QWidget signal sent when menu action selected - how do I prevent this?

In more than one of my Qt applications I've noticed that, whenever the menu bar is clicked, the last signal to have been sent from a widget within the GUI is re-sent before the menu action is invoked. Most of the time this doesn't matter; but on some occasions it matters very much.

In a few cases where the widget's signal is connected to one of its o开发者_StackOverflow社区wn slots, it's straightforward to begin the slot with a

if (hasFocus())
{
    // ...
}

...block so that such spurious signals, not generated by the user actually clicking on the widget, can be ignored.

However, I've recently identified that this behaviour is responsible for several related bugs where the spurious signals are passed on through several layers of the program before being acted upon, so simply checking whether a particular widget has focus is not trivial to implement.

My question, therefore, is:

• why on earth does clicking on a menu item cause a signal to be emitted from a widget elsewhere on the screen? I can't find this behaviour documented anywhere?

• how do I stop it?

Many thanks,

Stephen.

As you deduced yourself, the QLineEdit::editingFinished signal can be emitted twice if the user press enter and the QLineEdit loses the focus later. An old bug was created but behaviour has not been changed : https://bugreports.qt.io/browse/QTBUG-40.

Your solution is fine : overload QLineEdit and check if the value has changed before triggering the signal. You can use the flag QLineEdit::isModified for the check : it has a default value of false and is changed to true whenever the user changes the line edit's contents. It has to be manually reset to false when you emit the signal.

You can also do that check in the receiver object, just checking if the value has a meaning (is different from the last recorded value for example) and process it or not.
Therefore the polluting signals would not be a problem. It is in my opinion the cleanest solution, then you have a robust model. Because the user could also hit enter a few times with the same text value and you have to handle this anyway.

You can also use QLineEdit::returnPressed but this requires the user to always press enter to validate a value. Not always intuitive but forces the user to explicitly validate the inputs.