GUI event blocking

kshegunov

What about stack traceback when the second click causes "re-entrance" to the function, which I can break on? I'm wondering whether that might show me enough to get back, presumably through an event processing loop invocation, to where the code is allowing that to happen?

Yes, it should be possible, but I imagine it'd land you in the slot handling the actual button click signal. I'd start investigating from that slot anyway - there should be some hint in it. Probably a class is starting a thread internally or something like this. The other option is that someone is calling QCoreAppliction::processEvents intermittently to process the pending events, as already mentioned by @sierdzio, then you will actually need to find the code doing it and act accordingly. My best advice is to get into step-by-step debugging and inspect what's called where in that signal handler, it may be quite a lot of work, but I don't know of any quick and easy way.

As a "workaround", you could disable the button after it being clicked, and reenable it whenever the long operation has completed, so you don't get the problem with calling the heavy code twice.

JonB

@kshegunov
Thanks, I will investigate.

One more time, as nobody has said "yay" or "nay", do you think making one of the QSql... calls to access the database spins a thread/event loop which could be the cause???

As a "workaround", you could disable the button after it being clicked, and reenable it whenever the long operation has completed, so you don't get the problem with calling the heavy code twice.

As I said, I have already done this. Better than nothing. But not robust enough, as there are many other widgets on the window which user could interact with... And even is they do not re-invoke the long-running code, I cannot allow any other code to execute while one piece is already executing, for all the obvious reasons...

kshegunov

@JonB said in GUI event blocking:

do you think making one of the QSql... calls to access the database spins a thread/event loop which could be the cause???

Nope, I don't. As far as I have worked and looked at the SQL classes none of them (drivers) does use threading and/or event loops.

I cannot allow any other code to execute while one piece is already executing, for all the obvious reasons...

Then another option is to "quick-fix" it by setting a boolean flag just before the long operation starts, if it's raised then just quick return and do nothing, otherwise raise it and do the crunching. Whenever the operation's done, just lower the flag.

JonB

@kshegunov

Thanks for the heads-up on QSql calls not being the cause.

I cannot allow any other code to execute while one piece is already executing, for all the obvious reasons...

Then another option is to "quick-fix" it by setting a boolean flag just before the long operation starts, if it's raised then just quick return and do nothing, otherwise raise it and do the crunching. Whenever the operation's done, just lower the flag.

That's what I'm saying I've done. But I think (politely) you're missing the point on re-entrancy. You're concentrating only on preventing the long-runner from being re-invoked. Yes, I do that. But you need to think the other way round as well. While something is running, no other action is safe to perform. You mustn't, for example, close the dialog, or even request something from the database, or anything else you can think of, in response to a new UI action (let's pretend the window has 100 other buttons/views/widgets/tooltips/menus which do goodness knows what; I don't want to go through 100 functions testing for flag), because we don't know what state the application, or the database, or anything else, is in, until long-runner completes. Which is what we would get if clicking to run long-runner in the first place blocked, as it's "supposed" to.... And that's what I want to get back to.

kshegunov

@JonB said in GUI event blocking:

But I think (politely) you're missing the point on re-entrancy.

Probably, but then again I don't have the code at hand, so I'm just more or less guessing. Your point seems warranted though, so the only thing I can add here is: have a light and easy debugging. :)

JonB

@kshegunov
Thanks; as I said, my comment was intended "politely". I have tried to clarify the situation in previous post. Once a single UI operation is underway, it's not safe to allow any other one to proceed, unless you've written code in a very particular way. And I didn't even write this code, let alone know just what it does!

ambershark

@JonB said in GUI event blocking:

The question for me is: what might it be in the original code processing which is allowing the second UI interaction to proceed?

Well there are really only 2 things that can do this. Threading and events. So either the signal handler for the click is exiting after sending events or it's exiting after signalling a thread.

To be honest it should be pretty easy to debug. Just point a breakpoint in the signal handler for that click event, then step through it. Step over function calls and test how long they take. If everything exits quickly (what I'm guessing will happen) and the signal handler exits, before your data is processed, then you just start stepping in to each of the functions in that click handler until you see what they are doing.

At some point you will find one that signals something to do some work and exits (via thread or event). That is your problem. Address that area and you should be good to go. I would just use something easy like a modal progress dialog that locks up the gui automatically until you signal it to quit when you get back from the processing of the "click".

Oh and it always sucks working on someone else's code.. Especially confusing/bad code. When I get in messy code I use a tool called SourceTrail to navigate code.

https://www.sourcetrail.com

JonB

@ambershark
OK, now had a chance to break and examine stack.

Nobody said this code would be easy...

The "long running operation" involves producing HTML, and saving to PDF file (think of it like a report). I do this via QWebEngineView (partly because this is in shared code, at other times the generated HTML is displayed to the user and is editable prior to producing the PDF). In this case the view is never displayed interactively to the user, but it still follows that route.

Because QWebEngineView is asynchronous (its own thread, I believe), and here we need the HTML/PDF synchronously, before we can save the PDF to file we must await QWebEngineView finished loading (else we get empty PDF saved to file). I have code there like:

    def synchronousRenderHtml(self):
        # see synchronousWebViewLoaded() below
        self.rendered = False
        self.webView.loadFinished.connect(self.synchronousWebViewLoaded)
        #
        # see http://doc.qt.io/qt-5/qtwebenginewidgets-qtwebkitportingguide.html
        # because setHtml() is asynchronous, the call to make editable has to be moved into synchronousWebViewLoaded()
        htmlFile = filesystemfunctions.findFileInDataOrCodeDirectory(Paths.s166Html())
        self.webView.setHtml(self.html, QUrl.fromLocalFile(str(htmlFile)))
        #
        # see synchronousWebViewLoaded() below
        if not self.rendered:
            self.renderLoop.exec()

    def synchronousWebViewLoaded(self):
        # This function is called on self.webView.loadFinished() from synchronousRenderHtml() above
        # The code uses this because self.webView.setHtml() is asynchronous
        # so in effect this implements a "blocking" call (with event processing) in synchronousRenderHtml()
        # This is the only place in code where a QEventLoop is used explicitly
        # It's all very complicated, and I did wish this behaviour was not needed
        # but it seems, at minimum in the case where this dialog is used non-interactively, it is :(
        self.qWebEngineView.page().runJavaScript("document.documentElement.contentEditable = true")
        self.rendered = True
        # cause the self.renderLoop.exec() in synchronousRenderHtml() above to exit now
        self.renderLoop.quit()

Turns out, the re-entrant call for the top-level button being pressed a second time is coming from the self.renderLoop.exec() (which is awaiting the self.renderLoop.quit()).

Cutting a long story short, unless someone can suggest a better way than this self.renderLoop.exec() principle(?), I think I have to stick with that. Hence the button re-entrancy, and I'll have to live with the need to using a re-entrancy blocking flag....

kshegunov

@JonB said in GUI event blocking:

Cutting a long story short, unless someone can suggest a better way than this self.renderLoop.exec() principle(?),

Not in this particular case. But I have a suggestion:
Grab the mouse and keyboard in synchronousRenderHtml and discard those events in your event() override while self.rendered is false and you're owning the user input. In synchronousWebViewLoaded release back the mouse and keyboard. Tread carefully though, as grabbing the mouse and keyboard can make your system unusable (i.e. use -nograb with gdb to prevent you from cursing). ;)

JonB

@kshegunov
I did think about that "dirty" way of handling things.

However (apart from the debugging issue, I use Python + PyCharm debugger, not gdb, and I wouldn't hold your breath that it will have a -nograb functionality!), the problem will be which "event() override" you have in mind.

Because, as I said, the QWebEngineView, which produces the HTML/PDF asynchronously and has the synchronousRenderHtml() call, is not displayed to the user in this case, the UI events are being received to the original dialog where a button is pressed to produce the output. This in itself has no connection to the ("invisible") QWebEngineView --- it doesn't even know one is being used, the implementation is invisible to it. I would have to modify its event handling. And since that would break encapsulation, I really don't feel it would be a good idea to implement!

So I will soldier on with the "blocking flag" I currently have to prevent "re-entrancy", which at least works reasonably well in practice. At least I now understand why this whole behaviour is caused.

kshegunov

@JonB said in GUI event blocking:

the problem will be which "event() override" you have in mind.

That would be the event() override of the class (widget) in which you grab the input. When you grab the input events all of them are redirected to the widget that called the mentioned functions, so you'd need to filter them in that same class. Basically, you grab the input in the widget/dialog/w/e and filter the input there. Whenever the operation's done (i.e. you connect the finished signal to a slot in that same dialog you release the mouse/keyboard.

JonB

@kshegunov
Sorry, my friend, but as per the explanation I wrote above, I still don't understand which widget's events you mean.

Here is the architecture of how the code works:

The user starts from a dialog, which has buttons on it, like Run long-running report.
That button on that dialog makes call produceReportAndWaitForFinish().
Behind the scenes, produceReportAndWaitForFinish() goes: create invisible QWebEnginePage and await final HTML/PDF returned, invisible QWebEnginePage calls synchronousRenderHtml(), which in turn has the self.renderLoop.exec() & self.renderLoop.quit() inside itself (the QWebEnginePage), in order to produce the HTML/PDF.
You want me to place the "event filter" there (inside the QWebEnginePage), when I call self.renderLoop.exec().
However, I don't think that (the QWebEnginePage) receives any inputs. The inputs (which would cause re-entrancy) are directed to buttons on the original top-level dialog where the user originally clicked the Run long-running report button, which I don't want him to click again.

If that is correct(?), that is why I don't want to fiddle inside QWebEnginePage with the events directed to the top-level dialog, because of encapsulation/separation of code. The QWebEnginePage does not know if it was invoked from a dialog in the first place, and the dialog has no idea that a QWebEnginePage (which will do threads/events) is involved in the production of the HTML/PDF.

kshegunov

@JonB said in GUI event blocking:

If that is correct

Nope. I mean exactly the dialog with the buttons. See (untested) code below, which I hope will clear this up:

class DialogWithButtons : public QDialog
{
public:
    DialogWithButtons(QWidget * parent);

public slots:
    void produceReportAndWaitForFinish();
    void reportProduced();

protected:
    bool event(QEvent *) override;
private:
    QPushButton startLongRunningOpButton;
    bool inputBlocked;
}

DialogWithButtons::DialogWithButtons(QWidget * parent)
    : QWidget(parent), startLongRunningOpButton(this), inputBlocked(false)
{
    // Just connect the button
    QObject::connect(&startLongRunningOpButton, &QPushButton::clicked, this, &DialogWithButtons::produceReportAndWaitForFinish);
}

void DialogWithButtons::produceReportAndWaitForFinish()
{
    inputBlocked = true;
    grabMouse();
    grabKeyboard();

    // Create/Init the webengine and so on, put to render and all that goodness. 
    QWebEngineView webEngine;

    // Connect the handlers & QEventLoop blocking
    QObject::connect(&webEngine, &QWebEngineView::loadFinished, this, &DialogWithButtons::reportProduced);

    QEventLoop loop;
    QObject::connect(&webEngine, &QWebEngineView::loadFinished, &loop, &QEventLoop::quit);
    loop.exec();   // Wait for processing to finish
}

void DialogWithButtons::reportProduced()
{
     releaseMouse();
     releaseKeyboard();
     inputBlocked = false;
}

bool DialogWithButtons::event(QEvent * e)
{
    if (inputBlocked && dynamic_cast<QInputEvent *>(e))
        return true;   // Filter out input events - we receive all of them if `inputBlocked` is true
 
    return QDialog::event(e);  // Input was not blocked or event was not UI input - delegate to the default implementation
}

JonB

@kshegunov
Yes, OK, I thought so.

The problem is, my produceReportAndWaitForFinish() in the top-level dialog does not have any kind of

    // Create/Init the webengine and so on, put to render and all that goodness. 
    QWebEngineView webEngine;

inside it (and I don't want it to). It has more like:

    //  Call *completely opaque* ReportProducer to generate HTML/PDF
    //  Here we have *no knowledge* of how ReportProducer functions
    //  and to maintain code separation/encapsulation we do not want to know here
    //  We might completely change how ReportProducer works at any time
    //  without affecting any code/behaviour here in this dialog.
    ReportProducer rp;
    rp.doWhateverToProduceHtmlAndPdf()

So I do understand what your approach requires, but I hope you can see why I do not wish to go down that route. But thanks for your suggestion anyway!

kshegunov

@JonB said in GUI event blocking:

The problem is, my produceReportAndWaitForFinish() in the top-level dialog does not have any kind of
[snip]
inside it (and I don't want it to). It has more like:

This is of no consequence, I've put the event loop/QWebEngineView here only for completeness. You can connect the grab/release slots directly from the button/custom class that generates the pdf respectively. Basically, the same as what you have now, but including 1 event override and putting in the grabMouse and releaseMouse at the appropriate places.

JonB

@kshegunov
Hmm, so you mean in my dialog button:

blockInputs()
ReportProducer rp;
rp.doWhateverToProduceHtmlAndPdf()
releaseInputs()

Right?

kshegunov

Yep, that should work.

JonB

@kshegunov
So, to be pedantic, and hoping I'm not annoying you, I want precisely that code/behaviour wrapped around every single button handler I have anywhere in my code, just in case the code the button invokes happens to create a thread or process events which could cause re-entrance. Which was the reason I posted this thread in the very first place.

There is supposed to be nothing special about my doWhateverToProduceHtmlAndPdf(), it's a complete accident/coincidence that it happens to use something (a QWebEngineView) which itself has to spin an event loop to achieve its work. I might change that tomorrow. Which is why I wanted to isolate anything about its event behaviour down inside the class which creates the QWebEngineView and not up at the caller dialog level. This approach will work in this particular case, but not in other random places in code where a button's actions might happen to spin an event loop, and that bothers me....

Ah well, such are the dirty practicalities of coding...!

EDIT: Bear with me, I'm going to post a new thread to ask a particular question which would satisfy me, just to know if it's possible.... See https://forum.qt.io/topic/88481/is-it-possible-to-intercept-a-modal-dialog-s-event-loop

kshegunov

@JonB said in GUI event blocking:

This approach will work in this particular case, but not in other random places in code where a button's actions might happen to spin an event loop, and that bothers me....

You could in principle adapt it to the general case. Something along:

class UiGuard : public QObject
{
public:
    UiGuard(QWidget * ui)
        : widget(ui)
    {
        widget->grabMouse();
        widget->grabKeyboard();
        widget->installEventFilter(this);
    }

    UiGuard::~UiGuard() override
    {
        widget->releaseKeyboard();
        widget->releaseMouse();
    }

    bool eventFilter(QObject *, QEvent * event) override
    {
        return dynamic_cast<QInputEvent *>(event);  // Eat up the input events
    }

private:
    QWidget * widget;
};

And then you use in the places you have threads and/or event loops spinning, by just creating an instance:

// ...
{
    UiGuard guard(this);  // Prevent input events while in this method
    ReportProducer rp;
    rp.doWhateverToProduceHtmlAndPdf()
}

JonB

@kshegunov
Yes indeed, but as we said:

And then you use in the places you have threads and/or event loops spinning, by just creating an instance:

In principle I have no idea where code might have "threads and/or event loops". (Seriously, it took me days & debugging to even discover this particular button invoked code which did that, I had no idea it did! Which is my point.) So in effect I'd like this around every single UI slot handler (or probably 99% of them but not 1%), and I'm not prepared to edit thousands of places to do that! :)