focus(3tk) Tk Built-In Commands focus(3tk)
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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 when-
ever 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 de-
scendant of that top-level to receive the focus); when the window man-
ager 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 nor-
mally 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 im-
plicit 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 con-
taining the application'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 applica-
tions using multiple displays.
focus window
If the application currently has the input focus on window's
display, this command resets the input focus for window's dis-
play 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 appli-
cation does not currently have the input focus for the display.
This command should be used sparingly, if at all. In normal us-
age, an application should not claim the focus for itself; in-
stead, it should wait for the window manager to give it the fo-
cus. If window is an empty string then the command does noth-
ing.
focus -lastfor window
Returns the name of the most recent window to have the input fo-
cus 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 containing the window with the focus.
However, Tk generates FocusIn and FocusOut events just as if the X fo-
cus 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 be-
fore 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(3tk)
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