E2 - signals, slots and events

signals and slots

signals

notifications sent out by widgets when something happens

this can be due to pressing a button, changing input text, etc.
it can also send data for additional context
custom signals can be made too

slots

they are the receivers of signals

any function/method can be used as a slot
many Qt functions have built-in slots

import sys
from PyQt6.QtWidgets import QApplication, QMainWindow, QPushButton

class MainWindow(QMainWindow):

    def __init__(self):
        super().__init__()
        
        self.button_is_checked = True ## to store the button status

        self.setWindowTitle("My App")
        
        button = QPushButton("Press Me!")
        button.setCheckable(True)
        button.clicked.connect(self.the_button_was_clicked)
        button.setChecked(self.button_is_checked)
        
        self.setCentralWidget(button)

    def the_button_was_clicked(self):
	    self.button_is_checked = True
    
        print("Clicked!")


app = QApplication(sys.argv)

window = MainWindow()
window.show()

app.exec()

here, upon clicking the button (signal), the_button_was_clicked method (slot) is called

changing the interface

class MainWindow(QMainWindow):

    def __init__(self):
        super().__init__()

        self.setWindowTitle("My App")

        self.button = QPushButton("Press Me!")
        self.button.clicked.connect(self.the_button_was_clicked)
        
        self.setCentralWidget(self.button)
           

    def the_button_was_clicked(self):
        self.button.setText("You already clicked me.")
        self.button.setEnabled(False)

        self.setWindowTitle("New App")

here, clicking the button changes the window title and the button text, and makes it unclickable

connecting widgets together directly

Qt widgets can be directly connected to another

from PyQt6.QtWidgets import QApplication, QMainWindow, QLabel, QLineEdit, QVBoxLayout, QWidget

import sys


class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()

        self.setWindowTitle('my app')

        self.label = QLabel()

        self.input = QLineEdit()
        self.input.textChanged.connect(self.label.setText)

        layout = QVBoxLayout()
        layout.addWidget(self.input)
        layout.addWidget(self.label)
        
        container = QWidget()
        container.setLayout(layout)

        self.setCentralWidget(container)


app = QApplication(sys.argv)

window = MainWindow()
window.show()


app.exec()

here, the label is directly connected to input

events

any interaction the user has with a Qt application

these events are represented using event objects that package up information about what happened
these are then passed to specific event handlers
the event handlers for mouse events:

event handler	event type moved
`mouseMoveEvent`	mouse moved
`mousePressEvent`	mouse button pressed
`mouseReleaseEvent`	mouse button released
`mouseDoubleClickEvent`	double click detected

these are only registered when a mouse button is pressed
setMouseTracking(True) changes this behaviour
all mouse events are tracked with the QMouseEvent object