source: trunk/doc/src/howtos/timers.qdoc@ 564

/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation ([email protected])
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial Usage
** Licensees holding valid Qt Commercial licenses may use this file in
** accordance with the Qt Commercial License Agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and Nokia.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file.  Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Nokia gives you certain additional
** rights.  These rights are described in the Nokia Qt LGPL Exception
** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 3.0 as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL included in the
** packaging of this file.  Please review the following information to
** ensure the GNU General Public License version 3.0 requirements will be
** met: http://www.gnu.org/copyleft/gpl.html.
**
** If you have questions regarding the use of this file, please contact
** Nokia at [email protected].
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \page timers.html
    \title Timers
    \brief How to use timers in your application.

    \ingroup best-practices

    QObject, the base class of all Qt objects, provides the basic
    timer support in Qt. With QObject::startTimer(), 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 QObject::killTimer()
    with the timer ID.

    For this mechanism to work, the application must run in an event
    loop. You start an event loop with QApplication::exec(). When a
    timer fires, the application sends a QTimerEvent, 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.

    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 QThread::exec(). Qt uses the object's
    \l{QObject::thread()}{thread affinity} to determine which thread
    will deliver the QTimerEvent. 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.

    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 98 has 55
    millisecond accuracy; other systems that we have tested can handle
    1 millisecond intervals.

    The main API for the timer functionality is QTimer. That class
    provides regular timers that emit a signal when the timer fires, and
    inherits QObject so that it fits well into the ownership structure
    of most GUI programs. The normal way of using it is like this:

    \snippet doc/src/snippets/timers/timers.cpp 0
    \snippet doc/src/snippets/timers/timers.cpp 1
    \snippet doc/src/snippets/timers/timers.cpp 2

    The QTimer object is made into a child of this widget so that,
    when this widget is deleted, the timer is deleted too.
    Next, its \l{QTimer::}{timeout()} 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.

    QTimer also provides a static function for single-shot timers.
    For example:

    \snippet doc/src/snippets/timers/timers.cpp 3

    200 milliseconds (0.2 seconds) after this line of code is
    executed, the \c updateCaption() slot will be called.

    For QTimer to work, you must have an event loop in your
    application; that is, you must call QCoreApplication::exec()
    somewhere. Timer events will be delivered only while the event
    loop is running.

    In multithreaded applications, you can use QTimer in any thread
    that has an event loop. To start an event loop from a non-GUI
    thread, use QThread::exec(). Qt uses the timer's
    \l{QObject::thread()}{thread affinity} to determine which thread
    will emit the \l{QTimer::}{timeout()} 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.

    The \l{widgets/analogclock}{Analog Clock} example shows how to use
    QTimer to redraw a widget at regular intervals. From \c{AnalogClock}'s
    implementation:

    \snippet examples/widgets/analogclock/analogclock.cpp 0
    \snippet examples/widgets/analogclock/analogclock.cpp 2
    \snippet examples/widgets/analogclock/analogclock.cpp 3
    \snippet examples/widgets/analogclock/analogclock.cpp 4
    \snippet examples/widgets/analogclock/analogclock.cpp 5
    \snippet examples/widgets/analogclock/analogclock.cpp 6
    \dots
    \snippet examples/widgets/analogclock/analogclock.cpp 7

    Every second, QTimer will call the QWidget::update() slot to
    refresh the clock's display.

    If you already have a QObject subclass and want an easy
    optimization, you can use QBasicTimer instead of QTimer. With
    QBasicTimer, you must reimplement
    \l{QObject::timerEvent()}{timerEvent()} in your QObject subclass
    and handle the timeout there. The \l{widgets/wiggly}{Wiggly}
    example shows how to use QBasicTimer.
*/