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<td >Qt 5.12</td><td ><a href="qtwidgets-index.html">Qt Widgets</a></td><td >Event Transitions Example</td></tr></table><table class="buildversion"><tr>
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<h1 class="title">Event Transitions Example</h1>
<span class="subtitle"></span>
<!-- $$$statemachine/eventtransitions-brief -->
<p>The Event Transitions example shows how to use event transitions, a feature of <a href="../qtcore/statemachine-api.html">The State Machine Framework</a>.</p>
<!-- @@@statemachine/eventtransitions -->
<!-- $$$statemachine/eventtransitions-description -->
<div class="descr"> <a name="details"></a>
<p>The Event Transitions Example illustrates how states change when a user enters or leaves the area of a button. The states are handled by a <a href="../qtcore/qstatemachine.html">QStateMachine</a> object. The screen consists of a <a href="qvboxlayout.html">QVBoxLayout</a> with a central button.</p>
<p>When the mouse is outside the button, the text in the button displays "Outside". When the mouse enters the button, it displays "Inside".</p>
<div class="border"><p class="centerAlign"><img src="images/transitions.png" alt="" /></p></div><pre class="cpp">
<span class="keyword">class</span> Window : <span class="keyword">public</span> <span class="type"><a href="qwidget.html">QWidget</a></span>
{
<span class="keyword">public</span>:
Window(<span class="type"><a href="qwidget.html">QWidget</a></span> <span class="operator">*</span>parent <span class="operator">=</span> <span class="number">0</span>)
: <span class="type"><a href="qwidget.html">QWidget</a></span>(parent)
{
<span class="type"><a href="qpushbutton.html">QPushButton</a></span> <span class="operator">*</span>button <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qpushbutton.html">QPushButton</a></span>(<span class="keyword">this</span>);
button<span class="operator">-</span><span class="operator">></span>setSizePolicy(<span class="type"><a href="qsizepolicy.html">QSizePolicy</a></span><span class="operator">::</span>Expanding<span class="operator">,</span> <span class="type"><a href="qsizepolicy.html">QSizePolicy</a></span><span class="operator">::</span>Expanding);
<span class="type"><a href="qvboxlayout.html">QVBoxLayout</a></span> <span class="operator">*</span>layout <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qvboxlayout.html">QVBoxLayout</a></span>;
layout<span class="operator">-</span><span class="operator">></span>addWidget(button);
layout<span class="operator">-</span><span class="operator">></span>setContentsMargins(<span class="number">80</span><span class="operator">,</span> <span class="number">80</span><span class="operator">,</span> <span class="number">80</span><span class="operator">,</span> <span class="number">80</span>);
setLayout(layout);
</pre>
<p>The <code>Window</code> class's constructors begins by creating a button. This button is added to <code>layout</code>, which is a <a href="qvboxlayout.html">QVBoxLayout</a> object. Then two states are created: <code>s1</code> is the state "Outside", and <code>s2</code> is the state "Inside".</p>
<pre class="cpp">
<span class="type"><a href="../qtcore/qstatemachine.html">QStateMachine</a></span> <span class="operator">*</span>machine <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="../qtcore/qstatemachine.html">QStateMachine</a></span>(<span class="keyword">this</span>);
<span class="type"><a href="../qtcore/qstate.html">QState</a></span> <span class="operator">*</span>s1 <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="../qtcore/qstate.html">QState</a></span>();
s1<span class="operator">-</span><span class="operator">></span>assignProperty(button<span class="operator">,</span> <span class="string">"text"</span><span class="operator">,</span> <span class="string">"Outside"</span>);
<span class="type"><a href="../qtcore/qstate.html">QState</a></span> <span class="operator">*</span>s2 <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="../qtcore/qstate.html">QState</a></span>();
s2<span class="operator">-</span><span class="operator">></span>assignProperty(button<span class="operator">,</span> <span class="string">"text"</span><span class="operator">,</span> <span class="string">"Inside"</span>);
</pre>
<p>State <code>s1</code> is the state "Outside" and state <code>s2</code> is state "Inside".</p>
<pre class="cpp">
<span class="type"><a href="../qtcore/qeventtransition.html">QEventTransition</a></span> <span class="operator">*</span>enterTransition <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="../qtcore/qeventtransition.html">QEventTransition</a></span>(button<span class="operator">,</span> <span class="type"><a href="../qtcore/qevent.html">QEvent</a></span><span class="operator">::</span>Enter);
enterTransition<span class="operator">-</span><span class="operator">></span>setTargetState(s2);
s1<span class="operator">-</span><span class="operator">></span>addTransition(enterTransition);
</pre>
<p>When the button receives an event of type <a href="../qtcore/qevent.html#Type-enum">QEvent::Enter</a> and the state machine is in state <code>s1</code>, the machine will transition to state <code>s2</code>.</p>
<pre class="cpp">
<span class="type"><a href="../qtcore/qeventtransition.html">QEventTransition</a></span> <span class="operator">*</span>leaveTransition <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="../qtcore/qeventtransition.html">QEventTransition</a></span>(button<span class="operator">,</span> <span class="type"><a href="../qtcore/qevent.html">QEvent</a></span><span class="operator">::</span>Leave);
leaveTransition<span class="operator">-</span><span class="operator">></span>setTargetState(s1);
s2<span class="operator">-</span><span class="operator">></span>addTransition(leaveTransition);
</pre>
<p>When the button receives an event of type <a href="../qtcore/qevent.html#Type-enum">QEvent::Leave</a> and the state machine is in state <code>s2</code>, the machine will transition back to state <code>s1</code>.</p>
<pre class="cpp">
<span class="type"><a href="../qtcore/qstate.html">QState</a></span> <span class="operator">*</span>s3 <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="../qtcore/qstate.html">QState</a></span>();
s3<span class="operator">-</span><span class="operator">></span>assignProperty(button<span class="operator">,</span> <span class="string">"text"</span><span class="operator">,</span> <span class="string">"Pressing..."</span>);
<span class="type"><a href="../qtcore/qeventtransition.html">QEventTransition</a></span> <span class="operator">*</span>pressTransition <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="../qtcore/qeventtransition.html">QEventTransition</a></span>(button<span class="operator">,</span> <span class="type"><a href="../qtcore/qevent.html">QEvent</a></span><span class="operator">::</span>MouseButtonPress);
pressTransition<span class="operator">-</span><span class="operator">></span>setTargetState(s3);
s2<span class="operator">-</span><span class="operator">></span>addTransition(pressTransition);
<span class="type"><a href="../qtcore/qeventtransition.html">QEventTransition</a></span> <span class="operator">*</span>releaseTransition <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="../qtcore/qeventtransition.html">QEventTransition</a></span>(button<span class="operator">,</span> <span class="type"><a href="../qtcore/qevent.html">QEvent</a></span><span class="operator">::</span>MouseButtonRelease);
releaseTransition<span class="operator">-</span><span class="operator">></span>setTargetState(s2);
s3<span class="operator">-</span><span class="operator">></span>addTransition(releaseTransition);
</pre>
<p>Next, state <code>s3</code> is created. <code>s3</code> will be entered when the button receives an event of type <a href="../qtcore/qevent.html#Type-enum">QEvent::MouseButtonPress</a> and the state machine is in state <code>s2</code>. When the button receives an event of type <a href="../qtcore/qevent.html#Type-enum">QEvent::MouseButtonRelease</a> and the state machine is in state <code>s3</code>, the machine will revert to state <code>s2</code>.</p>
<pre class="cpp">
machine<span class="operator">-</span><span class="operator">></span>addState(s1);
machine<span class="operator">-</span><span class="operator">></span>addState(s2);
machine<span class="operator">-</span><span class="operator">></span>addState(s3);
machine<span class="operator">-</span><span class="operator">></span>setInitialState(s1);
machine<span class="operator">-</span><span class="operator">></span>start();
}
};
</pre>
<p>Finally, the states are added to the machine as top-level states, the initial state is set to be <code>s1</code> ("Outside"), and the machine is started.</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><span class="operator">*</span>argv)
{
<span class="type"><a href="qapplication.html">QApplication</a></span> app(argc<span class="operator">,</span> argv);
Window window;
window<span class="operator">.</span>resize(<span class="number">300</span><span class="operator">,</span> <span class="number">300</span>);
window<span class="operator">.</span>show();
<span class="keyword">return</span> app<span class="operator">.</span>exec();
}
</pre>
<p>The main() function constructs a Window object that displays the <a href="qvboxlayout.html">QVBoxLayout</a> object <code>layout</code> with its <code>button</code>.</p>
<p>Files:</p>
<ul>
<li><a href="qtwidgets-statemachine-eventtransitions-eventtransitions-pro.html">statemachine/eventtransitions/eventtransitions.pro</a></li>
<li><a href="qtwidgets-statemachine-eventtransitions-main-cpp.html">statemachine/eventtransitions/main.cpp</a></li>
</ul>
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<!-- @@@statemachine/eventtransitions -->
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