Signals are a neat feature of Qt that allow you to pass messages between different components in your applications.

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Qt 5 Documentation — QMainWindow Signals. As you can see, alongside the two QMainWindow signals, there are 4 signals inherited from QWidget and 2 signals inherited from Object. If you click through to the QWidget signal documentation you can see a.windowTitleChanged signal implemented here. Next we'll demonstrate that signal within our. Signals & Slots 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. 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.

Signals are connected to slots which are functions (or methods) which will be run every time the signal fires. Many signals also transmit data, providing information about the state change or widget that fired them. The receiving slot can use this data to perform different actions in response to the same signal.

However, there is a limitation: the signal can only emit the data it was designed to. So for example, a QAction has a .triggered that fires when that particular action has been activated. The triggered signal emits a single piece of data -- the checked state of the action after being triggered.

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. Using Qt Designer's Signal/Slot Editor.

For non-checkable actions, this value will always be False

The receiving function does not know whichQAction triggered it, or receiving any other data about it.

This is usually fine. You can tie a particular action to a unique function which does precisely what that action requires. Sometimes however you need the slot function to know more than that QAction is giving it. This could be the object the signal was triggered on, or some other associated metadata which your slot needs to perform the intended result of the signal.

This is a powerful way to extend or modify the built-in signals provided by Qt.

Intercepting the signal

Instead of connecting signal directly to the target function, youinstead use an intermediate function to intercept the signal, modify the signal data and forward that on to your actual slot function.

This slot function must accept the value sent by the signal (here the checked state) and then call the real slot, passing any additional data with the arguments.

Rather than defining this intermediate function, you can also achieve the same thing using a lambda function. As above, this accepts a single parameter checked and then calls the real slot.

In both examples the <additional args> can be replaced with anything you want to forward to your slot. In the example below we're forwarding the QAction object action to the receiving slot.

Our handle_trigger slot method will receive both the original checked value and the QAction object. Or receiving slot can look something like this

Below are a few examples using this approach to modify the data sent with the MainWindow.windowTitleChanged signal.

• PyQt5
• PySide2

The .setWindowTitle call at the end of the __init__ block changes the window title and triggers the .windowTitleChanged signal, which emits the new window title as a str. We've attached a series of intermediate slot functions (as lambda functions) which modify this signal and then call our custom slots with different parameters.

Running this produces the following output.

The intermediate functions can be as simple or as complicated as you like -- as well as discarding/adding parameters, you can also perform lookups to modify signals to different values.

In the following example a checkbox signal Qt.Checked or Qt.Unchecked is modified by an intermediate slot into a bool value.

• PyQt5
• PySide2

In this example we've connected the .stateChange signal to result in two ways -- a) with a intermediate function which calls the .result method with True or False depending on the signal parameter, and b) with a dictionary lookup within an intermediate lambda.

Running this code will output True or False to the command line each time the state is changed (once for each time we connect to the signal).

QCheckbox triggering 2 slots, with modified signal data

Trouble with loops

One of the most common reasons for wanting to connect signals in this way is when you're building a series of objects and connecting signals programmatically in a loop. Unfortunately then things aren't always so simple.

If you try and construct intercepted signals while looping over a variable, and want to pass the loop variable to the receiving slot, you'll hit a problem. For example, in the following code we create a series of buttons, and use a intermediate function to pass the buttons value (0-9) with the pressed signal.

• PyQt5
• PySide2

If you run this you'll see the problem -- no matter which button you click on you get the same number (9) shown on the label. Why 9? It's the last value of the loop.

The problem is the line lambda: self.button_pressed(a) where we pass a to the final button_pressed slot. In this context, a is bound to the loop.

We are not passing the value of a when the button is created, but whatever value a has when the signal fires. Since the signal fires after the loop is completed -- we interact with the UI after it is created -- the value of a for every signal is the final value that a had in the loop: 9.

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So clicking any of them will send 9 to button_pressed

The solution is to pass the value in as a (re-)named parameter. This binds the parameter to the value of a at that point in the loop, creating a new, un-connected variable. The loop continues, but the bound variable is not altered.

This ensures the correct value whenever it is called.

You don't have to rename the variable, you could also choose to use the same name for the bound value.

The important thing is to use named parameters. Putting this into a loop, it would look like this:

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Running this now, you will see the expected behavior -- with the label updating to a number matching the button which is pressed.

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The working code is as follows:

• PyQt5
• PySide2

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 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 ).

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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:

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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