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<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#signals-and-handlers">Signals and Handlers</a></li>
<li class="level2"><a href="#connecting-signals-to-methods-and-signals">Connecting Signals to Methods and Signals</a></li>
<li class="level3"><a href="#signal-to-signal-connect">Signal to Signal Connect</a></li>
<li class="level1"><a href="#c-additions">C++ Additions</a></li>
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<h1 class="title">QML Signal and Handler Event System</h1>
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<p>QML utilizes Qt's <a href="metaobjects.html">meta-object</a> and <a href="signalsandslots.html">signals</a> systems. Signals and slots created using Qt in C++ are inheritely valid in QML.</p>
<a name="qml-signals-and-handlers"></a><a name="signals-and-handlers"></a>
<h2>Signals and Handlers</h2>
<p>Signals provide a way to notify other objects when an event has occurred. For example, the <a href="qml-mousearea.html">MouseArea</a> <tt>clicked</tt> signal notifies other objects that the mouse has been clicked within the area.</p>
<p>The syntax for defining a new signal is:</p>
<p><tt>signal <name>[([<type> <parameter name>[, ...]])]</tt></p>
<p>Attempting to declare two signals or methods with the same name in the same type block generates an error. However, a new signal may reuse the name of an existing signal on the type. (This should be done with caution, as the existing signal may be hidden and become inaccessible.)</p>
<p>Here are various examples of signal declarations:</p>
<pre class="qml"> <span class="type"><a href="qml-rectangle.html">Rectangle</a></span> {
signal <span class="type">trigger</span>
signal <span class="type">send</span> (string notice)
signal <span class="type">perform</span> (string task, variant object)
}</pre>
<p>If the signal has no parameters, the "<tt>()</tt>" brackets are optional. If parameters are used, the parameter types must be declared, as for the <tt>string</tt> and <tt>variant</tt> arguments of the <tt>perform</tt> signal.</p>
<p>Adding a signal to an item automatically adds a <i>signal handler</i> as well. The signal hander is named <tt>on<SignalName></tt>, with the first letter of the signal in uppercase. The previous signals have the following signal handlers:</p>
<pre class="qml"> <span class="name">onTrigger</span>: <span class="name">console</span>.<span class="name">log</span>(<span class="string">"trigger signal emitted"</span>)
<span class="name">onSend</span>: {
<span class="name">console</span>.<span class="name">log</span>(<span class="string">"send signal emitted with notice: "</span> <span class="operator">+</span> <span class="name">notice</span>)
}
<span class="name">onPerform</span>: <span class="name">console</span>.<span class="name">log</span>(<span class="string">"perform signal emitted"</span>)</pre>
<p>Further, each QML properties have a <tt><property_name>Changed</tt> signal and its corresponding <tt>on<property_name>Changed</tt> signal handler. As a result, property changes may notify other components for any changes.</p>
<pre class="qml"> <span class="type"><a href="qml-rectangle.html">Rectangle</a></span> {
<span class="name">id</span>: <span class="name">sprite</span>
<span class="name">width</span>: <span class="number">25</span>; <span class="name">height</span>: <span class="number">25</span>
<span class="name">x</span>: <span class="number">50</span>; <span class="name">y</span>: <span class="number">15</span>
<span class="name">onXChanged</span>: <span class="name">console</span>.<span class="name">log</span>(<span class="string">"x property changed, emitted xChanged signal"</span>)
<span class="name">onYChanged</span>: <span class="name">console</span>.<span class="name">log</span>(<span class="string">"y property changed, emitted yChanged signal"</span>)
}</pre>
<p>To emit a signal, invoke it as a method. The signal handler binding is similar to a property binding and it is invoked when the signal is emitted. Use the defined argument names to access the respective arguments.</p>
<pre class="qml"> <span class="type"><a href="qml-rectangle.html">Rectangle</a></span> {
<span class="name">id</span>: <span class="name">messenger</span>
signal <span class="type">send</span>( string person, string notice)
<span class="name">onSend</span>: {
<span class="name">console</span>.<span class="name">log</span>(<span class="string">"For "</span> <span class="operator">+</span> <span class="name">person</span> <span class="operator">+</span> <span class="string">", the notice is: "</span> <span class="operator">+</span> <span class="name">notice</span>)
}
<span class="name">Component</span>.onCompleted: <span class="name">messenger</span>.<span class="name">send</span>(<span class="string">"Tom"</span>, <span class="string">"the door is ajar."</span>)
}</pre>
<p>Note that the <tt>Component.onCompleted</tt> is an <a href="propertybinding.html#attached-signalhandlers">attached signal handler</a>; it is invoked when the <a href="qml-component.html">Component</a> initialization is complete.</p>
<a name="qml-connect-signals-to-method"></a><a name="connecting-signals-to-methods-and-signals"></a>
<h3>Connecting Signals to Methods and Signals</h3>
<p>Signal objects have a <tt>connect()</tt> method to a connect a signal either to a method or another signal. When a signal is connected to a method, the method is automatically invoked whenever the signal is emitted. (In Qt terminology, the method is a <i>slot</i> that is connected to the <i>signal</i>; all methods defined in QML are created as <a href="signalsandslots.html">Qt slots</a>.) This enables a signal to be received by a method instead of a <a href="qdeclarativeintroduction.html#signal-handlers">signal handler</a>.</p>
<pre class="qml"> <span class="type"><a href="qml-rectangle.html">Rectangle</a></span> {
<span class="name">id</span>: <span class="name">relay</span>
signal <span class="type">send</span>( string person, string notice)
<span class="name">onSend</span>: <span class="name">console</span>.<span class="name">log</span>(<span class="string">"Send signal to: "</span> <span class="operator">+</span> <span class="name">person</span> <span class="operator">+</span> <span class="string">", "</span> <span class="operator">+</span> <span class="name">notice</span>)
<span class="name">Component</span>.onCompleted: {
<span class="name">relay</span>.<span class="name">send</span>.<span class="name">connect</span>(<span class="name">sendToPost</span>)
<span class="name">relay</span>.<span class="name">send</span>.<span class="name">connect</span>(<span class="name">sendToTelegraph</span>)
<span class="name">relay</span>.<span class="name">send</span>.<span class="name">connect</span>(<span class="name">sendToEmail</span>)
<span class="name">relay</span>.<span class="name">send</span>(<span class="string">"Tom"</span>, <span class="string">"Happy Birthday"</span>)
}
<span class="keyword">function</span> <span class="name">sendToPost</span>(<span class="name">person</span>, notice) {
<span class="name">console</span>.<span class="name">log</span>(<span class="string">"Sending to post: "</span> <span class="operator">+</span> <span class="name">person</span> <span class="operator">+</span> <span class="string">", "</span> <span class="operator">+</span> <span class="name">notice</span>)
}
<span class="keyword">function</span> <span class="name">sendToTelegraph</span>(<span class="name">person</span>, notice) {
<span class="name">console</span>.<span class="name">log</span>(<span class="string">"Sending to telegraph: "</span> <span class="operator">+</span> <span class="name">person</span> <span class="operator">+</span> <span class="string">", "</span> <span class="operator">+</span> <span class="name">notice</span>)
}
<span class="keyword">function</span> <span class="name">sendToEmail</span>(<span class="name">person</span>, notice) {
<span class="name">console</span>.<span class="name">log</span>(<span class="string">"Sending to email: "</span> <span class="operator">+</span> <span class="name">person</span> <span class="operator">+</span> <span class="string">", "</span> <span class="operator">+</span> <span class="name">notice</span>)
}
}</pre>
<p>The <tt>connect()</tt> method is appropriate when connecting a JavaScript method to a signal.</p>
<p>There is a corresponding <tt>disconnect()</tt> method for removing connected signals.</p>
<a name="signal-to-signal-connect"></a>
<h4>Signal to Signal Connect</h4>
<p>By connecting signals to other signals, the <tt>connect()</tt> method can form different signal chains.</p>
<pre class="qml"> <span class="type"><a href="qml-rectangle.html">Rectangle</a></span> {
<span class="name">id</span>: <span class="name">forwarder</span>
<span class="name">width</span>: <span class="number">100</span>; <span class="name">height</span>: <span class="number">100</span>
signal <span class="type">send</span>()
<span class="name">onSend</span>: <span class="name">console</span>.<span class="name">log</span>(<span class="string">"Send clicked"</span>)
<span class="type"><a href="qml-mousearea.html">MouseArea</a></span> {
<span class="name">id</span>: <span class="name">mousearea</span>
<span class="name">anchors</span>.fill: <span class="name">parent</span>
<span class="name">onClicked</span>: <span class="name">console</span>.<span class="name">log</span>(<span class="string">"MouseArea clicked"</span>)
}
<span class="name">Component</span>.onCompleted: {
<span class="name">mousearea</span>.<span class="name">clicked</span>.<span class="name">connect</span>(<span class="name">send</span>)
}
}</pre>
<p>Whenever the <a href="qml-mousearea.html">MouseArea</a> <tt>clicked</tt> signal is emitted, the <tt>send</tt> signal will automatically be emitted as well.</p>
<pre class="cpp"> output:
MouseArea clicked
Send clicked</pre>
<a name="c-additions"></a>
<h2>C++ Additions</h2>
<p>Because QML uses Qt, a signal defined in C++ also works as a QML signal. The signal may be emitted in QML code or called as a method. In addition, the QML runtime automatically creates signal handlers for the C++ signals. For more signal control, the <tt>connect()</tt> method and the <a href="qml-connections.html">Connections</a> element may connect a C++ signal to another signal or method.</p>
<p>For complete information on how to call C++ functions in QML, read the <a href="declarative-cppextensions-referenceexamples-signal.html">Extending QML - Signal Support Example</a>.</p>
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