Strange errors from paintEngine

drmhkelley

In an app I've been developing, I've started to get a huge number of error messages related to QWidget::paintEngine and QPainter. I just noticed them for the first time this evening, bug am not really certain when they started. The errors are of the form shown below, and just about anything I do with the app triggers many of these errors.

How do I start to find the ultimate source of these errors? I haven't a clue what part of the app uses these components.

For reference, the the app is essentially a collection of text widgets - QLabels, QPushButtons,QLineEditors, QComboBoxes, QTextEditors, etc. packaged into various layout widgets.

What sorts of components need/use paintEngine and QPainter? I've not previously been aware of them.

Thanks for any clues where to begin.

QWidget::paintEngine: Should no longer be called
QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::translate: Painter not active
QPainter::save: Painter not active
QPainter::setClipRect: Painter not active
QPainter::pen: Painter not active
QPainter::setPen: Painter not active
QPainter::pen: Painter not active
QPainter::setPen: Painter not active
QPainter::restore: Unbalanced save/restore
QPainter::setPen: Painter not active

jsulm

@drmhkelley Do you call https://doc.qt.io/qt-5/qwidget.html#paintEngine anywhere in your app directly? Or do you use any third party libraries in your project?

drmhkelley

I don't knowingly call paint

jsulm

@drmhkelley What about my second question?

drmhkelley

I don't knowingly call paintEngine anywhere in my code. I do use third party libraries - some really third party and some where I am the third party. None of those libraries have any reason to interact with paintEngine directly.

jsulm

@drmhkelley Can you reproduce this issue with a simple Qt GUI application?

drmhkelley

Working on that part. I just ran another of my apps and it does not exhibit this behavior. It uses most of the same basic components and libraries, plus a bunch more. The second is a plotting app I use a lot, so I thought it might place more demands on many aspects of the the graphics sub-components. It does use most of the same text widgets.

jsulm

@drmhkelley I think one of the libs you're using causes this warnings. Try to disable one by one until there are no warnings any-more, then you know where the problem is.

Bonnie

Hi, have you ever used QPainter in any widgets then?

mrjj

@drmhkelley said

How do I start to find the ultimate source of these errors?

hi
You can also use the message handler trick
https://stackoverflow.com/questions/4954140/how-to-redirect-qdebug-qwarning-qcritical-etc-output

where you redirect all messages to your own function and there do a if/string compare with a breakpoint.
So when your breakpoint is triggered, you can see where the message comes from via the call stack.

drmhkelley

@Bonnie - I have never used QPainter, at least not directly. I found reference to in in one 3rd party package I use quite often, but it is not used in my the app currently under consideration. And one app that does use that package does not exhibit the symptom.

drmhkelley

@mrjj - Thank you for this suggestion. I have not tried this before, but it does sound like it will get me much closer much quicker.

Bonnie

@drmhkelley
I'm not sure what is your situation here, but from my experience, this kind of error usually caused by calling something like

QPainter p(this);
//use p to draw something

outside the paintEvent of a widget.

drmhkelley

@mrjj - Again, thanks very much for this suggestion. Solved my problem immediately. Had I been aware of this in the past, I would have addressed several glitches much more easily. Will now be a standard part of my toolkit.

As is too often the case, it solved my immediate problem, but now has me curious why I only now discovered that I should have been aware of the underlying problem long ago.

Detailed info follows.

It is often the case that the out-of-the-box QWidgets don't offer quite the capabilities I expect, so I've put wrappers around several to better meet my expectations. In this particular case, I wanted to intercept a couple of QEvents to process on my own while passing the bulk of QEvents on to Qt. I've done this for about a dozen QWidgets, including QLineEdit and QTextEdit. My version of the former I use a lot, the latter much less often.

My mistake was that for all such wrapper widgets, I passed the intercepted to QWidget, not specifically to QLineEdit or QTextEdit as I should have. In any event, Qt was quite unhappy that I passed events intended specifically for QTextEdit to QWidget.

I'm not surprised that this mistake would lead to unexpected outcomes. I am surprised that doing so for QTextEdit led to such different outcomes that QLineEdit, QLabel, QPushButton, QDateEdit, QSpinBox, QCalendar, QComboBox, etc.