C++ Qt 5 - Displaying Windows
Earlier this week, I posted an example of integrating.
In it I showed how to call a C++ method from QML, but finished my post with this statement.
I have now found a way to use signals and slots to do this, which I will describe here.
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.
This allows you to design and build a 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 qml slots and signals />This function might emit a signal letting others know its progress, which the progress bar could listen to and update the GUI.
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.
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.
Emitting a signal from QML and listening to it qml slots and signals C++ To create a signal in QML, simply add the following line to qml slots and signals object which will emit the signal.
I will emit the signal when the button is pressed, passing the value of the text field.
Here is the full QML document.
The signal is being emitted, but because no slots are listening to it nothing happens.
The header see more looks like this.
Add the following to main.
Then we can connect the windows submitTextField signal to the handleSubmitTextField slot.
Running the application now and you should get qml slots and signals debug message showing the text being passed to C++.
Emitting a signal from C++ and listening to more info 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.
This is important, for the reasons well described in this.
The reason for the QVariant https://separateschooleducation.info/slots-and/tablet-with-usb-and-card-slot.html 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.
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.
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.
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.
C++ Qt 4 - Signals and Slots
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