How to ignore key press events that don't output characters - qt

Question

I'm developing a virtual musical keyboard that allows you to press keys on your keyboard and have it play notes. So in my MainWindow class I have reimplemented keyPressEvent. I would like to ignore events generated by modifiers (Control, Alt, Shift, etc.), as well as events generated by other non-character keys such as Tab, Backspace, and Enter. Something like this would be ideal:
void MainWindow::keyPressEvent(QKeyEvent *event)
{
if (!event->key().isCharacter()))
{
event->ignore();
return;
}
// handle the event
}
Is there a way to do something like that (short of testing every possible non-character key individually)? If not, is there at least a way to easily ignore the event if it is a modifier?

Answer 1

Looks like you want QKeyEvent::text().

Answer 2

The accepted answer is unfortunately incomplete. If you press Ctrl+C, then QKeyEvent::text() will return "\x03" (^C "End of Text").
That is not an empty string.
I decided to look inside the Qt source code, since they need to handle this problem for QLineEdit as well:
In QLineEdit::keyPressEvent:
d->control->processKeyEvent(event);
src/widgets/widgets/qlineedit.cpp:1742
In QWidgetLineControl::processKeyEvent:
if (unknown
&& !isReadOnly()
&& isAcceptableInput(event)) {
insert(event->text());
src/widgets/widgets/qwidgetlinecontrol.cpp:1912
In QInputControl::isAcceptableInput:
bool QInputControl::isAcceptableInput(const QKeyEvent *event) const
{
const QString text = event->text();
if (text.isEmpty())
return false;
const QChar c = text.at(0);
// Formatting characters such as ZWNJ, ZWJ, RLM, etc. This needs to go before the
// next test, since CTRL+SHIFT is sometimes used to input it on Windows.
if (c.category() == QChar::Other_Format)
return true;
// QTBUG-35734: ignore Ctrl/Ctrl+Shift; accept only AltGr (Alt+Ctrl) on German keyboards
if (event->modifiers() == Qt::ControlModifier
|| event->modifiers() == (Qt::ShiftModifier | Qt::ControlModifier)) {
return false;
}
if (c.isPrint())
return true;
if (c.category() == QChar::Other_PrivateUse)
return true;
if (c.isHighSurrogate() && text.size() > 1 && text.at(1).isLowSurrogate())
return true;
if (m_type == TextEdit && c == u'\t')
return true;
return false;
}
src/gui/text/qinputcontrol.cpp:21
This is exactly what you need but you may want to change the check for \t and ignore that as well.