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 .

1. Qt Slot Signal Tutorial Arduino
2. Qt Slot Signal Tutorial Android Studio
3. Qt Slot Signal Tutorial Python
4. Qt Slot Signal Tutorial
5. Qt Public Slots

Introduction

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

The Qt signals/slots and property system are based on the ability to introspect the objects at runtime. Introspection means being able to list the methods and properties of an object and have all kinds of information about them such as the type of their arguments. QtScript and QML would have hardly been possible without that ability. In this tutorial, we’ll show you how to handle signals and slots using Qt for Python. Signals and slots is a Qt feature that lets your graphical widgets communicate with other graphical widgets or your python code. Our application creates a button that logs the Button clicked, Hello! Message to the python console each time you click it.

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.)

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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

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

How to use a QCheckBox check box widget in a Qt Creator C++ program or application. This tutorial demonstrates the use of the QCheckBox class that implements a check box widget in Qt.

The example application built in this part of the Qt Creator tutorial series is shown in the image below. The result of clicking a check box is printed to stdout and can be seen in Application Output window of Qt Creator or at the command line.

Comparing Qt with GTK: The equivalent widget to a Qt QCheckBox in GTK is a GtkCheckButton. See the GtkCheckButton tutorial that uses a check box in a GTK C program for more information.

Part 19 of the Qt Creator C++ Tutorial

QCheckBox Tutorial Steps

Two examples are presented in the tutorial steps that follow. Both examples perform the same basic function, but do so in different ways. The first example uses three separate handler functions, or slots, for each check box. The second example uses a single slot for all three check boxes. Follow the tutorial steps below to build the two QCheckBox example programs.

1. Create a New Qt Widgets Application

Open Qt Creator and start a new Qt Widgets Application called checkbox. Open the mainwindow.ui form and delete the default widgets, as we have done in previous parts of this tutorial series.

2. Place the QCheckBox Widgets

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Place three QCheckBox widgets in the main window as shown in the following image and described below.

In Qt Creator, find the Check Box widget under Buttons in the left widgets pane. Place three Check Box widgets on the main window by dragging and dropping them, one below the other.

Give each Check Box a objectName of chkbx_1, chkbx_2 and chkbx_3 from top to bottom.

Change the text of each Check Box to Option 1, Option 2 and Option 3 from top to bottom.

Group the three Check Box widgets together by dragging to select all of them. Now click the Lay Out Vertically icon on the top toolbar.

3. Edit the Main Window

Resize the main window to an appropriate size by first clicking the window to select it and then dragging the bottom right handle.

Change the windowTitle to Checkbox in the bottom right property pane under the QWidget section.

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4. QCheckBox Example 1 C++ Code

Before writing the C++ code, add a slot for the clicked() signal of each check box widget, as described below.

4.1 Add a Slot for Each Check Box

While still editing mainwindow.ui in Design mode, right-click the top QCheckBox. On the menu that pops up, click Go to slot… In the dialog box that pops up, select the clicked() signal and then click the OK button.

Do the same for the other two check boxes. To get back to Design view, double-click mainwindow.ui in the left pane of Qt Creator, under the Forms item.

4.2 Write the C++ Code

After adding the last slot for the clicked() signal, modify mainwindow.cpp as follows.

mainwindow.cpp

4.3 Build and Run Example 1

Build example 1 and run it from within Qt Creator. When any of the check boxes are clicked in the application, text is written to the command line which is captured in Qt Creator. A pane at the bottom of Qt Creator, called Application Output opens and displays the text that the application prints.

The image below shows the bottom pane from Qt Creator. It contains text printed by the application from check box clicks.

4.4 How the Program Works

The program is very simple and uses a QTextStream to send text to stdout to display the status of a check box. Part 7 of this tutorial series, Qt Console Application Programming covers using QTextStream.

Whenever a QCheckBox is clicked in the application, the isChecked() method is called to check whether the checkbox is checked or unchecked. The result is printed to stdout, which is captured by Qt Creator when the application is run from Qt Creator.

5. QCheckBox Example 2 C++ Code

In this second example that uses QCheckBox, a single slot handles all three check boxes. Code manually connects signals for each checkbox to a single slot.

5.1 Create a New Qt Widgets Application

Create a new Qt widgets application exactly as done in section 1 of this tutorial. This time call it checkbox2.

5.2 Write the C++ Code

In this example, we add the slot manually in the C code. Open mainwindow.h and add the slot as shown in the code listing that follows.

mainwindow.h

Now open mainwindow.cpp and modify the code as follows.

mainwindow.cpp

5.3 Build and Run Example 2

Save the modifications made to the code and form. Build the application in Qt Creator and run it.

Click a check box and see the status of all three check boxes displayed in the bottom pane of Qt Creator.

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5.4 How the Code Works

Qt Public Slots

A slot is manually added in mainwindow.h as follows.

The body of the onchkbx_clicked() slot is then added to mainwindow.cpp. In this slot, the state of each checkbox is checked using its isChecked() method. The QTextStream object writes check box states to stdout.

In the constructor, connect() connects the clicked() signal of each checkbox to the same onchkbx_clicked() slot.

The Signals and Slots Qt documentation page contains information on the Qt signal slot mechanism.

connect() is a member function of QObject class. Read more about QObject::connect in the Qt QObject class documentation.