CODEWITHSUNDEEP
pythonpyqtpyqt5
Andrei Cioara
Andrei Cioara

Reputation: 3664

How to emit custom Events to the Event Loop in PyQt

I am trying to emit custom events in PyQt. One widget would emit and another would listen to events, but the two widgets would not need to be related.

In JavaScript, I would achieve this by doing

// Component 1
document.addEventListener('Hello', () => console.log('Got it'))

// Component 2
document.dispatchEvent(new Event("Hello"))

Edit: I know about signals and slots, but only know how to use them between parent and child. How would I this mechanism (or other mechanism) between arbitrary unrelated widgets?

Upvotes: 6

Views: 8210

Answers (1)

eyllanesc
eyllanesc

Reputation: 243897

In PyQt the following instruction:

document.addEventListener('Hello', () => console.log('Got it'))

is equivalent

document.hello_signal.connect(lambda: print('Got it'))

In a similar way:

document.dispatchEvent(new Event("Hello"))

is equivalent

document.hello_signal.emit()

But the big difference is the scope of the "document" object, since the connection is between a global element. But in PyQt that element does not exist.

One way to emulate the behavior that you point out is by creating a global object:

globalobject.py

from PyQt5 import QtCore
import functools

@functools.lru_cache()
class GlobalObject(QtCore.QObject):
    def __init__(self):
        super().__init__()
        self._events = {}

    def addEventListener(self, name, func):
        if name not in self._events:
            self._events[name] = [func]
        else:
            self._events[name].append(func)

    def dispatchEvent(self, name):
        functions = self._events.get(name, [])
        for func in functions:
            QtCore.QTimer.singleShot(0, func)

main.py

from PyQt5 import QtCore, QtWidgets
from globalobject import GlobalObject


class MainWindow(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        super().__init__(parent)
        button = QtWidgets.QPushButton(text="Press me", clicked=self.on_clicked)
        self.setCentralWidget(button)

    @QtCore.pyqtSlot()
    def on_clicked(self):
        GlobalObject().dispatchEvent("hello")


class Widget(QtWidgets.QWidget):
    def __init__(self, parent=None):
        super().__init__(parent)
        GlobalObject().addEventListener("hello", self.foo)
        self._label = QtWidgets.QLabel()
        lay = QtWidgets.QVBoxLayout(self)
        lay.addWidget(self._label)

    @QtCore.pyqtSlot()
    def foo(self):
        self._label.setText("foo")


if __name__ == "__main__":
    import sys

    app = QtWidgets.QApplication(sys.argv)
    w1 = MainWindow()
    w2 = Widget()
    w1.show()
    w2.show()
    sys.exit(app.exec_())

Upvotes: 8

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