Cannot change value/text after setValue in paintEvent

[namasikanam]

Problem

In an Qt Widgets Application, I cannot change the value of my QSpinBox after I setValue for it in overloaded paintEvent of MainWindow.

Reproduce the issue

I can reproduce the issue in the following steps.

1. Create a QtWidget Application by Qt Creator
2. Create a QSpinBox by embedded Qt Designer
3. Add following code to mainwindow.h

void paintEvent(QPaintEvent *);

4. Add following code to mainwindow.cpp

void MainWindow::paintEvent(QPaintEvent *) { ui->spinBox->setValue(2); }

5. Build and run the application.
6. Try to change the value of the spinBox and you will find you cannot change the value.

I can move my function setValue to another place to avoid this issue, but why this happen? I can't find what mistake that I make.

Some trials

• I tested QDoubleSpinBox and QLineEdit, they behave similarly.
• Trying to setReadOnly(false), it doesn't work.
• When I try to change the value, I get signal of valueChanged(int) emiting, but setValue(int) does not work anymore.
• I read documentation about QPaintEvent and QAbstractSpinBox. I can't find anything useful.

Help me please!

Environments

OS: Windows10
Qt Compiler: Desktop Qt 5.11.1 MinGW 32bit

Source Code

The complete source code can be found in mainwindow.h, main.cpp, mainwindow.cpp, mainwindow.ui.

[Christian Ehrlicher]

First, you don't call the baseclass implementation of paintEvent() so where should the painting occour?
Second the documentation of QWidget::paintEvent() says that you should not call update() inside paintEvent:

Note: Generally, you should refrain from calling update() or repaint() inside a paintEvent(). For example, calling update() or repaint() on children inside a paintEvent() results in undefined behavior; the child may or may not get a paint event.

[SGaist]

Hi,

To add to @Christian-Ehrlicher, why are you using the paint event to change a value in another widget ?

[namasikanam]

@Christian-Ehrlicher Thank you. I tried to search paintEvent() in documentation but I failed to find it. (Suggest Qt Documentation to support search a function) As your first question, the baseclass called in main.cpp, which generated automatically by wizard.

[namasikanam]

@SGaist Thx for replying. I simplify a large project(my Qt assignment) to the routine in my question. At first, I change a value just because of convenience, and I don't think it could be any problem at all. I spend several hours to debug and simplify it, I think my effort deserve a reason, why I can't use the paint event to change a value.

[Christian Ehrlicher]

@namasikanam said in Cannot change value/text after setValue in paintEvent:

As your first question, the baseclass called in main.cpp, which generated automatically by wizard.

You code above does not call the base class paint event implementation...

[SGaist]

@namasikanam said in Cannot change value/text after setValue in paintEvent:

@SGaist Thx for replying. I simplify a large project(my Qt assignment) to the routine in my question. At first, I change a value just because of convenience, and I don't think it could be any problem at all. I spend several hours to debug and simplify it, I think my effort deserve a reason, why I can't use the paint event to change a value.

Nobody said you can't, however as the name suggest the paintEvent method goal is to draw the widget. Changing a property value in it doesn't particularly make sense. Depending on what you do in your application or with your system, it can be called episodically to several times a second.

[namasikanam]

@Christian-Ehrlicher Sorry, I misunderstood you before. I doesn't call the paint event of base class because I think the default paintEvent of QMainWindow does nothing.