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1 /**************************************************************************** |
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2 ** |
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3 ** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies). |
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37 ** |
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38 ** $QT_END_LICENSE$ |
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39 ** |
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40 ****************************************************************************/ |
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41 |
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42 /*! |
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43 \page timers.html |
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44 \title Timers |
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45 \brief How to use timers in your application. |
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46 |
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47 \ingroup best-practices |
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48 |
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49 QObject, the base class of all Qt objects, provides the basic |
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50 timer support in Qt. With QObject::startTimer(), you start a |
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51 timer with an interval in milliseconds as argument. The function |
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52 returns a unique integer timer ID. The timer will now fire at |
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53 regular intervals until you explicitly call QObject::killTimer() |
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54 with the timer ID. |
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55 |
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56 For this mechanism to work, the application must run in an event |
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57 loop. You start an event loop with QApplication::exec(). When a |
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58 timer fires, the application sends a QTimerEvent, and the flow of |
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59 control leaves the event loop until the timer event is processed. |
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60 This implies that a timer cannot fire while your application is |
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61 busy doing something else. In other words: the accuracy of timers |
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62 depends on the granularity of your application. |
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63 |
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64 In multithreaded applications, you can use the timer mechanism in |
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65 any thread that has an event loop. To start an event loop from a |
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66 non-GUI thread, use QThread::exec(). Qt uses the object's |
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67 \l{QObject::thread()}{thread affinity} to determine which thread |
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68 will deliver the QTimerEvent. Because of this, you must start and |
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69 stop all timers in the object's thread; it is not possible to |
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70 start timers for objects in another thread. |
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71 |
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72 The upper limit for the interval value is determined by the number |
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73 of milliseconds that can be specified in a signed integer |
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74 (in practice, this is a period of just over 24 days). The accuracy |
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75 depends on the underlying operating system. Windows 98 has 55 |
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76 millisecond accuracy; other systems that we have tested can handle |
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77 1 millisecond intervals. |
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78 |
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79 The main API for the timer functionality is QTimer. That class |
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80 provides regular timers that emit a signal when the timer fires, and |
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81 inherits QObject so that it fits well into the ownership structure |
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82 of most GUI programs. The normal way of using it is like this: |
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83 |
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84 \snippet doc/src/snippets/timers/timers.cpp 0 |
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85 \snippet doc/src/snippets/timers/timers.cpp 1 |
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86 \snippet doc/src/snippets/timers/timers.cpp 2 |
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87 |
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88 The QTimer object is made into a child of this widget so that, |
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89 when this widget is deleted, the timer is deleted too. |
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90 Next, its \l{QTimer::}{timeout()} signal is connected to the slot |
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91 that will do the work, it is started with a value of 1000 |
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92 milliseconds, indicating that it will time out every second. |
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93 |
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94 QTimer also provides a static function for single-shot timers. |
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95 For example: |
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96 |
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97 \snippet doc/src/snippets/timers/timers.cpp 3 |
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98 |
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99 200 milliseconds (0.2 seconds) after this line of code is |
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100 executed, the \c updateCaption() slot will be called. |
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101 |
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102 For QTimer to work, you must have an event loop in your |
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103 application; that is, you must call QCoreApplication::exec() |
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104 somewhere. Timer events will be delivered only while the event |
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105 loop is running. |
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106 |
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107 In multithreaded applications, you can use QTimer in any thread |
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108 that has an event loop. To start an event loop from a non-GUI |
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109 thread, use QThread::exec(). Qt uses the timer's |
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110 \l{QObject::thread()}{thread affinity} to determine which thread |
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111 will emit the \l{QTimer::}{timeout()} signal. Because of this, you |
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112 must start and stop the timer in its thread; it is not possible to |
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113 start a timer from another thread. |
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114 |
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115 The \l{widgets/analogclock}{Analog Clock} example shows how to use |
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116 QTimer to redraw a widget at regular intervals. From \c{AnalogClock}'s |
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117 implementation: |
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118 |
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119 \snippet examples/widgets/analogclock/analogclock.cpp 0 |
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120 \snippet examples/widgets/analogclock/analogclock.cpp 2 |
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121 \snippet examples/widgets/analogclock/analogclock.cpp 3 |
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122 \snippet examples/widgets/analogclock/analogclock.cpp 4 |
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123 \snippet examples/widgets/analogclock/analogclock.cpp 5 |
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124 \snippet examples/widgets/analogclock/analogclock.cpp 6 |
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125 \dots |
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126 \snippet examples/widgets/analogclock/analogclock.cpp 7 |
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127 |
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128 Every second, QTimer will call the QWidget::update() slot to |
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129 refresh the clock's display. |
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130 |
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131 If you already have a QObject subclass and want an easy |
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132 optimization, you can use QBasicTimer instead of QTimer. With |
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133 QBasicTimer, you must reimplement |
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134 \l{QObject::timerEvent()}{timerEvent()} in your QObject subclass |
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135 and handle the timeout there. The \l{widgets/wiggly}{Wiggly} |
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136 example shows how to use QBasicTimer. |
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137 */ |