Define Signals And Slots Pyqt
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Pyqt Signals And Slots

This video introduces signals and slots so that we can now respond to events. It also wraps up our window in a class. Here is the code for the tutorial: http. New-style Signal and Slot Support¶ This section describes the new style of connecting signals and slots introduced in PyQt v4.5. One of the key features of Qt is its use of signals and slots to communicate between objects. Their use encourages the development of reusable components. A signal is emitted when something of potential interest happens. This Is Our 7th Video And In This Video We Are Going To Cover The Basics Of PyQt5 Signal And Slots. The Definitive Guide to PyQt Programming (paperback).

Every GUI library provides the details of events that take place, such as mouse clicks and key presses. For example, if we have a button with the text Click Me, and the user clicks it, all kinds of information becomes available. The GUI library can tell us the coordinates of the mouse click relative to the button, relative to the button's parent widget, and relative to the screen; it can tell us the state of the Shift, Ctrl, Alt, and NumLock keys at the time of the click; and the precise time of the click and of the release; and so on. Similar information can be provided if the user 'clicked' the button without using the mouse. The user may have pressed the Tab key enough times to move the focus to the button and then pressed Spacebar, or maybe they pressed Alt+C. Although the outcome is the same in all these cases, each different means of clicking the button produces different events and different information.

The Qt library was the first to recognize that in almost every case, programmers don't need or even want all the low-level details: They don't care how the button was pressed, they just want to know that it was pressed so that they can respond appropriately. For this reason Qt, and therefore PyQt, provides two communication mechanisms: a low-level event-handling mechanism which is similar to those provided by all the other GUI libraries, and a high-level mechanism which Trolltech (makers of Qt) have called 'signals and slots'. We will look at the low-level mechanism in Chapter 10, and again in Chapter 11, but in this section we will focus on the high-level mechanism.

The connection mechanism uses a vector indexed by signals. But all the slots waste space in the vector and there are usually more slots than signals in an object. So from Qt 4.6, a new internal signal index which only includes the signal index is used. While developing with Qt, you only need to know about the absolute method index.

Every QObject—including all of PyQt's widgets since they derive from QWidget, a QObject subclass—supports the signals and slots mechanism. In particular, they are capable of announcing state changes, such as when a checkbox becomes checked or unchecked, and other important occurrences, for example when a button is clicked (by whatever means). All of PyQt's widgets have a set of predefined signals.

Whenever a signal is emitted, by default PyQt simply throws it away! To take notice of a signal we must connect it to a slot. In C++/Qt, slots are methods that must be declared with a special syntax; but in PyQt, they can be any callable we like (e.g., any function or method), and no special syntax is required when defining them.

Most widgets also have predefined slots, so in some cases we can connect a predefined signal to a predefined slot and not have to do anything else to get the behavior we want. PyQt is more versatile than C++/Qt in this regard, because we can connect not just to slots, but also to any callable, and from PyQt 4.2, it is possible to dynamically add 'predefined' signals and slots to QObjects. Let's see how signals and slots works in practice with the Signals and Slots program shown in Figure 4.6.

Figure 4.6 The Signals and Slots program

Both the QDial and QSpinBox widgets have valueChanged() signals that, when emitted, carry the new value. And they both have setValue() slots that take an integer value. We can therefore connect these two widgets to each other so that whichever one the user changes, will cause the other to be changed correspondingly:

class Form(QDialog):

def_init_(self, parent=None):

super(Form, self)._init_(parent)

dial.setNotchesVisible(True) spinbox = QSpinBox()

layout = QHBoxLayout() layout.addWidget(dial) layout.addWidget(spinbox) self.setLayout(layout)

self.connect(dial, SIGNAL('valueChanged(int)'), spinbox.setValue) self.connect(spinbox, SIGNAL('valueChanged(int)'), dial.setValue)

self.setWindowTitle('Signals and Slots')

Since the two widgets are connected in this way, if the user moves the dial—say to value 20—the dial will emit a valueChanged(20) signal which will, in turn, cause a call to the spinbox's setValue() slot with 20 as the argument. But then, since its value has now been changed, the spinbox will emit a valueChanged(20) signal which will in turn cause a call to the dial's setValue() slot with 20 as the argument. So it looks like we will get an infinite loop. But what happens is that the valueChanged() signal is not emitted if the value is not actually changed. This is because the standard approach to writing value-changing slots is to begin by comparing the new value with the existing one. If the values are the same, we do nothing and return; otherwise, we apply the change and emit a signal to announce the change of state. The connections are depicted in Figure 4.7.

SIGNAL('valueChanged(int)')

| setValue(int)

QDial

QSpinBox setValue(int) ; ! SIGNAL('valueChanged(int)')

Figure 4.7 The signals and slots connections

Now let's look at the general syntax for connections. We assume that the PyQt modules have been imported using the from .. import * syntax, and that s and w are QObjects, normally widgets, with s usually being self.

s.connect(w, SIGNAL('signalSignature'), functionName) s.connect(w, SIGNAL('signalSignature'), instance.methodName) s,connect(w, SIGNAL('signalSignature'), instance, SLOT('slotSignature'))

The signalSignature is the name of the signal and a (possibly empty) comma-separated list of parameter type names in parentheses. If the signal is a Qt signal, the type names must be the C++ type names, such as int and QString. C++ type names can be rather complex, with each type name possibly including one or more of const, *, and &. When we write them as signal (or slot) signatures we can drop any consts and &s, but must keep any *s. For example, almost every Qt signal that passes a QString uses a parameter type of const QString&, but in PyQt, just using QString alone is sufficient. On the other hand, the QListWidget has a signal with the signature itemActivated(QListWidgetItem*), and we must use this exactly as written.

PyQt signals are defined when they are actually emitted and can have any number of any type of parameters, as we will see shortly.

The slotSignature has the same form as a signalSignature except that the name is of a Qt slot. A slot may not have more arguments than the signal that is connected to it, but may have less; the additional parameters are then discarded. Corresponding signal and slot arguments must have the same types, so for example, we could not connect a QDial's valueChanged(int) signal to a QLineEdit's setText(QString) slot.

In our dial and spinbox example we used the instance.methodName syntax as we did with the example applications shown earlier in the chapter. But when the slot is actually a Qt slot rather than a Python method, it is more efficient to use the SLOT() syntax:

self.connect(dial, SIGNAL('valueChanged(int)'), spinbox, SLOT('setValue(int)')) self.connect(spinbox, SIGNAL('valueChanged(int)'), dial, SLOT('setValue(int)'))

We have already seen that it is possible to connect multiple signals to the same slot. 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.

Connections are made using QObject.connect(); they can be broken using QOb-ject.disconnect(). In practice, we rarely need to break connections ourselves since, for example, PyQt will automatically disconnect any connections involving an object

Pyqt Signals And Slots