Qt Signals And Slots Between Classes

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  1. A developer can choose to connect to a signal by creating a function (a slot) and calling the connect function to relate the signal to the slot. Qt's signals and slots mechanism does not require classes to have knowledge of each other, which makes it much easier to develop highly reusable classes.
  2. PyQt4 Signals and Qt Signals¶. Qt signals are statically defined as part of a C class. They are referenced using the QtCore.SIGNAL function. This method takes a single string argument that is the name of the signal and its C signature.

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.

The receivers of signals are called Slots in Qt terminology. 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.

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

Qt Signals And Slots Between Classes For Beginners

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.

Qt Signals And Slots Between Classes

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.

Qt Signals And Slots Between Classes Open

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.

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.

Signals

Finally we use QObject::connect to make the connection.

Run the application, and you should now see your text be converted to upper-case.

Next steps

Qt signals and slots between classes open

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.

Qt Signals And Slots Between Classes Near Me

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.