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focus(n) Tk Built-In Commands focus(n)
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NAME
focus - Manage the input focus
SYNOPSIS
focus
focus window
focus option ?arg arg ...?
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DESCRIPTION
The focus command is used to manage the Tk input focus. At any given time, one window on
each display is designated as the focus window; any key press or key release events for
the display are sent to that window. It is normally up to the window manager to redirect
the focus among the top-level windows of a display. For example, some window managers
automatically set the input focus to a top-level window whenever the mouse enters it;
others redirect the input focus only when the user clicks on a window. Usually the window
manager will set the focus only to top-level windows, leaving it up to the application to
redirect the focus among the children of the top-level.
Tk remembers one focus window for each top-level (the most recent descendant of that top-
level to receive the focus); when the window manager gives the focus to a top-level, Tk
automatically redirects it to the remembered window. Within a top-level Tk uses an
explicit focus model by default. Moving the mouse within a top-level does not normally
change the focus; the focus changes only when a widget decides explicitly to claim the
focus (e.g., because of a button click), or when the user types a key such as Tab that
moves the focus.
The Tcl procedure tk_focusFollowsMouse may be invoked to create an implicit focus model:
it reconfigures Tk so that the focus is set to a window whenever the mouse enters it. The
Tcl procedures tk_focusNext and tk_focusPrev implement a focus order among the windows of
a top-level; they are used in the default bindings for Tab and Shift-Tab, among other
things.
The focus command can take any of the following forms:
focus Returns the path name of the focus window on the display containing the applica-
tion's main window, or an empty string if no window in this application has the
focus on that display. Note: it is better to specify the display explicitly
using -displayof (see below) so that the code will work in applications using mul-
tiple displays.
focus window
If the application currently has the input focus on window's display, this command
resets the input focus for window's display to window and returns an empty string.
If the application does not currently have the input focus on window's display,
window will be remembered as the focus for its top-level; the next time the focus
arrives at the top-level, Tk will redirect it to window. If window is an empty
string then the command does nothing.
focus -displayof window
Returns the name of the focus window on the display containing window. If the
focus window for window's display is not in this application, the return value is
an empty string.
focus -force window
Sets the focus of window's display to window, even if the application does not cur-
rently have the input focus for the display. This command should be used spar-
ingly, if at all. In normal usage, an application should not claim the focus for
itself; instead, it should wait for the window manager to give it the focus. If
window is an empty string then the command does nothing.
focus -lastfor window
Returns the name of the most recent window to have the input focus among all the
windows in the same top-level as window. If no window in that top-level has ever
had the input focus, or if the most recent focus window has been deleted, then the
name of the top-level is returned. The return value is the window that will
receive the input focus the next time the window manager gives the focus to the
top-level.
QUIRKS
When an internal window receives the input focus, Tk does not actually set the X focus to
that window; as far as X is concerned, the focus will stay on the top-level window con-
taining the window with the focus. However, Tk generates FocusIn and FocusOut events just
as if the X focus were on the internal window. This approach gets around a number of
problems that would occur if the X focus were actually moved; the fact that the X focus is
on the top-level is invisible unless you use C code to query the X server directly.
EXAMPLE
To make a window that only participates in the focus traversal ring when a variable is
set, add the following bindings to the widgets before and after it in that focus ring:
button .before -text "Before"
button .middle -text "Middle"
button .after -text "After"
checkbutton .flag -variable traverseToMiddle -takefocus 0
pack .flag -side left
pack .before .middle .after
bind .before <Tab> {
if {!$traverseToMiddle} {
focus .after
break
}
}
bind .after <Shift-Tab> {
if {!$traverseToMiddle} {
focus .before
break
}
}
focus .before
KEYWORDS
events, focus, keyboard, top-level, window manager
Tk 4.0 focus(n)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
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