timers.qdoc

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
    \page timers.html
    \title Timers
    \brief How to use Qt timers in your application.
 
    \ingroup how-to
 
    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 integral timer ID. The timer will then fire at
    regular intervals until you explicitly call QObject::killTimer()
    with that timer ID.
 
    Instead of handling timer IDs directly, you can use \l QBasicTimer.
    QBasicTimer is a value-class,
    \l{http://en.cppreference.com/w/cpp/language/raii}{RAII} wrapper around
    a timer ID. You start the timer with \l{QBasicTimer::start()}, and stop
    it with \l{QBasicTimer::stop()} (the latter is also called upon destruction).
    To use QBasicTimer, you must reimplement \l{QObject::timerEvent()}{timerEvent()}
    in your class (which must be a sub-class of QObject), and handle the
    timer event there.
 
    For this mechanism to work, the application must run in an event
    loop. You can start an event loop with QCoreApplication::exec(). For
    \l {Qt Widgets} applications, see 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. The accuracy of timers also
    depends on the underlying operating system and hardware. Short intervals may
    be rounded or delayed by the system scheduler, and all timer types may time
    out later than requested if the system is busy. For more information about
    timer resolution and Qt::TimerType, see the QTimer documentation.
 
    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 main API for the timer functionality is \l QTimer. QTimer stores
    the interval in a signed integer, which limits the maximum interval it
    supports to the number of milliseconds that can fit in a signed integer
    (in practice, this is a period of around 24 days).
 
    Qt 6.8 introduced the \l QChronoTimer class. The main difference between
    the two classes is that QChronoTimer supports a larger interval range
    and a higher precision (\c std::chrono::nanoseconds). For QTimer the
    maximum supported interval is ±24 days, whereas for QChronoTimer it is
    ±292 years. If you only need millisecond resolution and ±24 days range,
    you can continue to use \l QTimer. Note that QChronoTimer exists mainly
    because QTimer's precision couldn't be changed to \c std::chrono::nanoseconds
    without breaking binary-compatibility.
 
    QTimer provides regular timers that emit a signal when the timer
    fires, and inherits from QObject so that it fits well into the ownership
    structure of most Qt programs. The normal way of using it is like this:
 
    \snippet timers/timers.cpp timer-interval-in-ctor
    \snippet timers/timers.cpp timer-setinterval
 
    The QTimer object is made into a child of the \c this object so
    that, when \c this is destroyed, the timer is destroyed too. Next, the
    \l{QTimer::}{timeout()} signal is connected to the slot that will
    do the work, the timer interval can be either passed to the constructor,
    or set later on with setInterval().
 
    QTimer also provides static functions for single-shot timers.
    For example:
 
    \snippet timers/timers.cpp qtimer-singleshot
 
    200ms 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 timers/analogclock.cpp analogclock-qtimer
 
    Every second, QTimer will call the QWidget::update() slot to
    refresh the clock's display.
*/