Pyqt Connect Signal To Multiple Slots
In this part of the PyQt4 programming tutorial, we will explore events and signals occurring in applications.
Sep 04, 2016 It can be difficult for newcomers to configure signal and slot in PyQt5 who have no prior experience in Qt programming. Signal-Slot is one of the fundamental topics of Qt one should have a. It is also possible to connect a single signal to multiple slots. Although rare, we can also connect a signal to another signal: In such cases, when the first signal is emitted, it will cause the signal it is connected to, to be emitted. It can be difficult for newcomers to configure signal and slot in PyQt5 who have no prior experience in Qt programming. Signal-Slot is one of the fundamental topics of Qt one should have a firm.
Events
All GUI applications are event-driven. Events are generated mainly by the user of an application. But they can be generated by other means as well: e.g. an Internet connection, a window manager, or a timer.When we call the application's exec_() method, the application enters the main loop. The main loop fetches events and sends them to the objects.
In the event model, there are three participants:
- event source
- event object
- event target
The event source is the object whose state changes. It generates events. The event object (event) encapsulates the state changes in the event source.The event target is the object that wants to be notified. Event source object delegates the task of handling an event to the event target.
PyQt4 has a unique signal and slot mechanism to deal with events. Signals and slots are used for communication between objects. A signal is emitted when a particular event occurs. A slot can be any Python callable.A slot is called when a signal connected to it is emitted.
New API
PyQt4.5 introduced a new style API for working with signals and slots.
This is the old style API.
The new style adheres more to the Python standards.
Signals & Slots
This is a simple example demonstrating signals and slots in PyQt4.
In our example, we display a QtGui.QLCDNumberand a QtGui.QSlider. We change the lcd number by dragging the slider knob.
Here we connect a valueChanged signal of the slider to thedisplay slot of the lcd number.
The sender is an object that sends a signal. The receiver is the object that receives the signal. The slot is the method that reacts to the signal.
Reimplementing event handler
Events in PyQt4 are processed often by reimplementing event handlers.
In our example, we reimplement the keyPressEvent() event handler.
If we click the Escape button, the application terminates.
Event sender
Sometimes it is convenient to know which widget is the sender of a signal. For this, PyQt4 has the sender()method.
We have two buttons in our example. In the buttonClicked() methodwe determine which button we have clicked by calling the sender() method.
Both buttons are connected to the same slot.
We determine the signal source by calling the sender() method.In the statusbar of the application, we show the label of the button being pressed.
Pyqt Slot
Emitting signals
Objects created from a QtCore.QObject can emit signals. In the following example we will see how we can emit custom signals.
We create a new signal called closeApp. This signal is emitted during a mouse press event. The signal is connected to theclose() slot of the QtGui.QMainWindow.
A signal is created with the QtCore.pyqtSignal() as a class attributeof the external Communicate class.
The custom closeApp signal is connected to the close() slot of the QtGui.QMainWindow.
When we click on the window with a mouse pointer, the closeApp signal is emitted. The application terminates.
In this part of the PyQt4 tutorial, we have covered signals and slots.
This section describes the new style of connecting signals and slotsintroduced in PyQt4 v4.5.
One of the key features of Qt is its use of signals and slots to communicatebetween objects. Their use encourages the development of reusable components.
A signal is emitted when something of potential interest happens. A slot is aPython callable. If a signal is connected to a slot then the slot is calledwhen the signal is emitted. If a signal isn’t connected then nothing happens.The code (or component) that emits the signal does not know or care if thesignal is being used.
The signal/slot mechanism has the following features.
- A signal may be connected to many slots.
- A signal may also be connected to another signal.
- Signal arguments may be any Python type.
- A slot may be connected to many signals.
- Connections may be direct (ie. synchronous) or queued (ie. asynchronous).
- Connections may be made across threads.
- Signals may be disconnected.
Unbound and Bound Signals¶
A signal (specifically an unbound signal) is an attribute of a class that is asub-class of QObject. When a signal is referenced as an attribute of aninstance of the class then PyQt4 automatically binds the instance to the signalin order to create a bound signal. This is the same mechanism that Pythonitself uses to create bound methods from class functions.
A bound signal has connect(), disconnect() and emit() methods thatimplement the associated functionality. It also has a signal attributethat is the signature of the signal that would be returned by Qt’s SIGNAL()macro.
A signal may be overloaded, ie. a signal with a particular name may supportmore than one signature. A signal may be indexed with a signature in order toselect the one required. A signature is a sequence of types. A type is eithera Python type object or a string that is the name of a C++ type. The name of aC++ type is automatically normalised so that, for example, QString can beused instead of the non-normalised constQString&.
If a signal is overloaded then it will have a default that will be used if noindex is given.
When a signal is emitted then any arguments are converted to C++ types ifpossible. If an argument doesn’t have a corresponding C++ type then it iswrapped in a special C++ type that allows it to be passed around Qt’s meta-typesystem while ensuring that its reference count is properly maintained.
Defining New Signals with pyqtSignal()¶
PyQt4 automatically defines signals for all Qt’s built-in signals. New signalscan be defined as class attributes using the pyqtSignal()factory.
PyQt4.QtCore.pyqtSignal(types[, name])¶Create one or more overloaded unbound signals as a class attribute.
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| Return type: | an unbound signal |
The following example shows the definition of a number of new signals:
New signals should only be defined in sub-classes of QObject. They must bepart of the class definition and cannot be dynamically added as classattributes after the class has been defined.
New signals defined in this way will be automatically added to the class’sQMetaObject. This means that they will appear in Qt Designer and can beintrospected using the QMetaObject API.
Overloaded signals should be used with care when an argument has a Python typethat has no corresponding C++ type. PyQt4 uses the same internal C++ class torepresent such objects and so it is possible to have overloaded signals withdifferent Python signatures that are implemented with identical C++ signatureswith unexpected results. The following is an example of this:
Connecting, Disconnecting and Emitting Signals¶
Signals are connected to slots using the connect() method of a boundsignal.
connect(slot[, type=PyQt4.QtCore.Qt.AutoConnection[, no_receiver_check=False]])¶Connect a signal to a slot. An exception will be raised if the connectionfailed.
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Signals are disconnected from slots using the disconnect() method of abound signal.
disconnect([slot])¶Disconnect one or more slots from a signal. An exception will be raised ifthe slot is not connected to the signal or if the signal has no connectionsat all.
| Parameters: | slot – the optional slot to disconnect from, either a Python callable oranother bound signal. If it is omitted then all slots connected to thesignal are disconnected. |
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Signals are emitted from using the emit() method of a bound signal.
emit(*args)¶Emit a signal.
| Parameters: | args – the optional sequence of arguments to pass to any connected slots. |
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The following code demonstrates the definition, connection and emit of asignal without arguments:
The following code demonstrates the connection of overloaded signals:
Connecting Signals Using Keyword Arguments¶
It is also possible to connect signals by passing a slot as a keyword argumentcorresponding to the name of the signal when creating an object, or using thepyqtConfigure() method of QObject. For example the following threefragments are equivalent:
The pyqtSlot() Decorator¶
Pyqt5 Signal Slot Example
Although PyQt4 allows any Python callable to be used as a slot when connectingsignals, it is sometimes necessary to explicitly mark a Python method as beinga Qt slot and to provide a C++ signature for it. PyQt4 provides thepyqtSlot() function decorator to do this.
PyQt4.QtCore.pyqtSlot(types[, name[, result]])¶Decorate a Python method to create a Qt slot.
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Connecting a signal to a decorated Python method also has the advantage ofreducing the amount of memory used and is slightly faster.
For example:
It is also possible to chain the decorators in order to define a Python methodseveral times with different signatures. For example:
The PyQt_PyObject Signal Argument Type¶
It is possible to pass any Python object as a signal argument by specifyingPyQt_PyObject as the type of the argument in the signature. For example:
This would normally be used for passing objects where the actual Python typeisn’t known. It can also be used to pass an integer, for example, so that thenormal conversions from a Python object to a C++ integer and back again are notrequired.
The reference count of the object being passed is maintained automatically.There is no need for the emitter of a signal to keep a reference to the objectafter the call to finished.emit(), even if a connection is queued.
Connecting Slots By Name¶
PyQt4 supports the QtCore.QMetaObject.connectSlotsByName() function thatis most commonly used by pyuic4 generated Python code toautomatically connect signals to slots that conform to a simple namingconvention. However, where a class has overloaded Qt signals (ie. with thesame name but with different arguments) PyQt4 needs additional information inorder to automatically connect the correct signal.
For example the QtGui.QSpinBox class has the following signals:
Pyqt Connect Signal With Arguments
When the value of the spin box changes both of these signals will be emitted.If you have implemented a slot called on_spinbox_valueChanged (whichassumes that you have given the QSpinBox instance the name spinbox)then it will be connected to both variations of the signal. Therefore, whenthe user changes the value, your slot will be called twice - once with aninteger argument, and once with a unicode or QString argument.
Pyqt Signal Disconnect
This also happens with signals that take optional arguments. Qt implementsthis using multiple signals. For example, QtGui.QAbstractButton has thefollowing signal:
Qt implements this as the following:
The pyqtSlot() decorator can be used to specify which ofthe signals should be connected to the slot.
For example, if you were only interested in the integer variant of the signalthen your slot definition would look like the following:
If you wanted to handle both variants of the signal, but with different Pythonmethods, then your slot definitions might look like the following:
The following shows an example using a button when you are not interested inthe optional argument:
Mixing New-style and Old-style Connections¶
The implementation of new-style connections is slightly different to theimplementation of old-style connections. An application can freely use bothstyles subject to the restriction that any individual new-style connectionshould only be disconnected using the new style. Similarly any individualold-style connection should only be disconnected using the old style.
You should also be aware that pyuic4 generates code that usesold-style connections.