Create Slot In Qt

In this tutorial we will learn How to use signal and slots in qt. File-New File or Project Applications-Qt Gui Application-Choose We keep the class as MainWindow as given by default. A number of standard slots are provided on Qt classes to allow you to wire together different parts of your application. However, you can also use any Python function as a slot, and therefore receive the message yourself. Load up a fresh copy of `MyAppwindow.py` and save it under a new name for this section.

This article is the most comprehensive description of signals and slots in QML compared to all previous articles on this site.

In this article, I will try to explain the following when working with Qt/QML + Qt/C++:

• ways to declare signals and slots, also called methods in the C ++ class, which will be registered in the QML layer
• ways to connect to signals of classes declared in C ++ as context
• work with Q_PROPERTY, which also requires signals and slots
• ways to connect signals and slots in QML
• etc.

Signals and slots from the C++ class

Let's create our first class that will work with signals and slots in QML. This is one of the very first examples that I have already shown, but I will repeat this example so that the article is as complete as possible.

In this example, I want to create an application that has one button and by pressing this button increases the counter that is inside the C++ class. This C++ class will be registered as a context property in the QML engine of our application.

App appearance will be next

AppCore.h

Declaring signals and slots in C ++ code will not differ much from the classical Qt/C++.

AppCore.cpp

As well as the implementation of the methods themselves.

main.cpp

main.qml

And now the most interesting. How to use an object loaded in a QML context and how to connect to its signals.

As you remember, we loaded the object into the context QML under the name appCore , we will use this object to access it. But to connect to the signal, we will need to use the QML type Connections .

Thus, you can access the object that was loaded into the context of the QML engine, call its slot, and process the signal from this object.

It is also not necessary to declare receiveFromQml() as a slot in this case. This method can also be declared as Q_INVOKABLE method.

Using Q_PROPERTY

The next option is to use the Q_PROPERTY macro. A classic property in Qt might look like this for our task

This property has the following components:

• type of property, as well as its name: int counter , which are bound to the variable int m_counter inside the class, this is the logic of code generation in Qt
• name of the method to read, matches the name of the property: counter
• method name for setting the value: setCounter
• signal that reports property changes: counterChanged

You can also pass additional parameters to this macro, but this is beyond the scope of this article. And also the property can be read only, that is, without a setter.

Now look at the full code using Q_PROPERTY

AppCore.h

AppCore.cpp

main.qml

Here you will see that connecting the property and accessing it has become easier thanks to the declarative style of QML code. Of course, you cannot always use properties, sometimes you just need to use signals, slots, and Q_INVOKABLE methods. But for variables like counter, properties are likely to be much more convenient.

Connecting signals inside QML files

Now consider the option of connecting signals and slots (functions) inside QML files. There will no longer be any C ++ code.

Among other things, you can use and disable signals from slots

Connect a signal to a signal

Also in QML there is still the ability to connect a signal to a signal, as in Qt/C++. Look at the following artificial example.

In this case, the counter will continue to increase when the button is pressed. But the button press signal is not connected directly to the counter increase function, but is forwarded through the signal.

Using Variables in Signals

QML also has the ability to use variables in signals.

Conclusion

For the most part, this entire article fits into several points:

• In C ++, to interact with the QML layer, you can use signals, slots, Q_INVOKABLE methods, as well as create properties using the Q_PROPERTY macro
• In order to respond to signals from objects, you can use the QML type Connections
• Q_PROPERTY obeys the declarative style of QML and, when a property is changed, it can automatically set new values, if the property has been added to any object in QML. In this case, the signal slot connections are set automatically.
• In QML, you can connect and disconnect signal / slot connections using the following syntax:
  • object1.signal.connect (object2.slot)
  • object1.signal.disconnect (object2.slot)
• Signals in QML can also be connected to other signals, as is done in Qt / C ++
• Signals in QML may also have arguments

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

Properties

Public Functions

Public Slots

Signals

Static Public Members

Reimplemented Protected Functions

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):

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:

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:

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:

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:

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:

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:

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:

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(intmsec)

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.

template <typename Functor> QMetaObject::Connection QTimer::callOnTimeout(Functorslot, Qt::ConnectionTypeconnectionType = Qt::AutoConnection)

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().

template <typename Functor> QMetaObject::Connection QTimer::callOnTimeout(const QObject *context, Functorslot, Qt::ConnectionTypeconnectionType = 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).

This function was introduced in Qt 5.12.

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

template <typename MemberFunction> QMetaObject::Connection QTimer::callOnTimeout(const QObject *receiver, MemberFunction *slot, Qt::ConnectionTypeconnectionType = Qt::AutoConnection)

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(intmsec, 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:

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(intmsec, Qt::TimerTypetimerType, 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] template <typename PointerToMemberFunction> void QTimer::singleShot(intmsec, const QObject *receiver, PointerToMemberFunctionmethod)

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] template <typename PointerToMemberFunction> void QTimer::singleShot(intmsec, Qt::TimerTypetimerType, const QObject *receiver, PointerToMemberFunctionmethod)

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().

Create Slot In Qt Download

[static] template <typename Functor> void QTimer::singleShot(intmsec, Functorfunctor)

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] template <typename Functor> void QTimer::singleShot(intmsec, Qt::TimerTypetimerType, Functorfunctor)

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] template <typename Functor, int> void QTimer::singleShot(intmsec, const QObject *context, Functorfunctor)

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] template <typename Functor, int> void QTimer::singleShot(intmsec, Qt::TimerTypetimerType, const QObject *context, Functorfunctor)

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::millisecondsmsec, 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::millisecondsmsec, Qt::TimerTypetimerType, 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::millisecondsmsec)

This is an overloaded function.

Qt Create Slot In Designer

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.

Create Slot In Qt File

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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