PyQt - how to detect and close UI if it's already running?

Question

I'm starting the UI from within Maya. If the UI hasn't been closed, running the UI again will completely freeze Maya (with the error "Event Loop is already running")

Manually closing the UI before re-running the script will prevent it from freezing up. But I guess that's not really practical.

Is there a way to detect if the UI I'm trying to run already exists? And possible force close it?

Answer 1

Here is a very simple PyQt5 solution using QLockFile:

from PyQt5 import QtCore, QtWidgets

lockfile = QtCore.QLockFile(QtCore.QDir.tempPath() + '/my_app_name.lock')

if lockfile.tryLock(100):
    app = QtWidgets.QApplication([])
    win = QtWidgets.QWidget()
    win.setGeometry(50, 50, 100, 100)
    win.show()
    app.exec()
else:
    print('app is already running')

---

There were a couple of fairly straightforward C++ solutions given on the Qt Wiki which no longer seem to exist. I ported one of them to PyQt, and have provided a sample script below. The original C++ solution has been split into two classes, because the messaging facility may not be needed.

PyQt5:

from PyQt5 import QtWidgets, QtCore, QtNetwork

class SingleApplication(QtWidgets.QApplication):
    messageAvailable = QtCore.pyqtSignal(object)

    def __init__(self, argv, key):
        super().__init__(argv)
        # cleanup (only needed for unix)
        QtCore.QSharedMemory(key).attach()
        self._memory = QtCore.QSharedMemory(self)
        self._memory.setKey(key)
        if self._memory.attach():
            self._running = True
        else:
            self._running = False
            if not self._memory.create(1):
                raise RuntimeError(self._memory.errorString())

    def isRunning(self):
        return self._running

class SingleApplicationWithMessaging(SingleApplication):
    def __init__(self, argv, key):
        super().__init__(argv, key)
        self._key = key
        self._timeout = 1000
        self._server = QtNetwork.QLocalServer(self)
        if not self.isRunning():
            self._server.newConnection.connect(self.handleMessage)
            self._server.listen(self._key)

    def handleMessage(self):
        socket = self._server.nextPendingConnection()
        if socket.waitForReadyRead(self._timeout):
            self.messageAvailable.emit(
                socket.readAll().data().decode('utf-8'))
            socket.disconnectFromServer()
        else:
            QtCore.qDebug(socket.errorString())

    def sendMessage(self, message):
        if self.isRunning():
            socket = QtNetwork.QLocalSocket(self)
            socket.connectToServer(self._key, QtCore.QIODevice.WriteOnly)
            if not socket.waitForConnected(self._timeout):
                print(socket.errorString())
                return False
            if not isinstance(message, bytes):
                message = message.encode('utf-8')
            socket.write(message)
            if not socket.waitForBytesWritten(self._timeout):
                print(socket.errorString())
                return False
            socket.disconnectFromServer()
            return True
        return False

class Window(QtWidgets.QWidget):
    def __init__(self):
        super().__init__()
        self.edit = QtWidgets.QLineEdit(self)
        self.edit.setMinimumWidth(300)
        layout = QtWidgets.QVBoxLayout(self)
        layout.addWidget(self.edit)

    def handleMessage(self, message):
        self.edit.setText(message)

if __name__ == '__main__':

    import sys

    key = 'app-name'

    # send commandline args as message
    if len(sys.argv) > 1:
        app = SingleApplicationWithMessaging(sys.argv, key)
        if app.isRunning():
            print('app is already running')
            app.sendMessage(' '.join(sys.argv[1:]))
            sys.exit(1)
    else:
        app = SingleApplication(sys.argv, key)
        if app.isRunning():
            print('app is already running')
            sys.exit(1)

    window = Window()
    app.messageAvailable.connect(window.handleMessage)
    window.show()

    sys.exit(app.exec_())

PyQt4:

# only needed for python2
import sip
sip.setapi('QString', 2)

from PyQt4 import QtGui, QtCore, QtNetwork

class SingleApplication(QtGui.QApplication):
    messageAvailable = QtCore.pyqtSignal(object)

    def __init__(self, argv, key):
        QtGui.QApplication.__init__(self, argv)
        # cleanup (only needed for unix)
        QtCore.QSharedMemory(key).attach()
        self._memory = QtCore.QSharedMemory(self)
        self._memory.setKey(key)
        if self._memory.attach():
            self._running = True
        else:
            self._running = False
            if not self._memory.create(1):
                raise RuntimeError(self._memory.errorString())

    def isRunning(self):
        return self._running

class SingleApplicationWithMessaging(SingleApplication):
    def __init__(self, argv, key):
        SingleApplication.__init__(self, argv, key)
        self._key = key
        self._timeout = 1000
        self._server = QtNetwork.QLocalServer(self)
        if not self.isRunning():
            self._server.newConnection.connect(self.handleMessage)
            self._server.listen(self._key)

    def handleMessage(self):
        socket = self._server.nextPendingConnection()
        if socket.waitForReadyRead(self._timeout):
            self.messageAvailable.emit(
                socket.readAll().data().decode('utf-8'))
            socket.disconnectFromServer()
        else:
            QtCore.qDebug(socket.errorString())

    def sendMessage(self, message):
        if self.isRunning():
            socket = QtNetwork.QLocalSocket(self)
            socket.connectToServer(self._key, QtCore.QIODevice.WriteOnly)
            if not socket.waitForConnected(self._timeout):
                print(socket.errorString())
                return False
            if not isinstance(message, bytes):
                message = message.encode('utf-8')
            socket.write(message)
            if not socket.waitForBytesWritten(self._timeout):
                print(socket.errorString())
                return False
            socket.disconnectFromServer()
            return True
        return False

class Window(QtGui.QWidget):
    def __init__(self):
        QtGui.QWidget.__init__(self)
        self.edit = QtGui.QLineEdit(self)
        self.edit.setMinimumWidth(300)
        layout = QtGui.QVBoxLayout(self)
        layout.addWidget(self.edit)

    def handleMessage(self, message):
        self.edit.setText(message)

if __name__ == '__main__':

    import sys

    key = 'app-name'

    # send commandline args as message
    if len(sys.argv) > 1:
        app = SingleApplicationWithMessaging(sys.argv, key)
        if app.isRunning():
            print('app is already running')
            app.sendMessage(' '.join(sys.argv[1:]))
            sys.exit(1)
    else:
        app = SingleApplication(sys.argv, key)
        if app.isRunning():
            print('app is already running')
            sys.exit(1)

    window = Window()
    app.messageAvailable.connect(window.handleMessage)
    window.show()

    sys.exit(app.exec_())