QTimer Class
The QTimer class provides repetitive and single-shot timers. More...
| Header: | #include <QTimer> |
| CMake: | find_package(Qt6 REQUIRED COMPONENTS Core) target_link_libraries(mytarget PRIVATE Qt6::Core) |
| qmake: | QT += core |
| Inherits: | QObject |
- List of all members, including inherited members
- QTimer is part of Event Classes.
Properties
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Public Functions
| QTimer(QObject *parent = nullptr) | |
| virtual | ~QTimer() |
| QBindable<bool> | bindableActive() |
| QBindable<int> | bindableInterval() |
| QBindable<bool> | bindableSingleShot() |
| QBindable<Qt::TimerType> | bindableTimerType() |
| QMetaObject::Connection | callOnTimeout(Functor &&slot) |
| QMetaObject::Connection | callOnTimeout(const QObject *context, Functor &&slot, Qt::ConnectionType connectionType = Qt::AutoConnection) |
| 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
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(Duration msec, const QObject *context, Functor &&functor) |
| void | singleShot(Duration msec, Qt::TimerType timerType, const QObject *context, Functor &&functor) |
| void | singleShot(Duration msec, Functor &&functor) |
| void | singleShot(Duration msec, Qt::TimerType timerType, 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, &Foo::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, &QTimer::timeout, this, &Foo::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, and Analog Clock.
Property Documentation
[bindable read-only] active : bool
Note: This property supports QProperty bindings.
This boolean property is true if the timer is running; otherwise false.
[bindable] interval : int
Note: This property supports QProperty bindings.
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().
See also singleShot.
[read-only] 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.
Access functions:
| int | remainingTime() const |
See also interval.
[bindable] singleShot : bool
Note: This property supports QProperty bindings.
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.
See also interval and singleShot().
[bindable] timerType : Qt::TimerType
Note: This property supports QProperty bindings.
controls the accuracy of the timer
The default value for this property is Qt::CoarseTimer.
See also Qt::TimerType.
Member Function Documentation
[static] template <typename Duration, typename Functor> void QTimer::singleShot(Duration msec, Functor &&functor)
[static] template <typename Duration, typename Functor> void QTimer::singleShot(Duration msec, Qt::TimerType timerType, Functor &&functor)
[static] template <typename Duration, typename Functor> void QTimer::singleShot(Duration msec, Qt::TimerType timerType, const QObject *context, Functor &&functor)
[static] template <typename Duration, typename Functor> void QTimer::singleShot(Duration msec, const QObject *context, Functor &&functor)
This static function calls functor after msec milliseconds.
It is very convenient to use this function because you do not need to bother with a timerEvent or create a local QTimer object.
If context is specified, then the functor will be called only if the context object has not been destroyed before the interval occurs. The functor will then be run the thread of context. The context's thread must have a running Qt event loop.
If functor is a member function of context, then the function will be called on the object.
The msec parameter can be an int or a std::chrono::milliseconds value.
Note: This function is reentrant.
See also start().
[explicit] QTimer::QTimer(QObject *parent = nullptr)
Constructs a timer with the given parent.
[virtual noexcept] QTimer::~QTimer()
Destroys the timer.
template <typename Functor> QMetaObject::Connection QTimer::callOnTimeout(Functor &&slot)
Creates a connection from the timer's timeout() signal to slot. Returns a handle to the connection.
This method is provided for convenience. It's equivalent to calling:
QObject::connect(timer, &QTimer::timeout, timer, slot, Qt::DirectConnection);
Note: This overload is not available when QT_NO_CONTEXTLESS_CONNECT is defined, instead use the callOnTimeout() overload that takes a context object.
See also QObject::connect() and timeout().
template <typename Functor> QMetaObject::Connection QTimer::callOnTimeout(const QObject *context, Functor &&slot, Qt::ConnectionType connectionType = Qt::AutoConnection)
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);
See also QObject::connect() and timeout().
std::chrono::milliseconds QTimer::intervalAsDuration() const
Returns the interval of this timer as a std::chrono::milliseconds object.
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.
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, QCoreApplication::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(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.
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.
See also start().
[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. This is equivalent to:
timer.setInterval(msec); timer.start();
Note: Keeping the event loop busy with a zero-timer is bound to cause trouble and highly erratic behavior of the UI.
[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.
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 is equivalent to:
timer.setInterval(msec); timer.start();
[slot] void QTimer::stop()
Stops the timer.
See also start().
[private 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().
[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.