Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks. Signals and slots are made possible by Qt's meta-object system .

Introduction

• 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.
• Very basically, signals and slots in Qt allow communication between objects. In Qt, a signal is emitted when an event occurs. A slot is a function that is called when a signal is emitted. For example, a push button emits a clicked signal when clicked by a user.
• Further properties of signal/slots. Qt provides the QObject::sender function, which returns a pointer to the object that sent the signal Note: if the slot was not activated by a signal, the return is undefined. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal.

Qt connects widgets by means of a nice designed scheme based on the idea that objectS may send signalS of different typeS to a single object instance: This is a screenshot of the example code running. Qt signals and slots: Signal « Qt « C. Related examples in the same category. A Slot Responds to a Signal: 2. Inherited slot: 3. Qt signal test: 4.

In GUI programming, when we change one widget, we often want another widget to be notified. More generally, we want objects of any kind to be able to communicate with one another. For example, if a user clicks a Close button, we probably want the window's close() function to be called.

Other toolkits achieve this kind of communication using callbacks. A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function (the callback) to the processing function. The processing function then calls the callback when appropriate. While successful frameworks using this method do exist, callbacks can be unintuitive and may suffer from problems in ensuring the type-correctness of callback arguments.

Signals and Slots

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 predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in.

Signals and slots in Qt

The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot. (In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments.) Since the signatures are compatible, the compiler can help us detect type mismatches when using the function pointer-based syntax. The string-based SIGNAL and SLOT syntax will detect type mismatches at runtime. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. They are completely type safe.

All classes that inherit from QObject or one of its subclasses (e.g., QWidget ) can contain signals and slots. Signals are emitted by objects when they change their state in a way that may be interesting to other objects. This is all the object does to communicate. It does not know or care whether anything is receiving the signals it emits. This is true information encapsulation, and ensures that the object can be used as a software component.

Slots can be used for receiving signals, but they are also normal member functions. Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it. This ensures that truly independent components can be created with Qt.

You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need. It is even possible to connect a signal directly to another signal. (This will emit the second signal immediately whenever the first is emitted.)

Together, signals and slots make up a powerful component programming mechanism.

Signals

Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.

When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned. The situation is slightly different when using queued connections ; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.

If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted.

Signals are automatically generated by the moc and must not be implemented in the .cpp file. They can never have return types (i.e. use void ).

A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types. If QScrollBar::valueChanged () were to use a special type such as the hypothetical QScrollBar::Range, it could only be connected to slots designed specifically for QScrollBar . Connecting different input widgets together would be impossible.

Slots

A slot is called when a signal connected to it is emitted. Slots are normal C++ functions and can be called normally; their only special feature is that signals can be connected to them.

Since slots are normal member functions, they follow the normal C++ rules when called directly. However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection. This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.

You can also define slots to be virtual, which we have found quite useful in practice.

Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant. In general, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls. This is the overhead required to locate the connection object, to safely iterate over all connections (i.e. checking that subsequent receivers have not been destroyed during the emission), and to marshall any parameters in a generic fashion. While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or delete , the signals and slots overhead is only responsible for a very small proportion of the complete function call costs. The same is true whenever you do a system call in a slot; or indirectly call more than ten functions. The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice.

Note that other libraries that define variables called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application. To solve this problem, #undef the offending preprocessor symbol.

Connecting the signal to the slot

Prior to the fifth version of Qt to connect the signal to the slot through the recorded macros, whereas in the fifth version of the recording has been applied, based on the signs.

Writing with macros:

Writing on the basis of indicators:

The advantage of the second option is that it is possible to determine the mismatch of signatures and the wrong slot or signal name of another project compilation stage, not in the process of testing applications.

An example of using signals and slots

For example, the use of signals and slots project was created, which in the main window contains three buttons, each of which is connected to the slot and these slots already transmit a signal in a single slot with the pressed button number.

Project Structure

Project Structure

According to the tradition of conducting lessons enclosing structure of the project, which is absolutely trivial and defaulted to the disgrace that will not even describe members of her classes and files.

mainwindow.h

Thus, the following three buttons - three slots, one signal at all three buttons, which is fed into the slot button and transmits the number buttons into a single slot that displays a message with the number buttons.

mainwindow.cpp

Signal Slots Qt Examples

A file in this logic is configured as described in the preceding paragraphs. Just check the code and go to the video page, there is shown in detail the whole process, demonstrated the application, and also shows what happens if we make coding a variety of errors.

Video

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

Signal Slot Qt Example

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.

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

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.