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<td >Qt 5.12</td><td ><a href="qtcore-index.html">Qt Core</a></td><td >Queued Custom Type Example</td></tr></table><table class="buildversion"><tr>
<td id="buildversion" width="100%" align="right"><a href="qtcore-index.html">Qt 5.12.6 Reference Documentation</a></td>
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<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#overview">Overview</a></li>
<li class="level1"><a href="#the-block-class">The Block Class</a></li>
<li class="level1"><a href="#the-window-class">The Window Class</a></li>
<li class="level1"><a href="#the-renderthread-class">The RenderThread Class</a></li>
<li class="level1"><a href="#registering-the-type">Registering the Type</a></li>
<li class="level1"><a href="#further-reading">Further Reading</a></li>
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<h1 class="title">Queued Custom Type Example</h1>
<span class="subtitle"></span>
<!-- $$$threads/queuedcustomtype-brief -->
<p>Demonstrates multi-thread programming using Qt.</p>
<!-- @@@threads/queuedcustomtype -->
<!-- $$$threads/queuedcustomtype-description -->
<div class="descr"> <a name="details"></a>
<p class="centerAlign"><img src="images/queuedcustomtype-example.png" alt="" /></p><p>Contents:</p>
<a name="overview"></a>
<h2 id="overview">Overview</h2>
<p>In the <a href="qtcore-tools-customtype-example.html">Custom Type Example</a>, we showed how to integrate custom types with the meta-object system, enabling them to be stored in <a href="qvariant.html">QVariant</a> objects, written out in debugging information and used in signal-slot communication.</p>
<p>In this example, we create a new value class, <code>Block</code>, and register it with the meta-object system to enable us to send instances of it between threads using queued signals and slots.</p>
<a name="the-block-class"></a>
<h2 id="the-block-class">The Block Class</h2>
<p>The <code>Block</code> class is similar to the <code>Message</code> class described in the <a href="qtcore-tools-customtype-example.html">Custom Type Example</a>. It provides the default constructor, copy constructor and destructor in the public section of the class that the meta-object system requires. It describes a colored rectangle.</p>
<pre class="cpp">
<span class="keyword">class</span> Block
{
<span class="keyword">public</span>:
Block();
Block(<span class="keyword">const</span> Block <span class="operator">&</span>other);
<span class="operator">~</span>Block();
Block(<span class="keyword">const</span> <span class="type"><a href="qrect.html">QRect</a></span> <span class="operator">&</span>rect<span class="operator">,</span> <span class="keyword">const</span> <span class="type"><a href="../qtgui/qcolor.html">QColor</a></span> <span class="operator">&</span>color);
<span class="type"><a href="../qtgui/qcolor.html">QColor</a></span> color() <span class="keyword">const</span>;
<span class="type"><a href="qrect.html">QRect</a></span> rect() <span class="keyword">const</span>;
<span class="keyword">private</span>:
<span class="type"><a href="qrect.html">QRect</a></span> m_rect;
<span class="type"><a href="../qtgui/qcolor.html">QColor</a></span> m_color;
};
Q_DECLARE_METATYPE(Block);
</pre>
<p>We will still need to register it with the meta-object system at run-time by calling the <a href="qmetatype.html#qRegisterMetaType-1">qRegisterMetaType</a>() template function before we make any signal-slot connections that use this type. Even though we do not intend to use the type with <a href="qvariant.html">QVariant</a> in this example, it is good practice to also declare the new type with <a href="qmetatype.html#Q_DECLARE_METATYPE">Q_DECLARE_METATYPE</a>().</p>
<p>The implementation of the <code>Block</code> class is trivial, so we avoid quoting it here.</p>
<a name="the-window-class"></a>
<h2 id="the-window-class">The Window Class</h2>
<p>We define a simple <code>Window</code> class with a public slot that accepts a <code>Block</code> object. The rest of the class is concerned with managing the user interface and handling images.</p>
<pre class="cpp">
<span class="keyword">class</span> Window : <span class="keyword">public</span> <span class="type"><a href="../qtwidgets/qwidget.html">QWidget</a></span>
{
Q_OBJECT
<span class="keyword">public</span>:
Window();
<span class="type">void</span> loadImage(<span class="keyword">const</span> <span class="type"><a href="../qtgui/qimage.html">QImage</a></span> <span class="operator">&</span>image);
<span class="keyword">public</span> <span class="keyword">slots</span>:
<span class="type">void</span> addBlock(<span class="keyword">const</span> Block <span class="operator">&</span>block);
<span class="keyword">private</span> <span class="keyword">slots</span>:
<span class="type">void</span> loadImage();
<span class="type">void</span> resetUi();
<span class="keyword">private</span>:
<span class="type"><a href="../qtwidgets/qlabel.html">QLabel</a></span> <span class="operator">*</span>label;
<span class="type"><a href="../qtgui/qpixmap.html">QPixmap</a></span> pixmap;
<span class="type"><a href="../qtwidgets/qpushbutton.html">QPushButton</a></span> <span class="operator">*</span>loadButton;
<span class="type"><a href="../qtwidgets/qpushbutton.html">QPushButton</a></span> <span class="operator">*</span>resetButton;
<span class="type"><a href="qstring.html">QString</a></span> path;
RenderThread <span class="operator">*</span>thread;
};
</pre>
<p>The <code>Window</code> class also contains a worker thread, provided by a <code>RenderThread</code> object. This will emit signals to send <code>Block</code> objects to the window's <code>addBlock(Block)</code> slot.</p>
<p>The parts of the <code>Window</code> class that are most relevant are the constructor and the <code>addBlock(Block)</code> slot.</p>
<p>The constructor creates a thread for rendering images, sets up a user interface containing a label and two push buttons that are connected to slots in the same class.</p>
<pre class="cpp">
Window<span class="operator">::</span>Window()
{
thread <span class="operator">=</span> <span class="keyword">new</span> RenderThread();
label <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="../qtwidgets/qlabel.html">QLabel</a></span>();
label<span class="operator">-</span><span class="operator">></span>setAlignment(<span class="type"><a href="qt.html">Qt</a></span><span class="operator">::</span>AlignCenter);
loadButton <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="../qtwidgets/qpushbutton.html">QPushButton</a></span>(tr(<span class="string">"&Load image..."</span>));
resetButton <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="../qtwidgets/qpushbutton.html">QPushButton</a></span>(tr(<span class="string">"&Stop"</span>));
resetButton<span class="operator">-</span><span class="operator">></span>setEnabled(<span class="keyword">false</span>);
connect(loadButton<span class="operator">,</span> SIGNAL(clicked())<span class="operator">,</span> <span class="keyword">this</span><span class="operator">,</span> SLOT(loadImage()));
connect(resetButton<span class="operator">,</span> SIGNAL(clicked())<span class="operator">,</span> thread<span class="operator">,</span> SLOT(stopProcess()));
connect(thread<span class="operator">,</span> SIGNAL(finished())<span class="operator">,</span> <span class="keyword">this</span><span class="operator">,</span> SLOT(resetUi()));
connect(thread<span class="operator">,</span> SIGNAL(sendBlock(Block))<span class="operator">,</span> <span class="keyword">this</span><span class="operator">,</span> SLOT(addBlock(Block)));
</pre>
<p>In the last of these connections, we connect a signal in the <code>RenderThread</code> object to the <code>addBlock(Block)</code> slot in the window.</p>
<pre class="cpp">
...
setWindowTitle(tr(<span class="string">"Queued Custom Type"</span>));
}
</pre>
<p>The rest of the constructor simply sets up the layout of the window.</p>
<p>The <code>addBlock(Block)</code> slot receives blocks from the rendering thread via the signal-slot connection set up in the constructor:</p>
<pre class="cpp">
<span class="type">void</span> Window<span class="operator">::</span>addBlock(<span class="keyword">const</span> Block <span class="operator">&</span>block)
{
<span class="type"><a href="../qtgui/qcolor.html">QColor</a></span> color <span class="operator">=</span> block<span class="operator">.</span>color();
color<span class="operator">.</span>setAlpha(<span class="number">64</span>);
<span class="type"><a href="../qtgui/qpainter.html">QPainter</a></span> painter;
painter<span class="operator">.</span>begin(<span class="operator">&</span>pixmap);
painter<span class="operator">.</span>fillRect(block<span class="operator">.</span>rect()<span class="operator">,</span> color);
painter<span class="operator">.</span>end();
label<span class="operator">-</span><span class="operator">></span>setPixmap(pixmap);
}
</pre>
<p>We simply paint these onto the label as they arrive.</p>
<a name="the-renderthread-class"></a>
<h2 id="the-renderthread-class">The RenderThread Class</h2>
<p>The <code>RenderThread</code> class processes an image, creating <code>Block</code> objects and using the <code>sendBlock(Block)</code> signal to send them to other components in the example.</p>
<pre class="cpp">
<span class="keyword">class</span> RenderThread : <span class="keyword">public</span> <span class="type"><a href="qthread.html">QThread</a></span>
{
Q_OBJECT
<span class="keyword">public</span>:
RenderThread(<span class="type"><a href="qobject.html">QObject</a></span> <span class="operator">*</span>parent <span class="operator">=</span> <span class="number">0</span>);
<span class="operator">~</span>RenderThread();
<span class="type">void</span> processImage(<span class="keyword">const</span> <span class="type"><a href="../qtgui/qimage.html">QImage</a></span> <span class="operator">&</span>image);
<span class="keyword">signals</span>:
<span class="type">void</span> sendBlock(<span class="keyword">const</span> Block <span class="operator">&</span>block);
<span class="keyword">public</span> <span class="keyword">slots</span>:
<span class="type">void</span> stopProcess();
<span class="keyword">protected</span>:
<span class="type">void</span> run();
<span class="keyword">private</span>:
bool m_abort;
<span class="type"><a href="../qtgui/qimage.html">QImage</a></span> m_image;
<span class="type"><a href="qmutex.html">QMutex</a></span> mutex;
};
</pre>
<p>The constructor and destructor are not quoted here. These take care of setting up the thread's internal state and cleaning up when it is destroyed.</p>
<p>Processing is started with the <code>processImage()</code> function, which calls the <code>RenderThread</code> class's reimplementation of the <a href="qthread.html#run">QThread::run</a>() function:</p>
<pre class="cpp">
<span class="type">void</span> RenderThread<span class="operator">::</span>processImage(<span class="keyword">const</span> <span class="type"><a href="../qtgui/qimage.html">QImage</a></span> <span class="operator">&</span>image)
{
<span class="keyword">if</span> (image<span class="operator">.</span>isNull())
<span class="keyword">return</span>;
m_image <span class="operator">=</span> image;
m_abort <span class="operator">=</span> <span class="keyword">false</span>;
start();
}
<span class="type">void</span> RenderThread<span class="operator">::</span>run()
{
<span class="type">int</span> size <span class="operator">=</span> <a href="qtglobal.html#qMax">qMax</a>(m_image<span class="operator">.</span>width()<span class="operator">/</span><span class="number">20</span><span class="operator">,</span> m_image<span class="operator">.</span>height()<span class="operator">/</span><span class="number">20</span>);
<span class="keyword">for</span> (<span class="type">int</span> s <span class="operator">=</span> size; s <span class="operator">></span> <span class="number">0</span>; <span class="operator">-</span><span class="operator">-</span>s) {
<span class="keyword">for</span> (<span class="type">int</span> c <span class="operator">=</span> <span class="number">0</span>; c <span class="operator"><</span> <span class="number">400</span>; <span class="operator">+</span><span class="operator">+</span>c) {
</pre>
<p>Ignoring the details of the way the image is processed, we see that the signal containing a block is emitted in the usual way:</p>
<pre class="cpp">
...
Block block(<span class="type"><a href="qrect.html">QRect</a></span>(x1<span class="operator">,</span> y1<span class="operator">,</span> x2 <span class="operator">-</span> x1 <span class="operator">+</span> <span class="number">1</span><span class="operator">,</span> y2 <span class="operator">-</span> y1 <span class="operator">+</span> <span class="number">1</span>)<span class="operator">,</span>
<span class="type"><a href="../qtgui/qcolor.html">QColor</a></span>(red<span class="operator">/</span>n<span class="operator">,</span> green<span class="operator">/</span>n<span class="operator">,</span> blue<span class="operator">/</span>n));
<span class="keyword">emit</span> sendBlock(block);
<span class="keyword">if</span> (m_abort)
<span class="keyword">return</span>;
msleep(<span class="number">10</span>);
}
}
}
</pre>
<p>Each signal that is emitted will be queued and delivered later to the window's <code>addBlock(Block)</code> slot.</p>
<a name="registering-the-type"></a>
<h2 id="registering-the-type">Registering the Type</h2>
<p>In the example's <code>main()</code> function, we perform the registration of the <code>Block</code> class as a custom type with the meta-object system by calling the <a href="qmetatype.html#qRegisterMetaType-1">qRegisterMetaType</a>() template function:</p>
<pre class="cpp">
<span class="type">int</span> main(<span class="type">int</span> argc<span class="operator">,</span> <span class="type">char</span> <span class="operator">*</span>argv<span class="operator">[</span><span class="operator">]</span>)
{
<span class="type"><a href="../qtwidgets/qapplication.html">QApplication</a></span> app(argc<span class="operator">,</span> argv);
<a href="qmetatype.html#qRegisterMetaType-1">qRegisterMetaType</a><span class="operator"><</span>Block<span class="operator">></span>();
Window window;
window<span class="operator">.</span>show();
window<span class="operator">.</span>loadImage(createImage(<span class="number">256</span><span class="operator">,</span> <span class="number">256</span>));
<span class="keyword">return</span> app<span class="operator">.</span>exec();
}
</pre>
<p>This call is placed here to ensure that the type is registered before any signal-slot connections are made that use it.</p>
<p>The rest of the <code>main()</code> function is concerned with setting a seed for the pseudo-random number generator, creating and showing the window, and setting a default image. See the source code for the implementation of the <code>createImage()</code> function.</p>
<a name="further-reading"></a>
<h2 id="further-reading">Further Reading</h2>
<p>This example showed how a custom type can be registered with the meta-object system so that it can be used with signal-slot connections between threads. For ordinary communication involving direct signals and slots, it is enough to simply declare the type in the way described in the <a href="qtcore-tools-customtype-example.html">Custom Type Example</a>.</p>
<p>In practice, both the <a href="qmetatype.html#Q_DECLARE_METATYPE">Q_DECLARE_METATYPE</a>() macro and the <a href="qmetatype.html#qRegisterMetaType-1">qRegisterMetaType</a>() template function can be used to register custom types, but <a href="qmetatype.html#qRegisterMetaType-1">qRegisterMetaType</a>() is only required if you need to perform signal-slot communication or need to create and destroy objects of the custom type at run-time.</p>
<p>More information on using custom types with Qt can be found in the <a href="custom-types.html">Creating Custom Qt Types</a> document.</p>
<p>Files:</p>
<ul>
<li><a href="qtcore-threads-queuedcustomtype-block-cpp.html">threads/queuedcustomtype/block.cpp</a></li>
<li><a href="qtcore-threads-queuedcustomtype-block-h.html">threads/queuedcustomtype/block.h</a></li>
<li><a href="qtcore-threads-queuedcustomtype-main-cpp.html">threads/queuedcustomtype/main.cpp</a></li>
<li><a href="qtcore-threads-queuedcustomtype-queuedcustomtype-pro.html">threads/queuedcustomtype/queuedcustomtype.pro</a></li>
<li><a href="qtcore-threads-queuedcustomtype-renderthread-cpp.html">threads/queuedcustomtype/renderthread.cpp</a></li>
<li><a href="qtcore-threads-queuedcustomtype-renderthread-h.html">threads/queuedcustomtype/renderthread.h</a></li>
<li><a href="qtcore-threads-queuedcustomtype-window-cpp.html">threads/queuedcustomtype/window.cpp</a></li>
<li><a href="qtcore-threads-queuedcustomtype-window-h.html">threads/queuedcustomtype/window.h</a></li>
</ul>
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