Build complex application behaviours using signals and slots, and override widget event handling with custom events.
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. QPushButton emits signals if an event occurs. To handle the button connect its appropriate signal to a slot: connect(mbutton, &QPushButton::released, this, &MainWindow::handleButton); Example. The following simple code snippet shows how to create and use QPushButton. It has been tested on Qt Symbian Simulator. An instance of QPushButton is. Qt's meta-object system provides a mechanism to automatically connect signals and slots between QObject subclasses and their children. As long as objects are defined with suitable object names, and slots follow a simple naming convention, this connection can be performed at run-time by the QMetaObject::connectSlotsByName.
As already described, every interaction the user has with a Qt application causes an Event. There are multiple types of event, each representing a difference type of interaction — e.g. mouse or keyboard events.
Events that occur are passed to the event-specific handler on the widget where the interaction occurred. For example, clicking on a widget will cause a
QMouseEvent to be sent to the
.mousePressEvent event handler on the widget. This handler can interrogate the event to find out information, such as what triggered the event and where specifically it occurred.
Qt has a unique signal and slot mechanism. This signal and slot mechanism is an extension to the C programming language. Signals and slots are used for communication between objects. A signal is emitted when a particular event occurs. A slot is a normal C method; it is called when a signal connected to it is emitted. QtCore.SIGNAL and QtCore.SLOT macros allow Python to interface with Qt signal and slot delivery mechanisms. This is the old way of using signals and slots. The example below uses the well known clicked signal from a QPushButton. The connect method has a non python-friendly syntax.
You can intercept events by subclassing and overriding the handler function on the class, as you would for any other function. You can choose to filter, modify, or ignore events, passing them through to the normal handler for the event by calling the parent class function with
However, imagine you want to catch an event on 20 different buttons. Subclassing like this now becomes an incredibly tedious way of catching, interpreting and handling these events.
Thankfully Qt offers a neater approach to receiving notification of things happening in your application: Signals.
Instead of intercepting raw events, signals allow you to 'listen' for notifications of specific occurrences within your application. While these can be similar to events — a click on a button — they can also be more nuanced — updated text in a box. Data can also be sent alongside a signal - so as well as being notified of the updated text you can also receive it.
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.
Load up a fresh copy of `MyApp_window.py` and save it under a new name for this section. The code is copied below if you don't have it yet.
First, let's look at the signals available for our
QMainWindow. You can find this information in the Qt documentation. Scroll down to the Signals section to see the signals implemented for this class.
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 application.
Qt 5 Documentation — Widget Signals
The code below gives a few examples of using the
Try commenting out the different signals and seeing the effect on what the slot prints.
We start by creating a function that will behave as a ‘slot’ for our signals.
Then we use .connect on the
.windowTitleChanged signal. We pass the function that we want to be called with the signal data. In this case the signal sends a string, containing the new window title.
If we run that, we see that we receive the notification that the window title has changed.
Next, let’s take a quick look at events. Thanks to signals, for most purposes you can happily avoid using events in Qt, but it’s important to understand how they work for when they are necessary.
As an example, we're going to intercept the
QMainWindow. This event is fired whenever a context menu is about to be shown, and is passed a single value
event of type
To intercept the event, we simply override the object method with our new method of the same name. So in this case we can create a method on our
MainWindow subclass with the name
contextMenuEvent and it will receive all events of this type.
If you add the above method to your
MainWindow class and run your program you will discover that right-clicking in your window now displays the message in the print statement.
Sometimes you may wish to intercept an event, yet still trigger the default (parent) event handler. You can do this by calling the event handler on the parent class using
super as normal for Python class methods.
This allows you to propagate events up the object hierarchy, handling only those parts of an event handler that you wish.
However, in Qt there is another type of event hierarchy, constructed around the UI relationships. Widgets that are added to a layout, within another widget, may opt to pass their events to their UI parent. In complex widgets with multiple sub-elements this can allow for delegation of event handling to the containing widget for certain events.
However, if you have dealt with an event and do not want it to propagate in this way you can flag this by calling
.accept() on the event.
Alternatively, if you do want it to propagate calling
.ignore() will achieve this.
In this section we've covered signals, slots and events. We've demonstratedsome simple signals, including how to pass less and more data using lambdas.We've created custom signals, and shown how to intercept events, pass onevent handling and use
.ignore() to hide/show eventsto the UI-parent widget. In the next section we will go on to takea look at two common features of the GUI — toolbars and menus.
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.
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.
Older 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. Callbacks have two fundamental flaws: Firstly, they are not type-safe. We can never be certain that the processing function will call the callback with the correct arguments. Secondly, the callback is strongly coupled to the processing function since the processing function must know which callback to call.
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.
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. 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.
A Small Example
A minimal C++ class declaration might read:
A small QObject-based class might read:
The QObject-based version has the same internal state, and provides public methods to access the state, but in addition it has support for component programming using signals and slots. This class can tell the outside world that its state has changed by emitting a signal,
valueChanged(), and it has a slot which other objects can send signals to.
All classes that contain signals or slots must mention Q_OBJECT at the top of their declaration. They must also derive (directly or indirectly) from QObject.
Slots are implemented by the application programmer. Here is a possible implementation of the
emit line emits the signal
valueChanged() from the object, with the new value as argument.
In the following code snippet, we create two
Counter objects and connect the first object's
valueChanged() signal to the second object's
setValue() slot using QObject::connect():
a emit a
valueChanged(12) signal, which
b will receive in its
setValue() slot, i.e.
b.setValue(12) is called. Then
b emits the same
valueChanged() signal, but since no slot has been connected to
valueChanged() signal, the signal is ignored.
Note that the
setValue() function sets the value and emits the signal only if
value != m_value. This prevents infinite looping in the case of cyclic connections (e.g., if
b.valueChanged() were connected to
By default, for every connection you make, a signal is emitted; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect() call. If you pass the Qt::UniqueConnectiontype, the connection will only be made if it is not a duplicate. If there is already a duplicate (exact same signal to the exact same slot on the same objects), the connection will fail and connect will return false
This example illustrates that objects can work together without needing to know any information about each other. To enable this, the objects only need to be connected together, and this can be achieved with some simple QObject::connect() function calls, or with
uic's automatic connections feature.
Building the Example
The C++ preprocessor changes or removes the
emit keywords so that the compiler is presented with standard C++.
By running the moc on class definitions that contain signals or slots, a C++ source file is produced which should be compiled and linked with the other object files for the application. If you use qmake, the makefile rules to automatically invoke
moc will be added to your project's makefile.
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. Only the class that defines a signal and its subclasses can emit the signal.
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
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.
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
delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires
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. On an i586-500, you can emit around 2,000,000 signals per second connected to one receiver, or around 1,200,000 per second connected to two receivers. 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
slots may cause compiler warnings and errors when compiled alongside a Qt-based application. To solve this problem,
#undef the offending preprocessor symbol.
The meta-object compiler (moc) parses the class declaration in a C++ file and generates C++ code that initializes the meta-object. The meta-object contains the names of all the signal and slot members, as well as pointers to these functions.
The meta-object contains additional information such as the object's class name. You can also check if an object inherits a specific class, for example:
The meta-object information is also used by qobject_cast<T>(), which is similar to QObject::inherits() but is less error-prone:
See Meta-Object System for more information.
A Real Example
Here is a simple commented example of a widget.
LcdNumber inherits QObject, which has most of the signal-slot knowledge, via QFrame and QWidget. It is somewhat similar to the built-in QLCDNumber widget.
The Q_OBJECT macro is expanded by the preprocessor to declare several member functions that are implemented by the
moc; if you get compiler errors along the lines of 'undefined reference to vtable for
LcdNumber', you have probably forgotten to run the moc or to include the moc output in the link command.
It's not obviously relevant to the moc, but if you inherit QWidget you almost certainly want to have the
parent argument in your constructor and pass it to the base class's constructor.
Some destructors and member functions are omitted here; the
moc ignores member functions.
LcdNumber emits a signal when it is asked to show an impossible value.
If you don't care about overflow, or you know that overflow cannot occur, you can ignore the
overflow() signal, i.e. don't connect it to any slot.
If on the other hand you want to call two different error functions when the number overflows, simply connect the signal to two different slots. Qt will call both (in the order they were connected).
A slot is a receiving function used to get information about state changes in other widgets.
LcdNumber uses it, as the code above indicates, to set the displayed number. Since
display() is part of the class's interface with the rest of the program, the slot is public.
Several of the example programs connect the valueChanged() signal of a QScrollBar to the
display() slot, so the LCD number continuously shows the value of the scroll bar.
display() is overloaded; Qt will select the appropriate version when you connect a signal to the slot. With callbacks, you'd have to find five different names and keep track of the types yourself.
Some irrelevant member functions have been omitted from this example.
Signals And Slots With Default Arguments
The signatures of signals and slots may contain arguments, and the arguments can have default values. Consider QObject::destroyed():
Qt Connect Signal Parent Slot
When a QObject is deleted, it emits this QObject::destroyed() signal. We want to catch this signal, wherever we might have a dangling reference to the deleted QObject, so we can clean it up. A suitable slot signature might be:
To connect the signal to the slot, we use QObject::connect() and the
SLOT() macros. The rule about whether to include arguments or not in the
SLOT() macros, if the arguments have default values, is that the signature passed to the
SIGNAL() macro must not have fewer arguments than the signature passed to the
All of these would work:
Qt Connect Parent Slot Machine
But this one won't work:
...because the slot will be expecting a QObject that the signal will not send. This connection will report a runtime error.
Advanced Signals and Slots Usage
For cases where you may require information on the sender of the signal, Qt provides the QObject::sender() function, which returns a pointer to the object that sent the signal.
The QSignalMapper class is provided for situations where many signals are connected to the same slot and the slot needs to handle each signal differently.
Suppose you have three push buttons that determine which file you will open: 'Tax File', 'Accounts File', or 'Report File'.
In order to open the correct file, you use QSignalMapper::setMapping() to map all the clicked() signals to a QSignalMapper object. Then you connect the file's QPushButton::clicked() signal to the QSignalMapper::map() slot.
Then, you connect the mapped() signal to
readFile() where a different file will be opened, depending on which push button is pressed.
Note: The following code will compile and run, but due to signature normalization, the code will be slower.
Using Qt with 3rd Party Signals and Slots
It is possible to use Qt with a 3rd party signal/slot mechanism. You can even use both mechanisms in the same project. Just add the following line to your qmake project (.pro) file.
It tells Qt not to define the moc keywords
emit, because these names will be used by a 3rd party library, e.g. Boost. Then to continue using Qt signals and slots with the
no_keywords flag, simply replace all uses of the Qt moc keywords in your sources with the corresponding Qt macros Q_SIGNALS (or Q_SIGNAL), Q_SLOTS (or Q_SLOT), and Q_EMIT.
See also Meta-Object System and Qt's Property System.
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