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/****************************************************************************
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** Documentation of focus handling in Qt.
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** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
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/*!
    \page focus.html
    \title Keyboard Focus
    \ingroup architecture
    \ingroup gui-programming
    \brief An overview of the keyboard focus management and handling.

    \keyword keyboard focus

    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.

    \section1 Focus Motion

    The customs which have evolved for directing keyboard focus to a
    particular widget are these: 

    \list 1

    \o The user presses \key Tab (or \key Shift+Tab).
    \o The user clicks a widget.
    \o The user presses a keyboard shortcut.
    \o The user uses the mouse wheel.
    \o The user moves the focus to a window, and the application must
       determine which widget within the window should get the focus.
    \endlist

    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.

    \section2 Tab or Shift+Tab

    Pressing \key Tab is by far the most common way to move focus
    using the keyboard. (Sometimes in data-entry applications Enter
    does the same as \key{Tab}; this can easily be achieved in Qt by
    implementing an \l{Events and Event Filters}{event filter}.)

    Pressing \key Tab, in all window systems in common use today,
    moves the keyboard focus to the next widget in a circular
    per-window list. \key Tab moves focus along the circular list in
    one direction, \key Shift+Tab in the other. The order in which
    \key 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, \key Tab generally moves focus in the order of widget
    construction.) \l{Qt Designer} provides a means of visually
    changing the tab order.

    Since pressing \key 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,
    \key Tab could skip this field, and the dialog could use one of
    these mechanisms: 

    \list 1

    \o If the program can determine whether the field is needed, it can
    move focus there when the user finishes entry and presses \gui OK, or when
    the user presses Enter after finishing the other fields. Alternately,
    include the field in the tab order but disable it. Enable it if it
    becomes appropriate in view of what the user has set in the other
    fields.

    \o The label for the field can include a keyboard shortcut that moves
    focus to this field.

    \endlist

    Another exception to \key Tab support is text-entry widgets that
    must support the insertion of tabs; almost all text editors fall
    into this class. Qt treats \key Ctrl+Tab as \key Tab and \key
    Ctrl+Shift+Tab as \key 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 \key Ctrl+Tab for other purposes,
    and many users aren't aware of \key Ctrl+Tab anyway, this isn't a
    complete solution.

    \section2 The User Clicks a Widget

    This is perhaps even more common than pressing \key Tab on
    computers with a mouse or other pointing device.

    Clicking to move the focus is slightly more powerful than \key
    Tab. While it moves the focus \e 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.

    \section2 The User Presses a Keyboard Shortcut

    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. \key
    Alt+P to step to the \underline{P}rinting page. It is easy to
    overdo this: there are only a few keys, and it's also important
    to provide keyboard shortcuts for commands. \key Alt+P is also
    used for Paste, Play, Print, and Print Here in the \l{Standard
    Accelerator Keys} list, for example.

    \section2 The User Rotates the Mouse Wheel

    On Microsoft Windows, mouse wheel usage is always handled by the
    widget that has keyboard focus. On Mac OS X 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, Mac OS X, and X11.

    \section2 The User Moves the Focus to This Window

    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.
*/
