Old syntax Qt 5 continues to support the old string-based syntax for connecting signals and slots defined in a QObject or any class that inherits from QObject (including QWidget) connect (sender, SIGNAL (valueChanged (QString, QString)), receiver, SLOT (updateValue (QString))); New: connecting to QObject member. Browse other questions tagged c qt signals-slots undefined-reference or ask your own question. The Overflow Blog The Overflow #42: Bugs vs.

Using Layouts in Qt Designer ◦ Qt Designer's Buddy Editing Mode

In Qt Designer's signals and slots editing mode, you can connect objects in a form together using Qt's signals and slots mechanism. Both widgets and layouts can be connected via an intuitive connection interface, using the menu of compatible signals and slots provided by Qt Designer. When a form is saved, all connections are preserved so that they will be ready for use when your project is built.

For more information on Qt's signals and sltos mechanism, refer to the Signals and Slots document.

Connecting Objects

To begin connecting objects, enter the signals and slots editing mode by opening the Edit menu and selecting Edit Signals/Slots, or by pressing the F4 key.

All widgets and layouts on the form can be connected together. However, spacers just provide spacing hints to layouts, so they cannot be connected to other objects.

To make a connectionn, press the left mouse button and drag the cursor towards the object you want to connect it to. As you do this, a line will extend from the source object to the cursor. If the cursor is over another object on the form, the line will end with an arrow head that points to the destination object. This indicates that a connection will be made between the two objects when you release the mouse button.

You can abandon the connection at any point while you are dragging the connection path by pressing Esc.

The Configure Connection dialog (below) is displayed, showing signals from the source object and slots from the destination object that you can use.

To complete the connection, select a signal from the source object and a slot from the destination object, then click OK. Click Cancel if you wish to abandon the connection.

Note: If the Show all signals and slots checkbox is selected, all available signals from the source object will be shown. Otherwise, the signals and slots inherited from QWidget will be hidden.

You can make as many connections as you like between objects on the form; it is possible to connect signals from objects to slots in the form itself. As a result, the signal and slot connections in many dialogs can be completely configured from within Qt Designer.

Editing and Deleting Connections

By default, connection paths are created with two labels that show the signal and slot involved in the connection. These labels are usually oriented along the line of the connection. You can move them around inside their host widgets by dragging the red square at each end of the connection path.

Using Layouts in Qt Designer ◦ Qt Designer's Buddy Editing Mode

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.

Create one or more overloaded unbound signals as a class attribute.

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.

Qt Signal And Slots

Connect a signal to a slot. An exception will be raised if the connectionfailed.

Signals are disconnected from slots using the disconnect() method of abound signal.

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.

Signals are emitted from using the emit() method of a bound signal.

Emit a signal.

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¶

Qt Signals And Slots Tutorial

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¶

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.

Decorate a Python method to create a Qt slot.

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:

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:

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.

This also happens with signals that take optional arguments. Qt implementsthis using multiple signals. For example, QtGui.QAbstractButton has thefollowing signal:

Qt Signal Slot Reference Chart

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¶

Qt Signal Slot Example

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.

Qt Signal Slot Reference Generator

You should also be aware that pyuic4 generates code that usesold-style connections.