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168 lines
8.5 KiB
168 lines
8.5 KiB
14 years ago
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/doc/focus.doc:36 -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>Keyboard Focus Overview</title>
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<font color="#004faf">Home</font></a>
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<font color="#004faf">All Classes</font></a>
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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Keyboard Focus Overview</h1>
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<p> <!-- index keyboard focus --><a name="keyboard-focus"></a>
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<p> Qt's widgets handle keyboard focus in the ways that have become
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customary in GUIs.
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<p> The basic issue is that the user's keystrokes can be directed at any
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of several windows on the screen, and any of several widgets inside
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the intended window. When the user presses a key, they expect it to go
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to the right place, and the software must try to meet this
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expectation. The system must determine which application the keystroke
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is directed at, which window within that application, and which widget
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within that window.
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<p> <h2> Focus motion
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</h2>
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<a name="1"></a><p> The customs which have evolved for directing keyboard focus to a
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particular widget are these:
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<ol type=1>
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<p> <li> The user presses Tab (or Shift+Tab) (or sometimes Enter).
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<p> <li> The user clicks a widget.
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<p> <li> The user presses a keyboard shortcut.
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<p> <li> The user uses the mouse wheel.
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<p> <li> The user moves the focus to a window, and the application must
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determine which widget within the window should get the focus.
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<p> </ol>
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<p> Each of these motion mechanisms is different, and different types of
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widgets receive focus in only some of them. We'll cover each of them
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in turn.
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<p> <h3> Tab or Shift+Tab.
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</h3>
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<a name="1-1"></a><p> Pressing Tab is by far the most common way to move focus using the
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keyboard. Sometimes in data-entry applications Enter does the same as
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Tab. We will ignore that for the moment.
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<p> Pressing Tab, in all window systems in common use today, moves the
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keyboard focus to the next widget in a circular per-window list. Tab
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moves focus along the circular list in one direction, Shift+Tab in the
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other. The order in which Tab presses move from widget to widget is
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called the tab order.
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<p> In Qt, this list is kept in the <a href="qfocusdata.html">QFocusData</a> class. There is one
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<a href="qfocusdata.html">QFocusData</a> object per window, and widgets automatically append
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themselves to the end of it when <a href="qwidget.html#setFocusPolicy">QWidget::setFocusPolicy</a>() is
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called with an appropriate <a href="qwidget.html#FocusPolicy-enum">QWidget::FocusPolicy</a>. You can customize
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the tab order using <a href="qwidget.html#setTabOrder">QWidget::setTabOrder</a>(). (If you don't, Tab
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generally moves focus in the order of widget construction.) <a href="designer-manual.html">Qt Designer</a> provides a means of visually
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changing the tab order.
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<p> Since pressing Tab is so common, most widgets that can have focus
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should support tab focus. The major exception is widgets that are
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rarely used, and where there is some keyboard accelerator or error
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handler that moves the focus.
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<p> For example, in a data entry dialog, there might be a field that is
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only necessary in one per cent of all cases. In such a dialog, Tab
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could skip this field, and the dialog could use one of these
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mechanisms:
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<p> <ol type=1>
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<p> <li> If the program can determine whether the field is needed, it can
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move focus there when the user finishes entry and presses OK, or when
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the user presses Enter after finishing the other fields. Alternately,
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include the field in the tab order but disable it. Enable it if it
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becomes appropriate in view of what the user has set in the other
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fields.
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<p> <li> The label for the field can include a keyboard shortcut that moves
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focus to this field.
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<p> </ol>
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<p> Another exception to Tab support is text-entry widgets that must
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support the insertion of tabs; almost all text editors fall into this
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class. Qt treats Control+Tab as Tab and Control+Shift+Tab as
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Shift+Tab, and such widgets can reimplement <a href="qwidget.html#event">QWidget::event</a>() and
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handle Tab before calling <a href="qwidget.html#event">QWidget::event</a>() to get normal processing of
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all other keys. However, since some systems use Control+Tab for other
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purposes, and many users aren't aware of Control+Tab anyway, this
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isn't a complete solution.
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<p> <h3> The user clicks a widget.
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</h3>
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<a name="1-2"></a><p> This is perhaps even more common than pressing Tab on computers with a
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mouse or other pointing device.
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<p> Clicking to move the focus is slightly more powerful than Tab. While
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it moves the focus <em>to</em> a widget, for editor widgets it also moves
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the text cursor (the widget's internal focus) to the spot where the
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mouse is clicked.
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<p> Since it is so common and people are used to it, it's a good idea to
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support it for most widgets. However, there is also an important
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reason to avoid it: you may not want to remove focus from the widget
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where it was.
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<p> For example, in a word processor, when the user clicks the 'B' (bold)
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tool button, what should happen to the keyboard focus? Should it
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remain where it was, almost certainly in the editing widget, or should
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it move to the 'B' button?
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<p> We advise supporting click-to-focus for widgets that support text
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entry, and to avoid it for most widgets where a mouse click has a
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different effect. (For buttons, we also recommend adding a keyboard
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shortcut: <a href="qbutton.html">QButton</a> and its subclasses make this very easy.)
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<p> In Qt, only the <a href="qwidget.html#setFocusPolicy">QWidget::setFocusPolicy</a>() function affects
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click-to-focus.
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<p> <h3> The user presses a keyboard shortcut.
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</h3>
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<a name="1-3"></a><p> It's not unusual for keyboard shortcuts to move the focus. This can
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happen implicitly by opening modal dialogs, but also explicitly using
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focus accelerators such as those provided by <a href="qlabel.html#setBuddy">QLabel::setBuddy</a>(), <a href="qgroupbox.html">QGroupBox</a> and <a href="qtabbar.html">QTabBar</a>.
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<p> We advise supporting shortcut focus for all widgets that the user may
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want to jump to. For example, a tab dialog can have keyboard shortcuts
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for each of its pages, so the user can press e.g. Alt+P to step to the
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<u>P</u>rinting page. But don't overdo this: there are only a few
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keys, and it's also important to provide keyboard shortcuts for
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commands. Alt+P is also used for Paste, Play, Print and Print Here in
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the <a href="accelerators.html">standard list of shortcuts</a>, for
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example.
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<p> <h3> The user uses the mouse wheel.
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</h3>
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<a name="1-4"></a><p> On Microsoft Windows, mouse wheel usage is always handled by the
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widget that has keyboard focus. On Mac OS X and X11, it's handled by
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the widget that gets other mouse events.
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<p> The way Qt handles this platform difference is by letting widgets move
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the keyboard focus when the wheel is used. With the right focus policy
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on each widget, applications can work idiomatically correctly on
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Windows, Mac OS X, and X11.
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<p> <h3> The user moves the focus to this window.
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</h3>
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<a name="1-5"></a><p> In this situation the application must determine which widget within
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the window should receive the focus.
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<p> This can be simple: if the focus has been in this window before, then
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the last widget to have focus should regain it. Qt does this
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automatically.
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<p> If focus has never been in this window before and you know where focus
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should start out, call <a href="qwidget.html#setFocus">QWidget::setFocus</a>() on the widget which
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should receive focus before you <a href="qwidget.html#show">QWidget::show</a>() it. If you don't,
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Qt will pick a suitable widget.
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<p>
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<!-- eof -->
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<p><address><hr><div align=center>
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<table width=100% cellspacing=0 border=0><tr>
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<td>Copyright © 2007
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<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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<td align=right><div align=right>Qt 3.3.8</div>
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</table></div></address></body>
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</html>
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