Qt's widgets handle keyboard focus in the ways that have become customary in GUIs.
The basic issue is that the user's key strokes can be directed at any of several windows on the screen, and any of several widgets inside the intended window. When the user presses a key, they expect it to go to the right place, and the software must try to meet this expectation. The system must determine which application the key stroke is directed at, which window within that application, and which widget within that window.
The customs which have evolved for directing keyboard focus to a particular widget are these:
Each of these motion mechanisms is different, and different types of widgets receive focus in only some of them. We'll cover each of them in turn.
Pressing Tab is by far the most common way to move focus using the keyboard. (Sometimes in data-entry applications Enter does the same as Tab; this can easily be achieved in Qt by implementing an event filter.)
Pressing Tab, in all window systems in common use today, moves the keyboard focus to the next widget in a circular per-window list. Tab moves focus along the circular list in one direction, Shift+Tab in the other. The order in which Tab presses move from widget to widget is called the tab order.
You can customize the tab order using QWidget::setTabOrder(). (If you don't, Tab generally moves focus in the order of widget construction.) Qt Designer provides a means of visually changing the tab order.
Since pressing Tab is so common, most widgets that can have focus should support tab focus. The major exception is widgets that are rarely used, and where there is some keyboard accelerator or error handler that moves the focus.
For example, in a data entry dialog, there might be a field that is only necessary in one per cent of all cases. In such a dialog, Tab could skip this field, and the dialog could use one of these mechanisms:
Another exception to Tab support is text-entry widgets that must support the insertion of tabs; almost all text editors fall into this class. Qt treats Ctrl+Tab as Tab and Ctrl+Shift+Tab as Shift+Tab, and such widgets can reimplement QWidget::event() and handle Tab before calling QWidget::event() to get normal processing of all other keys. However, since some systems use Ctrl+Tab for other purposes, and many users aren't aware of Ctrl+Tab anyway, this isn't a complete solution.
This is perhaps even more common than pressing Tab on computers with a mouse or other pointing device.
Clicking to move the focus is slightly more powerful than Tab. While it moves the focus to a widget, for editor widgets it also moves the text cursor (the widget's internal focus) to the spot where the mouse is clicked.
Since it is so common and people are used to it, it's a good idea to support it for most widgets. However, there is also an important reason to avoid it: you may not want to remove focus from the widget where it was.
For example, in a word processor, when the user clicks the 'B' (bold) tool button, what should happen to the keyboard focus? Should it remain where it was, almost certainly in the editing widget, or should it move to the 'B' button?
We advise supporting click-to-focus for widgets that support text entry, and to avoid it for most widgets where a mouse click has a different effect. (For buttons, we also recommend adding a keyboard shortcut: QAbstractButton and its subclasses make this very easy.)
In Qt, only the QWidget::setFocusPolicy() function affects click-to-focus.
It's not unusual for keyboard shortcuts to move the focus. This can happen implicitly by opening modal dialogs, but also explicitly using focus accelerators such as those provided by QLabel::setBuddy(), QGroupBox, and QTabBar.
We advise supporting shortcut focus for all widgets that the user may want to jump to. For example, a tab dialog can have keyboard shortcuts for each of its pages, so the user can press e.g. Alt+P to step to the Printing page. It is easy to overdo this: there are only a few keys, and it's also important to provide keyboard shortcuts for commands. Alt+P is also used for Paste, Play, Print, and Print Here in the Standard Accelerator Keys list, for example.
On Microsoft Windows, mouse wheel usage is always handled by the widget that has keyboard focus. On macOS and X11, it's handled by the widget that gets other mouse events.
The way Qt handles this platform difference is by letting widgets move the keyboard focus when the wheel is used. With the right focus policy on each widget, applications can work idiomatically correctly on Windows, macOS, and X11.
In this situation the application must determine which widget within the window should receive the focus.
This can be simple: If the focus has been in this window before, then the last widget to have focus should regain it. Qt does this automatically.
If focus has never been in this window before and you know where focus should start out, call QWidget::setFocus() on the widget which should receive focus before you call QWidget::show() it. If you don't, Qt will pick a suitable widget.