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last modified July 16, 2020
In this part of the Qt5 C++ programming tutorial we talk about events and signals.
Events are an important part in any GUI program. All GUI applications are event-driven. An application reacts to different event types which are generated during its life. Events are generated mainly by the user of an application. But they can be generated by other means as well, e.g. Internet connection, window manager, or a timer. In the event model, there are three participants:
Qt event loop feature overview. Receives and translates GUI events ‒ NSEvents (on Mac) ‒ MSG (on Windows) ‒ BPS events (on Blackberry) ‒ X11 events (using XCB) ‒ Wayland events ‒ Many others. Manages the event queue (priority queue). Allows for event filtering. Integrates with: ‒ Generic Unix select(2) ‒ Glib ‒ Mac. An abstract view of some signals and slots connections In Qt we have an alternative to the callback technique. We use signals and slots. A signal is emitted when a particular event occurs. Qt’s widgets have many pre-defined signals, but we can always subclass to add our own. A slot is a function that is called in reponse to a particular signal. When a signal is emitted and there’s a slot connected to that signal an event is posted in the receiving thread’s event loop that the slot should be invoked. Consequently, when the event is processed the slot will be invoked. So you can see that without event loop there’s no slots!
*event source
*event object
*event target
*The main event loop receives events from the window system and dispatches these to the application widgets. Generally speaking, no user interaction can take place before calling exec. As a special case, modal widgets like QMessageBox can be used before calling exec, because modal widgets use their own local event loop.
*As of Qt4, you can use QThread to start your own event loops. This might sound somewhat uninteresting at first, but it means you can have your own signals and slots outside the main thread. The Trolls created a new way to connect signals to slots such that signals can actually cross thread boundaries. I can now emit a signal in one thread.
The event source is the object whose state changes. It generates Events. The event object (Event) encapsulates the state changes in the event source.The event target is the object that wants to be notified. Event source object delegates the task of handling an event to the event target.
When we call the application’s exec() method, the applicationenters the main loop. The main loop fetches events and sends them to the objects. Qt has a unique signal and slot mechanism. This signal and slot mechanism is an extension to the C++ programming language.
Signals and slots are used for communication between objects. A signal is emitted when a particular event occurs. A slot is a normal C++ method;it is called when a signal connected to it is emitted.Click
The first example shows a very simple event handling example. We have one push button. By clicking on the push button, we terminate the application. click.h
This is the header file.
We display a QPushButton on the window.
The connect() method connects a signal to the slot. When we click on the Quit button, the clicked signal is generated. The qApp is a global pointer to the application object. It is defined in the <QApplication> header file. The quit() method is called when the clicked signal is emitted. main.cpp
This is the main file. KeyPress
In the following example, we react to a key press.
This is the keypress.h header file. keypress.cpp
The application terminates if we press the Escape key.
One of the ways of working with events in Qt5 is to reimplement an event handler. The QKeyEvent is an event object, which holds information about what has happened. In our case, we use the event object to determine which key was actually pressed.
This is the main file. QMoveEvent
The QMoveEvent class contains event parameters for move events.Move events are sent to widgets that have been moved.move.h
This is the move.h header file.
In our code programming example, we react to a move event. We determine the current x, y coordinates of the upper left corner of the client area of the window and set those values to the title of the window.
We use the QMoveEvent object to determine the x, y values.
We convert the integer values to strings.
The setWindowTitle() method sets the text to the title of the window. main.cpp
This is the main file. Disconnecting a signal
A signal can be disconnected from the slot. The next example shows how we can accomplish this. Qt Signals And Slots Without Event Loop Downloaddisconnect.h
In the header file, we have declared two slots. The slots is not a C++ keyword, it is a Qt5 extension. These extensions are handled by the preprocessor, before the code is compiled. When we use signals and slots in our classes, we must provide a Q_OBJECT macro at the beginning of the class definition. Otherwise, the preprocessor would complain.
In our example, we have a button and a check box. The check box connects and disconnects a slot from the buttons clicked signal. This example must be executed from the command line.
Here we connect signals to our user defined slots.
If we click on the Click button, we send the ’Button clicked’ text to the terminal window.
Inside the onCheck() slot, we connect or disconnect the onClick() slot from the Click button, depending on the received state.main.cpp
This is the main file. Timer
A timer is used to implement single shot or repetitive tasks. A good example where we use a timer is a clock; each second we must update our label displaying the current time.
This is the header file. timer.cpp
In our example, we display a current local time on the window.
To display a time, we use a label widget.
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Here we determine the current local time. We set it to the label widget.
Casino millau. We start the timer. Every 1000 ms a timer event is generated.
To work with timer events, we must reimplement the timerEvent() method. Qt Signals And Slots Without Event Loop Free
This is the main file.
This chapter was dedicated to events and signals in Qt5.
I wrote about multi-threading in Qt a while back, and how to use QThread to wrap a blocking function call “lock free”. Today we’ll talk about how to pass data into your thread. This approach can be used, for example, to tell your QThread to stop.
There are two ways to use QThread, with and without an event loop, and the preferred method for talking to a QThread depends on which of them you use.
Way 1: Without an event loop
When you’re not using an event loop, the thread’s run() method often looks like this:
Since this method does not use an event loop, there is no way to deliver a signal to your thread. So if you want to pass data into the thread, you have to use a good old fashioned mutex. Let’s look at an example showing you how to stop your thread:
In this case, we have to check the stopRequested variable in a timely manner in our thread’s run() method. The longer you run between checks, the longer it will take your thread to actually stop.
Outside observers can use the finished() signal to know when your thread is actually done. So if you are in a QMainWindow, for example, and a closeEvent() happens, you can ignore the event, call MyThread::stop(), and then when the QThread::finished() signal arrives, you can actually close the window.
The downside is that the stop() call will actually block while it tries to acquire the mutex. Given the way this code is written, the blocking will probably be very short, but hey, I hate blocking. Let’s see if we can dig up a better way to do this.
Way 2: With an event loop
If you have an event loop, you can use Qt’s meta-objects to talk to your thread. Let’s look at the same example as before, only this time with no locking or blocking.Qt Signals And Slots Without Event Loop Youtube
Look mom! No locks! Now we have killed our thread, safely and gracefully. There is no chance of blocking, and we learned something about QMetaObject.
A couple items to note:
*The doSomething() method is left as an exercise for the reader, but be careful about the QTimer interval. I used 0, which means it will be firing almost constantly.
*The stop() method must be a slot in MyThread (and not just a regular method) or else invokeMethod() will return false and not actually re-invoke stop() for you.
*You can pass arguments to your thread this way, but it requires a bit more fun with QMetaObject::invokeMethod().
*You can reduce this whole thing to a magical macro that you could put at the top of your stop() method, saving you from having to write if(currentThread() this) at the top of every method. Hint: use the __FUNCTION__ macro.
*To run this example code, you’ll need to #include these files: QThread, QTimer, QMetaObject, QMutexLocker, and QMutex
*To call quit(), it may not actually be necessary to be running on the same QThread (it works for me without the QMetaObject), but this will be required when you start passing in data to your thread. Without it, your program could do unpredictable naughty things. I can’t find anything in the docs about whether quit() is thread safe.Qt Signals And Slots Without Event Loops
Fullcalendar slot height. I’ve found this QMetaObject approach the most effective and easiest way to pass data to QThreads safely, without blocking and without locks.
Happy threading!
Register here: http://gg.gg/vahar
https://diarynote.indered.space
last modified July 16, 2020
In this part of the Qt5 C++ programming tutorial we talk about events and signals.
Events are an important part in any GUI program. All GUI applications are event-driven. An application reacts to different event types which are generated during its life. Events are generated mainly by the user of an application. But they can be generated by other means as well, e.g. Internet connection, window manager, or a timer. In the event model, there are three participants:
Qt event loop feature overview. Receives and translates GUI events ‒ NSEvents (on Mac) ‒ MSG (on Windows) ‒ BPS events (on Blackberry) ‒ X11 events (using XCB) ‒ Wayland events ‒ Many others. Manages the event queue (priority queue). Allows for event filtering. Integrates with: ‒ Generic Unix select(2) ‒ Glib ‒ Mac. An abstract view of some signals and slots connections In Qt we have an alternative to the callback technique. We use signals and slots. A signal is emitted when a particular event occurs. Qt’s widgets have many pre-defined signals, but we can always subclass to add our own. A slot is a function that is called in reponse to a particular signal. When a signal is emitted and there’s a slot connected to that signal an event is posted in the receiving thread’s event loop that the slot should be invoked. Consequently, when the event is processed the slot will be invoked. So you can see that without event loop there’s no slots!
*event source
*event object
*event target
*The main event loop receives events from the window system and dispatches these to the application widgets. Generally speaking, no user interaction can take place before calling exec. As a special case, modal widgets like QMessageBox can be used before calling exec, because modal widgets use their own local event loop.
*As of Qt4, you can use QThread to start your own event loops. This might sound somewhat uninteresting at first, but it means you can have your own signals and slots outside the main thread. The Trolls created a new way to connect signals to slots such that signals can actually cross thread boundaries. I can now emit a signal in one thread.
The event source is the object whose state changes. It generates Events. The event object (Event) encapsulates the state changes in the event source.The event target is the object that wants to be notified. Event source object delegates the task of handling an event to the event target.
When we call the application’s exec() method, the applicationenters the main loop. The main loop fetches events and sends them to the objects. Qt has a unique signal and slot mechanism. This signal and slot mechanism is an extension to the C++ programming language.
Signals and slots are used for communication between objects. A signal is emitted when a particular event occurs. A slot is a normal C++ method;it is called when a signal connected to it is emitted.Click
The first example shows a very simple event handling example. We have one push button. By clicking on the push button, we terminate the application. click.h
This is the header file.
We display a QPushButton on the window.
The connect() method connects a signal to the slot. When we click on the Quit button, the clicked signal is generated. The qApp is a global pointer to the application object. It is defined in the <QApplication> header file. The quit() method is called when the clicked signal is emitted. main.cpp
This is the main file. KeyPress
In the following example, we react to a key press.
This is the keypress.h header file. keypress.cpp
The application terminates if we press the Escape key.
One of the ways of working with events in Qt5 is to reimplement an event handler. The QKeyEvent is an event object, which holds information about what has happened. In our case, we use the event object to determine which key was actually pressed.
This is the main file. QMoveEvent
The QMoveEvent class contains event parameters for move events.Move events are sent to widgets that have been moved.move.h
This is the move.h header file.
In our code programming example, we react to a move event. We determine the current x, y coordinates of the upper left corner of the client area of the window and set those values to the title of the window.
We use the QMoveEvent object to determine the x, y values.
We convert the integer values to strings.
The setWindowTitle() method sets the text to the title of the window. main.cpp
This is the main file. Disconnecting a signal
A signal can be disconnected from the slot. The next example shows how we can accomplish this. Qt Signals And Slots Without Event Loop Downloaddisconnect.h
In the header file, we have declared two slots. The slots is not a C++ keyword, it is a Qt5 extension. These extensions are handled by the preprocessor, before the code is compiled. When we use signals and slots in our classes, we must provide a Q_OBJECT macro at the beginning of the class definition. Otherwise, the preprocessor would complain.
In our example, we have a button and a check box. The check box connects and disconnects a slot from the buttons clicked signal. This example must be executed from the command line.
Here we connect signals to our user defined slots.
If we click on the Click button, we send the ’Button clicked’ text to the terminal window.
Inside the onCheck() slot, we connect or disconnect the onClick() slot from the Click button, depending on the received state.main.cpp
This is the main file. Timer
A timer is used to implement single shot or repetitive tasks. A good example where we use a timer is a clock; each second we must update our label displaying the current time.
This is the header file. timer.cpp
In our example, we display a current local time on the window.
To display a time, we use a label widget.
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Here we determine the current local time. We set it to the label widget.
Casino millau. We start the timer. Every 1000 ms a timer event is generated.
To work with timer events, we must reimplement the timerEvent() method. Qt Signals And Slots Without Event Loop Free
This is the main file.
This chapter was dedicated to events and signals in Qt5.
I wrote about multi-threading in Qt a while back, and how to use QThread to wrap a blocking function call “lock free”. Today we’ll talk about how to pass data into your thread. This approach can be used, for example, to tell your QThread to stop.
There are two ways to use QThread, with and without an event loop, and the preferred method for talking to a QThread depends on which of them you use.
Way 1: Without an event loop
When you’re not using an event loop, the thread’s run() method often looks like this:
Since this method does not use an event loop, there is no way to deliver a signal to your thread. So if you want to pass data into the thread, you have to use a good old fashioned mutex. Let’s look at an example showing you how to stop your thread:
In this case, we have to check the stopRequested variable in a timely manner in our thread’s run() method. The longer you run between checks, the longer it will take your thread to actually stop.
Outside observers can use the finished() signal to know when your thread is actually done. So if you are in a QMainWindow, for example, and a closeEvent() happens, you can ignore the event, call MyThread::stop(), and then when the QThread::finished() signal arrives, you can actually close the window.
The downside is that the stop() call will actually block while it tries to acquire the mutex. Given the way this code is written, the blocking will probably be very short, but hey, I hate blocking. Let’s see if we can dig up a better way to do this.
Way 2: With an event loop
If you have an event loop, you can use Qt’s meta-objects to talk to your thread. Let’s look at the same example as before, only this time with no locking or blocking.Qt Signals And Slots Without Event Loop Youtube
Look mom! No locks! Now we have killed our thread, safely and gracefully. There is no chance of blocking, and we learned something about QMetaObject.
A couple items to note:
*The doSomething() method is left as an exercise for the reader, but be careful about the QTimer interval. I used 0, which means it will be firing almost constantly.
*The stop() method must be a slot in MyThread (and not just a regular method) or else invokeMethod() will return false and not actually re-invoke stop() for you.
*You can pass arguments to your thread this way, but it requires a bit more fun with QMetaObject::invokeMethod().
*You can reduce this whole thing to a magical macro that you could put at the top of your stop() method, saving you from having to write if(currentThread() this) at the top of every method. Hint: use the __FUNCTION__ macro.
*To run this example code, you’ll need to #include these files: QThread, QTimer, QMetaObject, QMutexLocker, and QMutex
*To call quit(), it may not actually be necessary to be running on the same QThread (it works for me without the QMetaObject), but this will be required when you start passing in data to your thread. Without it, your program could do unpredictable naughty things. I can’t find anything in the docs about whether quit() is thread safe.Qt Signals And Slots Without Event Loops
Fullcalendar slot height. I’ve found this QMetaObject approach the most effective and easiest way to pass data to QThreads safely, without blocking and without locks.
Happy threading!
Register here: http://gg.gg/vahar
https://diarynote.indered.space
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