Редактирование: PyQt FAQ Events and Signals

In this part of the PyQt4 programming tutorial, we will explore events and singnals occuring in applications.

== Events ==
Events are an important part in any GUI program. Events are generated by users or by the system. When we call the application's <i>exec_()</i> method, the application enters the main loop. The main loop fetches events and sends them to the objects. Trolltech has introduced a unique signal and slot mechanism.

== Signals &amp; Slots ==
Signals are emitted, when users click on the button, drag a slider etc. Signals can be emitted also by the environment. For example, when a clock ticks. A slot is a method, that reacts to a signal. In python, a slot can be any python callable. 
<source lang="python">
#!/usr/bin/python

# sigslot.py

import sys
from PyQt4 import QtGui, QtCore


class SigSlot(QtGui.QWidget):
    def __init__(self, parent=None):
        QtGui.QWidget.__init__(self, parent)

        self.setWindowTitle('signal & slot')

        lcd = QtGui.QLCDNumber(self)
        slider = QtGui.QSlider(QtCore.Qt.Horizontal, self)

        vbox = QtGui.QVBoxLayout()
        vbox.addWidget(lcd)
        vbox.addWidget(slider)

        self.setLayout(vbox)
        self.connect(slider,  QtCore.SIGNAL('valueChanged(int)'), lcd, 
		QtCore.SLOT('display(int)') )

        self.resize(250, 150)


app = QtGui.QApplication(sys.argv)
qb = SigSlot()
qb.show()
sys.exit(app.exec_())
</source>

In our example, we display an lcd number and a slider. We change the lcd number by dragging the slider. 

<source lang="python">
 self.connect(slider,  QtCore.SIGNAL('valueChanged(int)'), lcd, QtCore.SLOT('display(int)') )
</source>

Here we connect a <i>valueChanged()</i> signal of the slider to the <i>display()</i> slot of the lcd number.

The connect method has four parameters. The <b>sender</b> is an object that sends a signal. The <b>signal</b> is the signal, which is emitted. The <b>receiver</b> is the object, that receives the signal. Finally the <b>slot</b> is the method, that reacts to the signal.

[[image: pyqt_faq_signalslot.jpg | center]]

== Reimplementing event handler ==
Events in PyQt are processed mainly by reimplementing event handlers .
<source lang="python">
#!/usr/bin/python

# escape.py

import sys
from PyQt4 import QtGui, QtCore

class Escape(QtGui.QWidget):
    def __init__(self, parent=None):
        QtGui.QWidget.__init__(self, parent)

        self.setWindowTitle('escape')
        self.resize(250, 150)
        self.connect(self, QtCore.SIGNAL('closeEmitApp()'), QtCore.SLOT('close()') )


    def keyPressEvent(self, event):
        if event.key() == QtCore.Qt.Key_Escape:
            self.close()

app = QtGui.QApplication(sys.argv)
qb = Escape()
qb.show()
sys.exit(app.exec_())
</source>

In our example, we reimplement the <i>keyPressEvent()</i> event handler.

<source lang="python">
 def keyPressEvent(self, event):
     if event.key() == QtCore.Qt.Key_Escape:
         self.close()
</source>
If we click the escape button, we close the application.

== Emitting signals ==
Objects created from <i>QtCore.QObject</i> can emit signals. 
If we click on the button, a clicked() signal is generated. In the following example we will see, how we can emit signals.

<source lang="python">

#!/usr/bin/python

# emit.py

import sys
from PyQt4 import QtGui, QtCore


class Emit(QtGui.QWidget):
    def __init__(self, parent=None):
        QtGui.QWidget.__init__(self, parent)

        self.setWindowTitle('emit')
        self.resize(250, 150)
        self.connect(self, QtCore.SIGNAL('closeEmitApp()'), QtCore.SLOT('close()') )

    def mousePressEvent(self, event):
        self.emit(QtCore.SIGNAL('closeEmitApp()'))

app = QtGui.QApplication(sys.argv)
qb = Emit()
qb.show()
sys.exit(app.exec_())
</source>

We create a new signal called <i>closeEmitApp()</i>. This signal is emitted, during a mouse press event. 

<source lang="python">
 def mousePressEvent(self, event):
     self.emit(QtCore.SIGNAL('closeEmitApp()'))
</source>

Emitting a signal with the <i>emit()</i> method.


<source lang="python">
 self.connect(self, QtCore.SIGNAL('closeEmitApp()'), QtCore.SLOT('close()') )
</source>

Here we connect the manually created <i>closeEmitApp()</i> signal with the <i>close()</i> slot.

[[Категория:Qt]]
[[Категория:Python]]