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153 lines
6.2 KiB
153 lines
6.2 KiB
14 years ago
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/doc/object.doc:86 -->
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>Timers</title>
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<style type="text/css"><!--
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fn { margin-left: 1cm; text-indent: -1cm; }
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--></style>
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<body>
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<table border="0" cellpadding="0" cellspacing="0" width="100%">
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<tr bgcolor="#E5E5E5">
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<td valign=center>
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<a href="index.html">
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<font color="#004faf">Home</font></a>
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| <a href="classes.html">
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<font color="#004faf">All Classes</font></a>
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| <a href="mainclasses.html">
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<font color="#004faf">Main Classes</font></a>
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<font color="#004faf">Annotated</font></a>
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<font color="#004faf">Grouped Classes</font></a>
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<font color="#004faf">Functions</font></a>
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</td>
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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Timers</h1>
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<p> <a href="qobject.html">QObject</a>, the base class of all Qt objects, provides the basic timer
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support in Qt. With <a href="qobject.html#startTimer">QObject::startTimer</a>(), you start a timer with
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an <em>interval</em> in milliseconds as argument. The function returns a
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unique integer timer id. The timer will now "fire" every <em>interval</em>
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milliseconds, until you explicitly call <a href="qobject.html#killTimer">QObject::killTimer</a>() with
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the timer id.
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<p> For this mechanism to work, the application must run in an event
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loop. You start an event loop with <a href="qapplication.html#exec">QApplication::exec</a>(). When a
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timer fires, the application sends a <a href="qtimerevent.html">QTimerEvent</a>, and the flow of
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control leaves the event loop until the timer event is processed. This
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implies that a timer cannot fire while your application is busy doing
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something else. In other words: the accuracy of timers depends on the
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granularity of your application.
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<p> There is practically no upper limit for the interval value (more than
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one year is possible). The accuracy depends on the underlying operating
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system. Windows 95/98 has 55 millisecond (18.2 times per second)
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accuracy; other systems that we have tested (UNIX X11 and Windows NT)
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can handle 1 millisecond intervals.
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<p> The main API for the timer functionality is <a href="qtimer.html">QTimer</a>. That class
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provides regular timers that emit a signal when the timer fires, and
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inherits <a href="qobject.html">QObject</a> so that it fits well into the ownership structure
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of most GUI programs. The normal way of using it is like this:
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<pre>
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<a href="qtimer.html">QTimer</a> * counter = new <a href="qtimer.html">QTimer</a>( this );
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connect( counter, SIGNAL(<a href="qtimer.html#timeout">timeout</a>()),
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this, SLOT(updateCaption()) );
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counter-><a href="qtimer.html#start">start</a>( 1000 );
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</pre>
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<p> The counter timer is made into a child of this widget, so that when
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this widget is deleted, the timer is deleted too. Next, its timeout
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signal is connected to the slot that will do the work, and finally
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it's started.
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<p> <a href="qtimer.html">QTimer</a> also provides a simple one-shot timer API. <a href="qbutton.html">QButton</a> uses this
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to show the button being pressed down and then (0.1 seconds later) be
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released when the keyboard is used to "press" a button, for example:
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<p> <pre>
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QTimer::<a href="qtimer.html#singleShot">singleShot</a>( 100, this, SLOT(animateTimeout()) );
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</pre>
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<p> 0.1 seconds after this line of code is executed, the same button's
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animateTimeout() slot is called.
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<p> Here is an outline of a slightly longer example that combines object
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communication via signals and slots with a QTimer object. It
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demonstrates how to use timers to perform intensive calculations in a
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single-threaded application without blocking the user interface.
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<p> <pre>
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// The Mandelbrot class uses a QTimer to calculate the mandelbrot
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// set one scanline at a time without blocking the CPU. It
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// inherits QObject to use signals and slots. Calling start()
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// starts the calculation. The done() signal is emitted when it
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// has finished. Note that this example is not complete, just an
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// outline.
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class Mandelbrot : public <a href="qobject.html">QObject</a>
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{
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<a href="metaobjects.html#Q_OBJECT">Q_OBJECT</a> // required for signals/slots
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public:
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Mandelbrot( <a href="qobject.html">QObject</a> *parent=0, const char *name );
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...
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public slots:
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void start();
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signals:
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void done();
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private slots:
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void calculate();
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private:
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<a href="qtimer.html">QTimer</a> timer;
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...
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};
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//
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// Constructs and initializes a Mandelbrot object.
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//
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Mandelbrot::Mandelbrot( <a href="qobject.html">QObject</a> *parent=0, const char *name )
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: <a href="qobject.html">QObject</a>( parent, name )
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{
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<a href="qobject.html#connect">connect</a>( &timer, SIGNAL(<a href="qtimer.html#timeout">timeout</a>()), SLOT(calculate()) );
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...
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}
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//
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// Starts the calculation task. The internal calculate() slot
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// will be activated every 10 milliseconds.
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//
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void Mandelbrot::start()
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{
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if ( !timer.<a href="qtimer.html#isActive">isActive</a>() ) // not already running
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timer.<a href="qtimer.html#start">start</a>( 10 ); // timeout every 10 ms
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}
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//
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// Calculates one scanline at a time.
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// Emits the done() signal when finished.
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//
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void Mandelbrot::calculate()
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{
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... // perform the calculation for a scanline
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if ( finished ) { // no more scanlines
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timer.<a href="qtimer.html#stop">stop</a>();
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emit done();
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}
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}
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</pre>
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<p>
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<!-- eof -->
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<p><address><hr><div align=center>
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<table width=100% cellspacing=0 border=0><tr>
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<td>Copyright © 2007
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|
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<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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<td align=right><div align=right>Qt 3.3.8</div>
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</table></div></address></body>
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</html>
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