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<td >Qt 5.9</td><td ><a href="qtcore-index.html">Qt Core</a></td><td >Timers</td></tr></table><table class="buildversion"><tr>
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<h1 class="title">Timers</h1>
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<p><a href="qobject.html">QObject</a>, the base class of all Qt objects, provides the basic timer support in Qt. With <a href="qobject.html#startTimer">QObject::startTimer</a>(), you start a timer with an interval in milliseconds as argument. The function returns a unique integer timer ID. The timer will now fire at regular intervals until you explicitly call <a href="qobject.html#killTimer">QObject::killTimer</a>() with the timer ID.</p>
<p>For this mechanism to work, the application must run in an event loop. You start an event loop with <a href="../qtwidgets/qapplication.html#exec">QApplication::exec</a>(). When a timer fires, the application sends a <a href="qtimerevent.html">QTimerEvent</a>, and the flow of control leaves the event loop until the timer event is processed. This implies that a timer cannot fire while your application is busy doing something else. In other words: the accuracy of timers depends on the granularity of your application.</p>
<p>In multithreaded applications, you can use the timer mechanism in any thread that has an event loop. To start an event loop from a non-GUI thread, use <a href="qthread.html#exec">QThread::exec</a>(). Qt uses the object's <a href="qobject.html#thread">thread affinity</a> to determine which thread will deliver the <a href="qtimerevent.html">QTimerEvent</a>. Because of this, you must start and stop all timers in the object's thread; it is not possible to start timers for objects in another thread.</p>
<p>The upper limit for the interval value is determined by the number of milliseconds that can be specified in a signed integer (in practice, this is a period of just over 24 days). The accuracy depends on the underlying operating system. Windows 2000 has 15 millisecond accuracy; other systems that we have tested can handle 1 millisecond intervals.</p>
<p>The main API for the timer functionality is <a href="qtimer.html">QTimer</a>. That class provides regular timers that emit a signal when the timer fires, and inherits <a href="qobject.html">QObject</a> so that it fits well into the ownership structure of most GUI programs. The normal way of using it is like this:</p>
<pre class="cpp">
<span class="type"><a href="qtimer.html">QTimer</a></span> <span class="operator">*</span>timer <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qtimer.html">QTimer</a></span>(<span class="keyword">this</span>);
connect(timer<span class="operator">,</span> SIGNAL(timeout())<span class="operator">,</span> <span class="keyword">this</span><span class="operator">,</span> SLOT(updateCaption()));
timer<span class="operator">-</span><span class="operator">></span>start(<span class="number">1000</span>);
</pre>
<p>The <a href="qtimer.html">QTimer</a> object is made into a child of this widget so that, when this widget is deleted, the timer is deleted too. Next, its <a href="qtimer.html#timeout">timeout()</a> signal is connected to the slot that will do the work, it is started with a value of 1000 milliseconds, indicating that it will time out every second.</p>
<p><a href="qtimer.html">QTimer</a> also provides a static function for single-shot timers. For example:</p>
<pre class="cpp">
<span class="type"><a href="qtimer.html">QTimer</a></span><span class="operator">::</span>singleShot(<span class="number">200</span><span class="operator">,</span> <span class="keyword">this</span><span class="operator">,</span> SLOT(updateCaption()));
</pre>
<p>200 milliseconds (0.2 seconds) after this line of code is executed, the <code>updateCaption()</code> slot will be called.</p>
<p>For <a href="qtimer.html">QTimer</a> to work, you must have an event loop in your application; that is, you must call <a href="qcoreapplication.html#exec">QCoreApplication::exec</a>() somewhere. Timer events will be delivered only while the event loop is running.</p>
<p>In multithreaded applications, you can use <a href="qtimer.html">QTimer</a> in any thread that has an event loop. To start an event loop from a non-GUI thread, use <a href="qthread.html#exec">QThread::exec</a>(). Qt uses the timer's <a href="qobject.html#thread">thread affinity</a> to determine which thread will emit the <a href="qtimer.html#timeout">timeout()</a> signal. Because of this, you must start and stop the timer in its thread; it is not possible to start a timer from another thread.</p>
<p>The <a href="../qtwidgets/qtwidgets-widgets-analogclock-example.html">Analog Clock</a> example shows how to use <a href="qtimer.html">QTimer</a> to redraw a widget at regular intervals. From <code>AnalogClock</code>'s implementation:</p>
<pre class="cpp">
AnalogClock<span class="operator">::</span>AnalogClock(<span class="type"><a href="../qtwidgets/qwidget.html">QWidget</a></span> <span class="operator">*</span>parent)
: <span class="type"><a href="../qtwidgets/qwidget.html">QWidget</a></span>(parent)
{
<span class="type"><a href="qtimer.html">QTimer</a></span> <span class="operator">*</span>timer <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qtimer.html">QTimer</a></span>(<span class="keyword">this</span>);
connect(timer<span class="operator">,</span> SIGNAL(timeout())<span class="operator">,</span> <span class="keyword">this</span><span class="operator">,</span> SLOT(update()));
timer<span class="operator">-</span><span class="operator">></span>start(<span class="number">1000</span>);
...
}
</pre>
<p>Every second, <a href="qtimer.html">QTimer</a> will call the <a href="../qtwidgets/qwidget.html#update">QWidget::update</a>() slot to refresh the clock's display.</p>
<p>If you already have a <a href="qobject.html">QObject</a> subclass and want an easy optimization, you can use <a href="qbasictimer.html">QBasicTimer</a> instead of <a href="qtimer.html">QTimer</a>. With <a href="qbasictimer.html">QBasicTimer</a>, you must reimplement <a href="qobject.html#timerEvent">timerEvent()</a> in your <a href="qobject.html">QObject</a> subclass and handle the timeout there. The <a href="../qtwidgets/qtwidgets-widgets-wiggly-example.html">Wiggly</a> example shows how to use <a href="qbasictimer.html">QBasicTimer</a>.</p>
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