PySide, Maya callbacks and order of execution

[Enrico Losavio]

Hi all,

I write here because I'm dealing with an issue that makes my maya crash all the times.

I have found a workaround (explained later), but I'm still curious to know if there's a better solution.

Basically I want to display some messages to the user using a modal dialog (QMessageBox.information()).

I have a function (called from a ui) that is used to import a file in the maya scene. This is working fine.

I also have a callback attached to that piece of ui, which is meant to update it (by deleting it and recreating it) every time a file is imported.

What happens is that when I click on the Import button, the file is imported, the QMessageBox is shown, the code execution is paused (because of the modal dialog), but the callback is fired anyways.

Since the callback deletes (and recreates) the dialog, when the code finally reaches the ui "self.close()" instruction, the ui object was already destroyed - thus resulting in a # RuntimeError: Internal C++ object (ImportFileUI) already deleted. # or more often than not a sudden maya crash.

I temporarily avoided this error by calling the QMessageBox.show() method, which does not halt the execution of the code.

However, rather than finding more workarounds, I would like to find a solution and get to the bottom of this issue.

Does anyone have an idea of how to make the callback "wait"? Are there better practices to deal with a similar situation?

I tried to delay the execution of the callback using "evalDeferred()" and even "processIdleEvents()", but none of them really made any difference.

I have put together a few lines of codes to recreate the issue, and I tested it in Maya 2018.7 on Windows10.

from PySide2 import QtCore, QtWidgets
from maya.OpenMayaUI import MQtUtil
from pymel import core as pm
from shiboken2 import wrapInstance

# this code is an overly simplified version of the original script
# its only purpose is to recreate the issue described in the post above
# paste it the script editor, *change the ui's file_path variable to an existing .ma file* , and run it
#
# the callback destroys the ui object, before the ui.close() instruction can be executed
# causing a # RuntimeError: Internal C++ object (ImportFileUI) already deleted. # or often even a maya crash
# how can this be prevented?

ui_name = 'ImportFileUI'

def get_maya_window():
    pointer = MQtUtil.mainWindow()
    return wrapInstance(long(pointer), QtWidgets.QMainWindow)

class ImportFileUI(QtWidgets.QDialog):
    def __init__(self, parent):
        super(ImportFileUI, self).__init__(parent)
        self.setObjectName(ui_name)

        self.file_path = r"C:\example_file.ma"
        self.btn = QtWidgets.QPushButton("Import File")
        self.btn.clicked.connect(self.onBtnClicked)
        self.setLayout(QtWidgets.QVBoxLayout())
        self.layout().addWidget(self.btn)

    def onBtnClicked(self):
        pm.importFile(self.file_path, force=True)
        QtWidgets.QMessageBox.information(get_maya_window(), "Info", "Scene was imported correctly")
        self.close()

def create_ui():
    if pm.control(ui_name, exists=True):
        pm.deleteUI(ui_name)
    ui = ImportFileUI(get_maya_window())
    ui.show()
    pm.scriptJob(e=["PostSceneRead", create_ui], parent=ui_name)

if __name__ == "__main__":
    create_ui()

Thank you for your help!

Enrico
 

[Justin Israel]

On Wed, Sep 23, 2020 at 4:55 AM Enrico Losavio <enrico...@gmail.com> wrote:

Hi all,

I write here because I'm dealing with an issue that makes my maya crash all the times.

I have found a workaround (explained later), but I'm still curious to know if there's a better solution.

Basically I want to display some messages to the user using a modal dialog (QMessageBox.information()).

I have a function (called from a ui) that is used to import a file in the maya scene. This is working fine.

I also have a callback attached to that piece of ui, which is meant to update it (by deleting it and recreating it) every time a file is imported.

What happens is that when I click on the Import button, the file is imported, the QMessageBox is shown, the code execution is paused (because of the modal dialog), but the callback is fired anyways.

Since the callback deletes (and recreates) the dialog, when the code finally reaches the ui "self.close()" instruction, the ui object was already destroyed - thus resulting in a # RuntimeError: Internal C++ object (ImportFileUI) already deleted. # or more often than not a sudden maya crash.

I temporarily avoided this error by calling the QMessageBox.show() method, which does not halt the execution of the code.

However, rather than finding more workarounds, I would like to find a solution and get to the bottom of this issue.

Does anyone have an idea of how to make the callback "wait"? Are there better practices to deal with a similar situation?

I tried to delay the execution of the callback using "evalDeferred()" and even "processIdleEvents()", but none of them really made any difference.

Your example code may be oversimplified, leading to me missing your actual intention. But based on your example it seems the question should really be "how can I avoid the dialog widget crashing when it tries to close after already being deleted?". If we look at the order of operations, there is really no logical sense in trying to delay the scriptJob. It is separate logic from the dialog which wants to happen in response to a Maya event. It is a misconception that your modal information dialog is pausing code execution. What it is actually doing is starting its own blocking event loop at that point, which means the entire application can still continue to function and process events. Only the next line in that scope of the main thread is paused. As soon as you start the modal dialog, the new event loop will process the scriptJob. 

So now that we have addressed why the scriptJob callback still gets executed before the end of that slot function scope, we could take a moment to look at the suggestion of delaying the callback. If the callback is defined outside the scope of this dialog, as in your simplified example, I think it makes less sense to somehow try and defer it until after some other UI logic in your dialog, because that defeats the point of the callback. You might as well just not use the callback at all, and trigger the create_ui function directly at the point you deem appropriate (after the QMessageBox modal dialog has returned). But I will leave that suggestion here for your consideration, if it makes sense to do so.

The simplest fix I can see to keeping the scriptJob callback as-is and just preventing the close from crashing is to use deleteLater just before you start the modal dialog:

    def onBtnClicked(self):
        pm.importFile(self.file_path, force=True)
        self.deleteLater()
        QtWidgets.QMessageBox.information(get_maya_window(), "Info", "Scene was imported correctly")

I don't know if this approach makes sense in the scope of your actual production code. But it fixes the issue in the simplified version. I figured if you already intended on destroying the dialog after the QMessageBox anyways, that this would be equivalent. If this isn't acceptable, then maybe you can expand your example to cover something closer to your intentions.

Justin

 

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[Enrico Losavio]

Hi Justin,

Thank you so much for your reply.

Sorry for not providing better code. It was meant mainly to recreate the issue rather than describe my intent. I tried to keep it as short and readable as possible.

The original ui needs to be updated any time a new scene is loaded/saved/imported, and since this can also happen outside my code I need the callback.

I tried also your solution of adding the self.deleteLater() instruction. It can be a good fix in some situations, but it would not suffice if more ui methods were called after the callback is fired.

I've been researching and testing a bit more. I tried playing around with threading but that only led me to a deadlock. However maybe I came up with a solution which seems to be a bit more flexible: I've found a function QApplication.activeModalWidget() to get any existing (top-level) modal dialogs. So when the callback is fired I check if a modal dialog exists, and if so I connect its finished signal to the callback function. In this context, the executeDeferred() function seems to actually delay the execution of the callback and prevent maya from crashing.

def create_ui():

    def callback():
        # checks whether a modal dialog exists
        modal_dialog = QtWidgets.QApplication.activeModalWidget()
        if modal_dialog:
            # connects the ui's 'finished' signal to the (deferred) callback
            modal_dialog.finished.connect(lambda: maya.utils.executeDeferred(callback))
            return
        # call the ui creation function (only if a modal dialog doesn't exist)
        maya.utils.executeDeferred(create_ui)

    if pm.control(ui_name, exists=True):

        pm.deleteUI(ui_name)
    ui = ImportFileUI(get_maya_window())
    ui.show()

    pm.scriptJob(e=["PostSceneRead", callback], parent=ui_name)

I've tested also adding a bunch of instructions between the QMessageBox and the self.close(), and still the callback waits until all the commands are executed!

 

def onBtnClicked(self):

    pm.importFile(self.file_path, force=True)

    QtWidgets.QMessageBox.information(get_maya_window(), "Info", "Scene was imported correctly")

    for i in range(1000):

        print(self.objectName, i)
    self.close()

To me this seems a good solution, but I'd be curious to hear any feedback!

Enrico

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[Justin Israel]

Seems like you got a good solution that suits your use case. Have you played with the idea of making your QMessageBox parented to your ImportFileUI, and then only having to do the connection if the active modal widget is a child of your target UI? Maybe it doesn't matter. But I thought it would mean if someone was using a normal file dialog, or some other tool did a file operation that triggered your scriptJob, that you wouldn't really care if the modal widget was some other widget as opposed to yours. The only reason you are doing all of this extra logic is because you are trying to delete your own known widget while it might be in the middle of an operation.

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