpp</a></li>
<li><a href="draganddrop-delayedencoding-sourcewidget-h.html">draganddrop/delayedencoding/sourcewidget.h</a></li>
<li><a href="draganddrop-delayedencoding-images-example-svg.html">draganddrop/delayedencoding/images/example.svg</a></li>
<li><a href="draganddrop-delayedencoding-main-cpp.html">draganddrop/delayedencoding/main.cpp</a></li>
<li><a href="draganddrop-delayedencoding-delayedencoding-pro.html">draganddrop/delayedencoding/delayedencoding.pro</a></li>
<li><a href="draganddrop-delayedencoding-delayedencoding-qrc.html">draganddrop/delayedencoding/delayedencoding.qrc</a></li>
</ul>
<p>Images:</p>
<ul>
<li><a href="images/used-in-examples/draganddrop/delayedencoding/images/drag.png">draganddrop/delayedencoding/images/drag.png</a></li>
</ul>
<p>The Delayed Encoding example shows how to delay preparing of data for drag and drop operations until a drop target is found.</p>
<p align="center"><img src="images/delayedecoding-example.png" /></p><p>The <b>Export</b> push button is pressed down to start the drag. The data for the drag and drop operation is not processed until the user of the application has found a valid drop target. This removes redundant processing if the operation is aborted. In our case, we have an SVG image that we wish to send as the <tt>"image/png"</tt> MIME type. It is the conversion from SVG to PNG we wish to delay - it can be quite expensive.</p>
<p>The example is implemented in two classes: <tt>SourceWidget</tt> and <tt>MimeData</tt>. The <tt>SourceWidget</tt> class sets up the GUI and starts the drag operation on request. The <tt>MimeData</tt> class, which inherits <a href="qmimedata.html">QMimeData</a>, sends a signal when a drop target is found. This signal is connected to a slot in <tt>SourceWidget</tt>, which does the conversion from SVG to PNG.</p>
<a name="sourcewidget-class-definition"></a>
<h2>SourceWidget Class Definition</h2>
<p>The <tt>SourceWidget</tt> class starts drag and drop operations and also does the image conversion.</p>
<pre> public slots:
void createData(const QString &mimetype);
void startDrag();
private:
QByteArray imageData;
QSvgWidget *imageLabel;
MimeData *mimeData;</pre>
<p>The <b>Export</b> push button is connected to the <tt>startDrag()</tt> slot. The <tt>createData()</tt> slot will be invoked when data for the drag and drop operation is to be created.</p>
<a name="sourcewidget-class-implementation"></a>
<h2>SourceWidget Class Implementation</h2>
<p>Let's start our code tour with a look at the <tt>startDrag()</tt> slot.</p>
<pre> void SourceWidget::startDrag()
{
mimeData = new MimeData;
connect(mimeData, SIGNAL(dataRequested(QString)),
this, SLOT(createData(QString)), Qt::DirectConnection);
QDrag *drag = new QDrag(this);
drag->setMimeData(mimeData);
drag->setPixmap(QPixmap(":/images/drag.png"));
drag->exec(Qt::CopyAction);
}</pre>
<p>We emit <tt>dataRequested()</tt> from <tt>MimeData</tt> when the operation has found a valid drop target.</p>
<p>We gallop along to <tt>createData()</tt>:</p>
<pre> 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);
}</pre>
<p>Fortunately, Qt provides <a href="qsvgrenderer.html">QSvgRenderer</a>, which can render the SVG image to any <a href="qpaintdevice.html">QPaintDevice</a>. Also, <a href="qimage.html">QImage</a> has no problems saving to the PNG format.</p>
<p>Finally, we can give the data to <tt>MimeData</tt>.</p>
<a name="mimedata-class-definition"></a>
<h2>MimeData Class Definition</h2>
<p>The <tt>MimeData</tt> class inherits <a href="qmimedata.html">QMimeData</a> and makes it possible to delay preparing of the data for a drag and drop operation.</p>
<pre> 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;
};</pre>
<p>We will look closer at <tt>retrieveData()</tt> and <tt>formats()</tt> in the next section.</p>
<a name="mimedata-class-implementation"></a>
<h2>MimeData Class Implementation</h2>
<pre> QStringList MimeData::formats() const
{
return QMimeData::formats() << "image/png";
}</pre>
<p>In the <tt>formats()</tt> function, we return the format of the data we provide. This is the <tt>image/png</tt> MIME type.</p>
<pre> QVariant MimeData::retrieveData(const QString &mimeType, QVariant::Type type)
const
{
emit dataRequested(mimeType);
return QMimeData::retrieveData(mimeType, type);
}</pre>
<p><tt>retrieveData()</tt> is reimplemented from <a href="qmimedata.html">QMimeData</a> and is called when the data is requested by the drag and drop operation. Fortunately for us, this happens when the operation is finishing, i.e., when a drop target has been found.</p>
<p>We emit the <tt>dataRequested()</tt> signal, which is picked up by <tt>SourceWidget</tt>. The <tt>SourceWidget</tt> (as already explained) sets the data on <tt>MimeData</tt> with <a href="qmimedata.html#setData">setData()</a>.</p>
<p /><address><hr /><div align="center">
<table width="100%" cellspacing="0" border="0"><tr class="address">
<td width="40%" align="left">Copyright © 2010 Nokia Corporation and/or its subsidiary(-ies)</td>
<td width="20%" align="center"><a href="trademarks.html">Trademarks</a></td>
<td width="40%" align="right"><div align="right">Qt 4.6.3</div></td>
</tr></table></div></address></body>
</html>
distrib
>
Mandriva
>
current
>
i586
>
media
>
main-updates
>
by-pkgid
>
8e6051afcdb111a0317a58fb64c2abf5
>
files
>
694
