Qt : keyboard focus

Question

I am writing a program in Qt that looks like this:

The main window is my class Window : QWidget, it has a QGridLayout containing four other widgets (Input_Menu : QWidget and Output_Menu : QWidget, and then two Canvas : QWidget)

I would like to trigger certain events when the user strikes a key. The problem is, the Window sometimes loses focus (it goes, say to Input_Menu, or maybe a button in Input_Menu...)

I have tried the following solutions, but they seem unsatisfactory (and dirty):

1. Give Window the focus whenever it loses it.
2. Tell each widget who could have the focus to trigger Window's KeyPressEvent function (or a clone of it) whenever it receives a keyboard event.

Ideally, I would like that if a widget receives an event (say a keyboard event) and doesn't know what to do with it, it should automatically call its parent's event handler. I would have hoped this to be a default feature of Qt but it doesn't look like it. On the other hand I am really confused about the whole focus thing, I don't really get what's going on. Can someone explain this to me: I have included a std::cout << "key pressed" << std::endl; in my Window::KeyPressEvent function. When I first run my program, it seems the focus is on the top QComboBox in Input_Menu: if I hit the Up/Down keys, I navigate in that box and no "key pressed" is showed in my console. If I hit most letters, nothing happens. But if I hit Left/Right keys, I do get a "key pressed" in my console!?

Thanks a lot in advance for your insights.

Answer 1

You can install an event filter on QApplication to filter the relevant QEvent::KeyPress events globally. From the Qt documentation:

It is also possible to filter all events for the entire application, by installing an event filter on the QApplication or QCoreApplication object. Such global event filters are called before the object-specific filters. This is very powerful, but it also slows down event delivery of every single event in the entire application; the other techniques discussed should generally be used instead.

Besides the performance considerations, remember to check if your window currently has the focus before you filter the key event, or you might break popup dialogs or input into other windows.

Answer 2

Actually, I found that for keys that are modifiers (such as Shift, Control), Qt supports finding out whether they are pressed.

Eg : if(QApplication::keyboardModifiers() == Qt::ShiftModifier) ...

This is good enough.