C Signal Slot
- PyQt Tutorial
- PyQt Useful Resources
- Download C Signal/Slot Library (sigslot) for free. Portable C type-safe, thread-safe signal/slot library for ISO C, Unix/BSD/Linux and Win32. Sigslot allows C code to use the signal/slot paradigm made popular by, for example, Qt.
- Qml Signal To C Slot This program demonstrates how QML and C can be connected through Qt signalsand slots. It does this through embedding C code as a context property in QMLrather than explicitly connecting signals and slots. When the program is started, the C part send a signal to QML, including aparameter.
간단한 신호/슬롯 시스템을 C로 만들지는 않으나 매개 변수와 함께 사용하려고하면 몇 가지 문제가 있습니다. 내 신호 클래스: 이 template class Signal private: typedef std::function Slot; public.
- Selected Reading
Unlike a console mode application, which is executed in a sequential manner, a GUI based application is event driven. Functions or methods are executed in response to user’s actions like clicking on a button, selecting an item from a collection or a mouse click etc., called events.
Widgets used to build the GUI interface act as the source of such events. Each PyQt widget, which is derived from QObject class, is designed to emit ‘signal’ in response to one or more events. The signal on its own does not perform any action. Instead, it is ‘connected’ to a ‘slot’. The slot can be any callable Python function.
In PyQt, connection between a signal and a slot can be achieved in different ways. Following are most commonly used techniques −
A more convenient way to call a slot_function, when a signal is emitted by a widget is as follows −
Suppose if a function is to be called when a button is clicked. Here, the clicked signal is to be connected to a callable function. It can be achieved in any of the following two techniques −
or
Example
In the following example, two QPushButton objects (b1 and b2) are added in QDialog window. We want to call functions b1_clicked() and b2_clicked() on clicking b1 and b2 respectively.
When b1 is clicked, the clicked() signal is connected to b1_clicked() function
When b2 is clicked, the clicked() signal is connected to b2_clicked() function
Example
The above code produces the following output −
Output
Earlier this week, I posted an example of integrating QML2 and C++. In it I showed how to call a C++ method from QML, but finished my post with this statement.
I’m still new to Qt, so this may not be the best way. It looks like you can also use signals, which would probably be better as it means your QML application isn’t tied to your C++ implementation, but I haven’t yet got that working.
I have now found a way to use signals and slots to do this, which I will describe here.
Signals and Slots
Signals and Slots are a feature of Qt used for communication between objects. When something happens to an object, it can emit a signal. Zero or more objects can listen for this signal using a slot, and act on it. The signal doesn’t know if anything is listening to it, and the slot doesn’t know what object called it.
This allows you to design and build a loosely coupled application, giving you the flexibility to change, add or remove features of one component without updating all its dependencies, so long as you continue to emit the same signals and listen on the same slots.
You can see why this might be useful in GUI programming. When a user enters some input, you may want to do a number of things with it. Maybe you want to update the GUI with a progress bar, then kick off a function to handle this input. This function might emit a signal letting others know its progress, which the progress bar could listen to and update the GUI. And so on.
Even outside of GUI programming this could be useful. You might have an object watching the filesystem for changes. When a change happens, you could emit a signal to let other objects know about this change. One object might run a process against this file, while another object updates a cache of the filesystem.
The example application
I’m going to create the same example application as I did before. It will contain a text field and a button. You enter some text in the text field, and once you click the button the text will be converted to upper-case. The conversion to upper-case will be done in C++, and the interface drawn in QML2.
The source of the finished application is available on GitHub.
Emitting a signal from QML and listening to it from C++
To create a signal in QML, simply add the following line to the object which will emit the signal.
Here I have created a signal, submitTextField, which will pass a string as an argument to any connecting slots (if they choose to receive it).
I’m going to add this signal to the Window. I will emit the signal when the button is pressed, passing the value of the text field. Here is the full QML document.
We can run that and click the button. The signal is being emitted, but because no slots are listening to it nothing happens.
Let’s create a C++ class to listen to this signal. I’m going to call it HandleTextField, and it will have a slot called handleSubmitTextField. The header file looks like this.
The class file has a simple implementation for handleSubmitTextField.
To connect the QML signal to the C++ slot, we use QObject::connect. Add the following to main.cpp.
We need an instance of HandleTextField, and the QML Window object. Then we can connect the windows submitTextField signal to the handleSubmitTextField slot. Running the application now and you should get a debug message showing the text being passed to C++.
Emitting a signal from C++ and listening to it from QML
Now we want to convert the string to upper-case and display it in the text field. Lets create a signal in our C++ class by adding the following to the header file.
Then change the handleSubmitTextField function to emit this signal with the upper-cased text.
Notice we are passing the text as a QVariant. This is important, for the reasons well described in this Stack Overflow answer.
C++ Signal Slot
The reason for the QVariant is the Script based approach of QML. The QVariant basically contains your data and a desription of the data type, so that the QML knows how to handle it properly. That’s why you have to specify the parameter in QML with String, int etc.. But the original data exchange with C++ remains a QVariant
We now need a slot in QML to connect this signal to. It will handle the updating of the text field, and is simply a function on the Window.
Finally we use QObject::connect to make the connection.
C Signal Sigusr1
Run the application, and you should now see your text be converted to upper-case.
Next steps
It feels like more work to use signals and slots, instead of method calls. But you should be able to see the benefits of a loosely coupled application, especially for larger applications.
I’m not sure a handler class for the text field is the best approach for this in practice. In a real application I think you would emit user actions in your GUI and have classes to implement your application logic without knowledge of where that data is coming from. As I gain more experience with Qt, I will update the example with the best practice.
Check out the full application on GitHub.
Cover image by Beverley Goodwin.