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<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">Queued Custom Type Example<br /><span class="subtitle"></span>
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<p>Files:</p>
<ul>
<li><a href="threads-queuedcustomtype-block-cpp.html">threads/queuedcustomtype/block.cpp</a></li>
<li><a href="threads-queuedcustomtype-block-h.html">threads/queuedcustomtype/block.h</a></li>
<li><a href="threads-queuedcustomtype-renderthread-cpp.html">threads/queuedcustomtype/renderthread.cpp</a></li>
<li><a href="threads-queuedcustomtype-renderthread-h.html">threads/queuedcustomtype/renderthread.h</a></li>
<li><a href="threads-queuedcustomtype-window-cpp.html">threads/queuedcustomtype/window.cpp</a></li>
<li><a href="threads-queuedcustomtype-window-h.html">threads/queuedcustomtype/window.h</a></li>
<li><a href="threads-queuedcustomtype-main-cpp.html">threads/queuedcustomtype/main.cpp</a></li>
<li><a href="threads-queuedcustomtype-queuedcustomtype-pro.html">threads/queuedcustomtype/queuedcustomtype.pro</a></li>
</ul>
<p>The Queued Custom Type example shows how to send custom types between threads with queued signals and slots.</p>
<p align="center"><img src="images/queuedcustomtype-example.png" /></p><p>Contents:</p>
<ul><li><a href="#overview">Overview</a></li>
<li><a href="#the-block-class">The Block Class</a></li>
<li><a href="#the-window-class">The Window Class</a></li>
<li><a href="#the-renderthread-class">The RenderThread Class</a></li>
<li><a href="#registering-the-type">Registering the Type</a></li>
<li><a href="#further-reading">Further Reading</a></li>
</ul>
<a name="overview"></a>
<h2>Overview</h2>
<p>In the <a href="tools-customtypesending.html">Custom Type Sending Example</a>, we showed how to use a custom type with signal-slot communication within the same thread.</p>
<p>In this example, we create a new value class, <tt>Block</tt>, 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>The Block Class</h2>
<p>The <tt>Block</tt> class is similar to the <tt>Message</tt> class described in the <a href="tools-customtype.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 Block
{
public:
Block();
Block(const Block &other);
~Block();
Block(const QRect &rect, const QColor &color);
QColor color() const;
QRect rect() const;
private:
QRect m_rect;
QColor 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">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 <tt>Block</tt> class is trivial, so we avoid quoting it here.</p>
<a name="the-window-class"></a>
<h2>The Window Class</h2>
<p>We define a simple <tt>Window</tt> class with a public slot that accepts a <tt>Block</tt> object. The rest of the class is concerned with managing the user interface and handling images.</p>
<pre> class Window : public QWidget
{
Q_OBJECT
public:
Window();
void loadImage(const QImage &image);
public slots:
void addBlock(const Block &block);
private slots:
void loadImage();
void resetUi();
private:
QLabel *label;
QPixmap pixmap;
QPushButton *loadButton;
QPushButton *resetButton;
QString path;
RenderThread *thread;
};</pre>
<p>The <tt>Window</tt> class also contains a worker thread, provided by a <tt>RenderThread</tt> object. This will emit signals to send <tt>Block</tt> objects to the window's <tt>addBlock(Block)</tt> slot.</p>
<p>The parts of the <tt>Window</tt> class that are most relevant are the constructor and the <tt>addBlock(Block)</tt> 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> Window::Window()
{
thread = new RenderThread();
label = new QLabel();
label->setAlignment(Qt::AlignCenter);
loadButton = new QPushButton(tr("&Load image..."));
resetButton = new QPushButton(tr("&Stop"));
resetButton->setEnabled(false);
connect(loadButton, SIGNAL(clicked()), this, SLOT(loadImage()));
connect(resetButton, SIGNAL(clicked()), thread, SLOT(stopProcess()));
connect(thread, SIGNAL(finished()), this, SLOT(resetUi()));
connect(thread, SIGNAL(sendBlock(Block)), this, SLOT(addBlock(Block)));</pre>
<p>In the last of these connections, we connect a signal in the <tt>RenderThread</tt> object to the <tt>addBlock(Block)</tt> slot in the window.</p>
<pre> ...
setWindowTitle(tr("Queued Custom Type"));
}</pre>
<p>The rest of the constructor simply sets up the layout of the window.</p>
<p>The <tt>addBlock(Block)</tt> slot receives blocks from the rendering thread via the signal-slot connection set up in the constructor:</p>
<pre> void Window::addBlock(const Block &block)
{
QColor color = block.color();
color.setAlpha(64);
QPainter painter;
painter.begin(&pixmap);
painter.fillRect(block.rect(), color);
painter.end();
label->setPixmap(pixmap);
}</pre>
<p>We simply paint these onto the label as they arrive.</p>
<a name="the-renderthread-class"></a>
<h2>The RenderThread Class</h2>
<p>The <tt>RenderThread</tt> class processes an image, creating <tt>Block</tt> objects and using the <tt>sendBlock(Block)</tt> signal to send them to other components in the example.</p>
<pre> class RenderThread : public QThread
{
Q_OBJECT
public:
RenderThread(QObject *parent = 0);
~RenderThread();
void processImage(const QImage &image);
signals:
void sendBlock(const Block &block);
public slots:
void stopProcess();
protected:
void run();
private:
bool m_abort;
QImage m_image;
QMutex 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 <tt>processImage()</tt> function, which calls the <tt>RenderThread</tt> class's reimplementation of the <a href="qthread.html#run">QThread::run</a>() function:</p>
<pre> void RenderThread::processImage(const QImage &image)
{
if (image.isNull())
return;
m_image = image;
m_abort = false;
start();
}
void RenderThread::run()
{
int size = qMax(m_image.width()/20, m_image.height()/20);
for (int s = size; s > 0; --s) {
for (int c = 0; c < 400; ++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> ...
Block block(QRect(x1, y1, x2 - x1 + 1, y2 - y1 + 1),
QColor(red/n, green/n, blue/n));
emit sendBlock(block);
if (m_abort)
return;
msleep(10);
}
}
}</pre>
<p>Each signal that is emitted will be queued and delivered later to the window's <tt>addBlock(Block)</tt> slot.</p>
<a name="registering-the-type"></a>
<h2>Registering the Type</h2>
<p>In the example's <tt>main()</tt> function, we perform the registration of the <tt>Block</tt> class as a custom type with the meta-object system by calling the <a href="qmetatype.html#qRegisterMetaType">qRegisterMetaType</a>() template function:</p>
<pre> int main(int argc, char *argv[])
{
QApplication app(argc, argv);
qRegisterMetaType<Block>();
qsrand(QTime::currentTime().elapsed());
Window window;
window.show();
window.loadImage(createImage(256, 256));
return app.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 <tt>main()</tt> 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 <tt>createImage()</tt> function.</p>
<a name="further-reading"></a>
<h2>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="tools-customtypesending.html">Custom Type Sending 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">qRegisterMetaType</a>() template function can be used to register custom types, but <a href="qmetatype.html#qRegisterMetaType">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>
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