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<title>Qt 4.8: Delayed Encoding Example</title>
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<a href="index.html" class="qtref"><span>Qt Reference Documentation</span></a>
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<li class="first"><a href="index.html">Home</a></li>
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<li><a href="all-examples.html">Examples</a></li>
<li>Delayed Encoding Example</li>
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<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#sourcewidget-class-definition">SourceWidget Class Definition</a></li>
<li class="level1"><a href="#sourcewidget-class-implementation">SourceWidget Class Implementation</a></li>
<li class="level1"><a href="#mimedata-class-definition">MimeData Class Definition</a></li>
<li class="level1"><a href="#mimedata-class-implementation">MimeData Class Implementation</a></li>
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<h1 class="title">Delayed Encoding Example</h1>
<span class="subtitle"></span>
<!-- $$$draganddrop/delayedencoding-description -->
<div class="descr"> <a name="details"></a>
<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 class="centerAlign"><img src="images/delayedecoding-example.png" alt="" /></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 class="cpp"> <span class="keyword">public</span> <span class="keyword">slots</span>:
<span class="type">void</span> createData(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>mimetype);
<span class="type">void</span> startDrag();
<span class="keyword">private</span>:
<span class="type"><a href="qbytearray.html">QByteArray</a></span> imageData;
<span class="type"><a href="qsvgwidget.html">QSvgWidget</a></span> <span class="operator">*</span>imageLabel;
MimeData <span class="operator">*</span>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 class="cpp"> <span class="type">void</span> SourceWidget<span class="operator">::</span>startDrag()
{
mimeData <span class="operator">=</span> <span class="keyword">new</span> MimeData;
connect(mimeData<span class="operator">,</span> SIGNAL(dataRequested(<span class="type"><a href="qstring.html">QString</a></span>))<span class="operator">,</span>
<span class="keyword">this</span><span class="operator">,</span> SLOT(createData(<span class="type"><a href="qstring.html">QString</a></span>))<span class="operator">,</span> <span class="type"><a href="qt.html">Qt</a></span><span class="operator">::</span>DirectConnection);
<span class="type"><a href="qdrag.html">QDrag</a></span> <span class="operator">*</span>drag <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qdrag.html">QDrag</a></span>(<span class="keyword">this</span>);
drag<span class="operator">-</span><span class="operator">></span>setMimeData(mimeData);
drag<span class="operator">-</span><span class="operator">></span>setPixmap(<span class="type"><a href="qpixmap.html">QPixmap</a></span>(<span class="string">":/images/drag.png"</span>));
drag<span class="operator">-</span><span class="operator">></span>exec(<span class="type"><a href="qt.html">Qt</a></span><span class="operator">::</span>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 class="cpp"> <span class="type">void</span> SourceWidget<span class="operator">::</span>createData(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>mimeType)
{
<span class="keyword">if</span> (mimeType <span class="operator">!</span><span class="operator">=</span> <span class="string">"image/png"</span>)
<span class="keyword">return</span>;
<span class="type"><a href="qimage.html">QImage</a></span> image(imageLabel<span class="operator">-</span><span class="operator">></span>size()<span class="operator">,</span> <span class="type"><a href="qimage.html">QImage</a></span><span class="operator">::</span>Format_RGB32);
<span class="type"><a href="qpainter.html">QPainter</a></span> painter;
painter<span class="operator">.</span>begin(<span class="operator">&</span>image);
imageLabel<span class="operator">-</span><span class="operator">></span>renderer()<span class="operator">-</span><span class="operator">></span>render(<span class="operator">&</span>painter);
painter<span class="operator">.</span>end();
<span class="type"><a href="qbytearray.html">QByteArray</a></span> data;
<span class="type"><a href="qbuffer.html">QBuffer</a></span> buffer(<span class="operator">&</span>data);
buffer<span class="operator">.</span>open(<span class="type"><a href="qiodevice.html">QIODevice</a></span><span class="operator">::</span>WriteOnly);
image<span class="operator">.</span>save(<span class="operator">&</span>buffer<span class="operator">,</span> <span class="string">"PNG"</span>);
buffer<span class="operator">.</span>close();
mimeData<span class="operator">-</span><span class="operator">></span>setData(<span class="string">"image/png"</span><span class="operator">,</span> 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="cpp"> <span class="keyword">class</span> MimeData : <span class="keyword">public</span> <span class="type"><a href="qmimedata.html">QMimeData</a></span>
{
Q_OBJECT
<span class="keyword">public</span>:
MimeData();
<span class="type"><a href="qstringlist.html">QStringList</a></span> formats() <span class="keyword">const</span>;
<span class="keyword">signals</span>:
<span class="type">void</span> dataRequested(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>mimeType) <span class="keyword">const</span>;
<span class="keyword">protected</span>:
<span class="type"><a href="qvariant.html">QVariant</a></span> retrieveData(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>mimetype<span class="operator">,</span> <span class="type"><a href="qvariant.html">QVariant</a></span><span class="operator">::</span>Type type) <span class="keyword">const</span>;
};</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 class="cpp"> <span class="type"><a href="qstringlist.html">QStringList</a></span> MimeData<span class="operator">::</span>formats() <span class="keyword">const</span>
{
<span class="keyword">return</span> <span class="type"><a href="qmimedata.html">QMimeData</a></span><span class="operator">::</span>formats() <span class="operator"><</span><span class="operator"><</span> <span class="string">"image/png"</span>;
}</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 class="cpp"> <span class="type"><a href="qvariant.html">QVariant</a></span> MimeData<span class="operator">::</span>retrieveData(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>mimeType<span class="operator">,</span> <span class="type"><a href="qvariant.html">QVariant</a></span><span class="operator">::</span>Type type)
<span class="keyword">const</span>
{
<span class="keyword">emit</span> dataRequested(mimeType);
<span class="keyword">return</span> <span class="type"><a href="qmimedata.html">QMimeData</a></span><span class="operator">::</span>retrieveData(mimeType<span class="operator">,</span> 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>
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