source: trunk/doc/src/timers.qdoc@ 432

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41
42/*!
43 \page timers.html
44 \title Timers
45 \ingroup architecture
46 \brief How to use timers in your application.
47
48 QObject, the base class of all Qt objects, provides the basic
49 timer support in Qt. With QObject::startTimer(), you start a
50 timer with an interval in milliseconds as argument. The function
51 returns a unique integer timer ID. The timer will now fire at
52 regular intervals until you explicitly call QObject::killTimer()
53 with the timer ID.
54
55 For this mechanism to work, the application must run in an event
56 loop. You start an event loop with QApplication::exec(). When a
57 timer fires, the application sends a QTimerEvent, and the flow of
58 control leaves the event loop until the timer event is processed.
59 This implies that a timer cannot fire while your application is
60 busy doing something else. In other words: the accuracy of timers
61 depends on the granularity of your application.
62
63 In multithreaded applications, you can use the timer mechanism in
64 any thread that has an event loop. To start an event loop from a
65 non-GUI thread, use QThread::exec(). Qt uses the the object's
66 \l{QObject::thread()}{thread affinity} to determine which thread
67 will deliver the QTimerEvent. Because of this, you must start and
68 stop all timers in the object's thread; it is not possible to
69 start timers for objects in another thread.
70
71 The upper limit for the interval value is determined by the number
72 of milliseconds that can be specified in a signed integer
73 (in practice, this is a period of just over 24 days). The accuracy
74 depends on the underlying operating system. Windows 98 has 55
75 millisecond accuracy; other systems that we have tested can handle
76 1 millisecond intervals.
77
78 The main API for the timer functionality is QTimer. That class
79 provides regular timers that emit a signal when the timer fires, and
80 inherits QObject so that it fits well into the ownership structure
81 of most GUI programs. The normal way of using it is like this:
82
83 \snippet doc/src/snippets/timers/timers.cpp 0
84 \snippet doc/src/snippets/timers/timers.cpp 1
85 \snippet doc/src/snippets/timers/timers.cpp 2
86
87 The QTimer object is made into a child of this widget so that,
88 when this widget is deleted, the timer is deleted too.
89 Next, its \l{QTimer::}{timeout()} signal is connected to the slot
90 that will do the work, it is started with a value of 1000
91 milliseconds, indicating that it will time out every second.
92
93 QTimer also provides a static function for single-shot timers.
94 For example:
95
96 \snippet doc/src/snippets/timers/timers.cpp 3
97
98 200 milliseconds (0.2 seconds) after this line of code is
99 executed, the \c updateCaption() slot will be called.
100
101 For QTimer to work, you must have an event loop in your
102 application; that is, you must call QCoreApplication::exec()
103 somewhere. Timer events will be delivered only while the event
104 loop is running.
105
106 In multithreaded applications, you can use QTimer in any thread
107 that has an event loop. To start an event loop from a non-GUI
108 thread, use QThread::exec(). Qt uses the the timer's
109 \l{QObject::thread()}{thread affinity} to determine which thread
110 will emit the \l{QTimer::}{timeout()} signal. Because of this, you
111 must start and stop the timer in its thread; it is not possible to
112 start a timer from another thread.
113
114 The \l{widgets/analogclock}{Analog Clock} example shows how to use
115 QTimer to redraw a widget at regular intervals. From \c{AnalogClock}'s
116 implementation:
117
118 \snippet examples/widgets/analogclock/analogclock.cpp 0
119 \snippet examples/widgets/analogclock/analogclock.cpp 2
120 \snippet examples/widgets/analogclock/analogclock.cpp 3
121 \snippet examples/widgets/analogclock/analogclock.cpp 4
122 \snippet examples/widgets/analogclock/analogclock.cpp 5
123 \snippet examples/widgets/analogclock/analogclock.cpp 6
124 \dots
125 \snippet examples/widgets/analogclock/analogclock.cpp 7
126
127 Every second, QTimer will call the QWidget::update() slot to
128 refresh the clock's display.
129
130 If you already have a QObject subclass and want an easy
131 optimization, you can use QBasicTimer instead of QTimer. With
132 QBasicTimer, you must reimplement
133 \l{QObject::timerEvent()}{timerEvent()} in your QObject subclass
134 and handle the timeout there. The \l{widgets/wiggly}{Wiggly}
135 example shows how to use QBasicTimer.
136*/
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