在 PyQt5/Pyside2 中窃取焦点

Question

I am creating a launcher, in the style of Albert , Alfred or uLauncher . My application runs in the background and shows up when a hotkey is pressed. I use pynput to listen to hotkeys. I cannot use PyQt5 hotkey's feature (can't I?) because I need to listen to keyboard events in the system scope, not only the application's scope.

When the shortcut is pressed, it calls the show() method of my widget. The only issue is that I can't get the focus back on my window, despite the use of raise_ , setFocus and activateWindow .

I found a (ugly) workaround that consists in openning a QMessageBox (+ tweaking its appearance to make it invisible, but I didn't put that in the example code) and closing it immediately after.

When I was working on Linux, that workaround was doing the job, and I was ready to forget how ugly it is for it does the job. But I switched to Windows (on which my app must run too), and now this cheeky trick seems to cause freeze then crash of my application. Karma? For sure.

Any ways, my application is useless if it cannot catch focus, so I'm asking two questions, and I'd be happy with only one being solved. :)

• Do you know why showing the QMessageBox causes a crash?
• Do you know any other way to get the focus back on my application?

Here is an example code to play with.

Thank you very much:)

EDIT : I just found out that even with deactivating the QMessageBox workaround, the application eventually crashes (after 5, 20, 30 calls of the hotkey). So the issue might as well be in the way I bind my shortcut to the GUI, I fear a thread issue, but this is way beyond my knowledge:/

import sys
from PyQt5.QtWidgets import QLineEdit, QApplication, QMessageBox
from PyQt5.QtCore import QSize, Qt, QEvent
from pynput import keyboard


class Launcher(QLineEdit):
    def __init__(self):
        super().__init__()
        self.setFixedSize(QSize(600, 50))
        self.setWindowFlags(Qt.FramelessWindowHint | Qt.WindowStaysOnTopHint)
        self.setPlaceholderText('Search...')

        self.installEventFilter(self)

        self.set_shortcut('<ctrl>+`')

    def set_shortcut(self, shortcut):
        def for_canonical(f):
            return lambda k: f(listener.canonical(k))

        hotkey = keyboard.HotKey(
            keyboard.HotKey.parse(shortcut),
            self.wake_up)

        listener = keyboard.Listener(
                on_press=for_canonical(hotkey.press),
                on_release=for_canonical(hotkey.release))

        listener.start()

    def wake_up(self):
        print('Waking up')
        self.show()
        self.cheeky_focus_stealer()

    def cheeky_focus_stealer(self):
        self.setFocus()
        self.raise_()
        self.activateWindow()

        # Working of linux, but causes freeze/crash on Windows 10
        message_box = QMessageBox(self)
        message_box.show()
        message_box.hide()

    def eventFilter(self, obj, event):
        if obj is self and event.type() == QEvent.KeyPress:
            if event.key() == Qt.Key_Escape:
                self.hide()
                return True

        return super().eventFilter(obj, event)


def main():
    app = QApplication(sys.argv)
    app.setQuitOnLastWindowClosed(False)

    window = Launcher()
    window.show()

    app.exec_()


if __name__ == "__main__":
    main()

Answer 1

I found my error, so I'm posting an updated piece of code here for it could be helpful to anyone trying to bind a global hotkey to a function that affects a GUI, aka two different thread communicating.

My mistake was indeed to bind the hotkey triggered action straight to my show() method, which implies that the pynput listenner thread will attempt to communnicate with the QApplication .

The trick is to use a pyqtSignal() and to ask it to trigger the show() method. The signal itself being trigger by the hotkey.

After doing that in a clean way, my cheeky_focus_stealer works again, because it is ran from the GUI thread.

import sys
from PyQt5.QtWidgets import QLineEdit, QApplication, QMessageBox
from PyQt5.QtCore import QSize, Qt, QEvent, QObject, pyqtSignal
from pynput import keyboard


class Forwarder(QObject):
    signal = pyqtSignal()


class Launcher(QLineEdit):
    def __init__(self):
        super().__init__()
        self.setFixedSize(QSize(600, 50))
        self.setWindowFlags(Qt.FramelessWindowHint | Qt.WindowStaysOnTopHint)
        self.setPlaceholderText('Search...')

        self.installEventFilter(self)

        self.set_shortcut('<ctrl>+`')

    def set_shortcut(self, shortcut):
        # The forwarder must be parented to the Launcher
        forwarder = Forwarder(parent=self)
        forwarder.signal.connect(self.wake_up)

        def for_canonical(f):
            return lambda k: f(listener.canonical(k))

        hotkey = keyboard.HotKey(
            keyboard.HotKey.parse(shortcut),
            forwarder.signal.emit)

        listener = keyboard.Listener(
                on_press=for_canonical(hotkey.press),
                on_release=for_canonical(hotkey.release))

        listener.start()

    def wake_up(self):
        print('Waking up')
        self.show()
        self.cheeky_focus_stealer()

    def cheeky_focus_stealer(self):
        self.setFocus()
        self.raise_()
        self.activateWindow()

        # Working of linux, but causes freeze/crash on Windows 10
        message_box = QMessageBox(self)
        message_box.show()
        message_box.hide()

    def eventFilter(self, obj, event):
        if obj is self and event.type() == QEvent.KeyPress:
            if event.key() == Qt.Key_Escape:
                self.hide()
                return True

        return super().eventFilter(obj, event)


def main():
    app = QApplication(sys.argv)
    app.setQuitOnLastWindowClosed(False)

    window = Launcher()
    window.show()

    app.exec_()


if __name__ == "__main__":
    main()