Qt Documentation

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QTimer Class

The QTimer class provides repetitive and single-shot timers. More...

Header: #include <QTimer>
qmake: QT += core
Inherits: QObject

Properties

Public Functions

QTimer(QObject *parent = nullptr)
virtual ~QTimer()
QMetaObject::Connection callOnTimeout(Functor slot, Qt::ConnectionType connectionType = ...)
QMetaObject::Connection callOnTimeout(const QObject *context, Functor slot, Qt::ConnectionType connectionType = ...)
QMetaObject::Connection callOnTimeout(const QObject *receiver, MemberFunction *slot, Qt::ConnectionType connectionType = ...)
int interval() const
std::chrono::milliseconds intervalAsDuration() const
bool isActive() const
bool isSingleShot() const
int remainingTime() const
std::chrono::milliseconds remainingTimeAsDuration() const
void setInterval(int msec)
void setInterval(std::chrono::milliseconds value)
void setSingleShot(bool singleShot)
void setTimerType(Qt::TimerType atype)
void start(std::chrono::milliseconds msec)
int timerId() const
Qt::TimerType timerType() const

Public Slots

void start()
void start(int msec)
void stop()

Signals

void timeout()

Static Public Members

void singleShot(int msec, const QObject *receiver, const char *member)
void singleShot(int msec, Qt::TimerType timerType, const QObject *receiver, const char *member)
void singleShot(int msec, const QObject *receiver, PointerToMemberFunction method)
void singleShot(int msec, Qt::TimerType timerType, const QObject *receiver, PointerToMemberFunction method)
void singleShot(int msec, Functor functor)
void singleShot(int msec, Qt::TimerType timerType, Functor functor)
void singleShot(int msec, const QObject *context, Functor functor)
void singleShot(int msec, Qt::TimerType timerType, const QObject *context, Functor functor)
void singleShot(std::chrono::milliseconds msec, const QObject *receiver, const char *member)
void singleShot(std::chrono::milliseconds msec, Qt::TimerType timerType, const QObject *receiver, const char *member)

Reimplemented Protected Functions

virtual void timerEvent(QTimerEvent *e) override

Detailed Description

The QTimer class provides a high-level programming interface for timers. To use it, create a QTimer, connect its timeout() signal to the appropriate slots, and call start(). From then on, it will emit the timeout() signal at constant intervals.

Example for a one second (1000 millisecond) timer (from the Analog Clock example):

    QTimer *timer = new QTimer(this);
    connect(timer, &QTimer::timeout, this, QOverload<>::of(&AnalogClock::update));
    timer->start(1000);

From then on, the update() slot is called every second.

You can set a timer to time out only once by calling setSingleShot(true). You can also use the static QTimer::singleShot() function to call a slot after a specified interval:

    QTimer::singleShot(200, this, SLOT(updateCaption()));

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 thread affinity to determine which thread will emit the 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.

As a special case, a QTimer with a timeout of 0 will time out as soon as possible, though the ordering between zero timers and other sources of events is unspecified. Zero timers can be used to do some work while still providing a snappy user interface:

    QTimer *timer = new QTimer(this);
    connect(timer, SIGNAL(timeout()), this, SLOT(processOneThing()));
    timer->start();

From then on, processOneThing() will be called repeatedly. It should be written in such a way that it always returns quickly (typically after processing one data item) so that Qt can deliver events to the user interface and stop the timer as soon as it has done all its work. This is the traditional way of implementing heavy work in GUI applications, but as multithreading is nowadays becoming available on more and more platforms, we expect that zero-millisecond QTimer objects will gradually be replaced by QThreads.

Accuracy and Timer Resolution

The accuracy of timers depends on the underlying operating system and hardware. Most platforms support a resolution of 1 millisecond, though the accuracy of the timer will not equal this resolution in many real-world situations.

The accuracy also depends on the timer type. For Qt::PreciseTimer, QTimer will try to keep the accuracy at 1 millisecond. Precise timers will also never time out earlier than expected.

For Qt::CoarseTimer and Qt::VeryCoarseTimer types, QTimer may wake up earlier than expected, within the margins for those types: 5% of the interval for Qt::CoarseTimer and 500 ms for Qt::VeryCoarseTimer.

All timer types may time out later than expected if the system is busy or unable to provide the requested accuracy. In such a case of timeout overrun, Qt will emit timeout() only once, even if multiple timeouts have expired, and then will resume the original interval.

Alternatives to QTimer

An alternative to using QTimer is to call QObject::startTimer() for your object and reimplement the QObject::timerEvent() event handler in your class (which must inherit QObject). The disadvantage is that timerEvent() does not support such high-level features as single-shot timers or signals.

Another alternative is QBasicTimer. It is typically less cumbersome than using QObject::startTimer() directly. See Timers for an overview of all three approaches.

Some operating systems limit the number of timers that may be used; Qt tries to work around these limitations.

See also QBasicTimer, QTimerEvent, QObject::timerEvent(), Timers, Analog Clock Example, and Wiggly Example.

Property Documentation

active : const bool

This boolean property is true if the timer is running; otherwise false.

This property was introduced in Qt 4.3.

Access functions:

bool isActive() const

interval : int

This property holds the timeout interval in milliseconds

The default value for this property is 0. A QTimer with a timeout interval of 0 will time out as soon as all the events in the window system's event queue have been processed.

Setting the interval of an active timer changes its timerId().

Access functions:

int interval() const
void setInterval(int msec)
void setInterval(std::chrono::milliseconds value)

See also singleShot.

remainingTime : const int

This property holds the remaining time in milliseconds

Returns the timer's remaining value in milliseconds left until the timeout. If the timer is inactive, the returned value will be -1. If the timer is overdue, the returned value will be 0.

This property was introduced in Qt 5.0.

Access functions:

int remainingTime() const

See also interval.

singleShot : bool

This property holds whether the timer is a single-shot timer

A single-shot timer fires only once, non-single-shot timers fire every interval milliseconds.

The default value for this property is false.

Access functions:

bool isSingleShot() const
void setSingleShot(bool singleShot)

See also interval and singleShot().

timerType : Qt::TimerType

controls the accuracy of the timer

The default value for this property is Qt::CoarseTimer.

Access functions:

Qt::TimerType timerType() const
void setTimerType(Qt::TimerType atype)

See also Qt::TimerType.

Member Function Documentation

QTimer::QTimer(QObject *parent = nullptr)

Constructs a timer with the given parent.

[slot] void QTimer::start()

This function overloads start().

Starts or restarts the timer with the timeout specified in interval.

If the timer is already running, it will be stopped and restarted.

If singleShot is true, the timer will be activated only once.

[slot] void QTimer::start(int msec)

Starts or restarts the timer with a timeout interval of msec milliseconds.

If the timer is already running, it will be stopped and restarted.

If singleShot is true, the timer will be activated only once.

[slot] void QTimer::stop()

Stops the timer.

See also start().

[signal] void QTimer::timeout()

This signal is emitted when the timer times out.

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

See also interval, start(), and stop().

[virtual] QTimer::~QTimer()

Destroys the timer.

QMetaObject::Connection QTimer::callOnTimeout(Functor slot, Qt::ConnectionType connectionType = ...)

This is an overloaded function.

Creates a connection of type connectionType from the timeout() signal to slot, and returns a handle to the connection.

This method is provided for convenience. It's equivalent to calling QObject::connect(timer, &QTimer::timeout, timer, slot, connectionType).

This function was introduced in Qt 5.12.

See also QObject::connect() and timeout().

QMetaObject::Connection QTimer::callOnTimeout(const QObject *context, Functor slot, Qt::ConnectionType connectionType = ...)

This function overloads callOnTimeout().

Creates a connection from the timeout() signal to slot to be placed in a specific event loop of context, and returns a handle to the connection.

This method is provided for convenience. It's equivalent to calling QObject::connect(timer, &QTimer::timeout, context, slot, connectionType).

This function was introduced in Qt 5.12.

See also QObject::connect() and timeout().

QMetaObject::Connection QTimer::callOnTimeout(const QObject *receiver, MemberFunction *slot, Qt::ConnectionType connectionType = ...)

This function overloads callOnTimeout().

Creates a connection from the timeout() signal to the slot in the receiver object. Returns a handle to the connection.

This method is provided for convenience. It's equivalent to calling QObject::connect(timer, &QTimer::timeout, receiver, slot, connectionType).

This function was introduced in Qt 5.12.

See also QObject::connect() and timeout().

std::chrono::milliseconds QTimer::intervalAsDuration() const

Returns the interval of this timer as a std::chrono::milliseconds object.

This function was introduced in Qt 5.8.

See also interval.

bool QTimer::isActive() const

Returns true if the timer is running (pending); otherwise returns false.

Note: Getter function for property active.

std::chrono::milliseconds QTimer::remainingTimeAsDuration() const

Returns the time remaining in this timer object as a std::chrono::milliseconds object. If this timer is due or overdue, the returned value is std::chrono::milliseconds::zero(). If the remaining time could not be found or the timer is not active, this function returns a negative duration.

This function was introduced in Qt 5.8.

See also remainingTime().

[static] void QTimer::singleShot(int msec, const QObject *receiver, const char *member)

This static function calls a slot after a given time interval.

It is very convenient to use this function because you do not need to bother with a timerEvent or create a local QTimer object.

Example:

#include <QApplication>
#include <QTimer>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QTimer::singleShot(600000, &app, SLOT(quit()));
    ...
    return app.exec();
}

This sample program automatically terminates after 10 minutes (600,000 milliseconds).

The receiver is the receiving object and the member is the slot. The time interval is msec milliseconds.

Note: This function is reentrant.

See also setSingleShot() and start().

[static] void QTimer::singleShot(int msec, Qt::TimerType timerType, const QObject *receiver, const char *member)

This is an overloaded function.

This static function calls a slot after a given time interval.

It is very convenient to use this function because you do not need to bother with a timerEvent or create a local QTimer object.

The receiver is the receiving object and the member is the slot. The time interval is msec milliseconds. The timerType affects the accuracy of the timer.

Note: This function is reentrant.

See also start().

[static] void QTimer::singleShot(int msec, const QObject *receiver, PointerToMemberFunction method)

This is an overloaded function.

This static function calls a member function of a QObject after a given time interval.

It is very convenient to use this function because you do not need to bother with a timerEvent or create a local QTimer object.

The receiver is the receiving object and the method is the member function. The time interval is msec milliseconds.

If receiver is destroyed before the interval occurs, the method will not be called. The function will be run in the thread of receiver. The receiver's thread must have a running Qt event loop.

Note: This function is reentrant.

This function was introduced in Qt 5.4.

See also start().

[static] void QTimer::singleShot(int msec, Qt::TimerType timerType, const QObject *receiver, PointerToMemberFunction method)

This is an overloaded function.

This static function calls a member function of a QObject after a given time interval.

It is very convenient to use this function because you do not need to bother with a timerEvent or create a local QTimer object.

The receiver is the receiving object and the method is the member function. The time interval is msec milliseconds. The timerType affects the accuracy of the timer.

If receiver is destroyed before the interval occurs, the method will not be called. The function will be run in the thread of receiver. The receiver's thread must have a running Qt event loop.

Note: This function is reentrant.

This function was introduced in Qt 5.4.

See also start().

[static] void QTimer::singleShot(int msec, Functor functor)

This is an overloaded function.

This static function calls functor after a given time interval.

It is very convenient to use this function because you do not need to bother with a timerEvent or create a local QTimer object.

The time interval is msec milliseconds.

Note: This function is reentrant.

This function was introduced in Qt 5.4.

See also start().

[static] void QTimer::singleShot(int msec, Qt::TimerType timerType, Functor functor)

This is an overloaded function.

This static function calls functor after a given time interval.

It is very convenient to use this function because you do not need to bother with a timerEvent or create a local QTimer object.

The time interval is msec milliseconds. The timerType affects the accuracy of the timer.

Note: This function is reentrant.

This function was introduced in Qt 5.4.

See also start().

[static] void QTimer::singleShot(int msec, const QObject *context, Functor functor)

This is an overloaded function.

This static function calls functor after a given time interval.

It is very convenient to use this function because you do not need to bother with a timerEvent or create a local QTimer object.

The time interval is msec milliseconds.

If context is destroyed before the interval occurs, the method will not be called. The function will be run in the thread of context. The context's thread must have a running Qt event loop.

Note: This function is reentrant.

This function was introduced in Qt 5.4.

See also start().

[static] void QTimer::singleShot(int msec, Qt::TimerType timerType, const QObject *context, Functor functor)

This is an overloaded function.

This static function calls functor after a given time interval.

It is very convenient to use this function because you do not need to bother with a timerEvent or create a local QTimer object.

The time interval is msec milliseconds. The timerType affects the accuracy of the timer.

If context is destroyed before the interval occurs, the method will not be called. The function will be run in the thread of context. The context's thread must have a running Qt event loop.

Note: This function is reentrant.

This function was introduced in Qt 5.4.

See also start().

[static] void QTimer::singleShot(std::chrono::milliseconds msec, const QObject *receiver, const char *member)

This is an overloaded function.

This static function calls a slot after a given time interval.

It is very convenient to use this function because you do not need to bother with a timerEvent or create a local QTimer object.

The receiver is the receiving object and the member is the slot. The time interval is given in the duration object msec.

Note: This function is reentrant.

This function was introduced in Qt 5.8.

See also start().

[static] void QTimer::singleShot(std::chrono::milliseconds msec, Qt::TimerType timerType, const QObject *receiver, const char *member)

This is an overloaded function.

This static function calls a slot after a given time interval.

It is very convenient to use this function because you do not need to bother with a timerEvent or create a local QTimer object.

The receiver is the receiving object and the member is the slot. The time interval is given in the duration object msec. The timerType affects the accuracy of the timer.

Note: This function is reentrant.

This function was introduced in Qt 5.8.

See also start().

void QTimer::start(std::chrono::milliseconds msec)

This is an overloaded function.

Starts or restarts the timer with a timeout of duration msec milliseconds.

If the timer is already running, it will be stopped and restarted.

If singleShot is true, the timer will be activated only once.

This function was introduced in Qt 5.8.

[override virtual protected] void QTimer::timerEvent(QTimerEvent *e)

Reimplements: QObject::timerEvent(QTimerEvent *event).

int QTimer::timerId() const

Returns the ID of the timer if the timer is running; otherwise returns -1.

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