focus(3)
NAME
focus - Manage the input focus
SYNOPSIS
$widget->focus $widget->focusOption $widget->focusNext $widget->focusPrev $widget->focusFollowsMouse
DESCRIPTION
The focus methods are used to manage the Tk input focus.
At any given time, one window on each display is desig
nated 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 applica
tion 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 method 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 methods focusNext and 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 methods can take any of the following forms:
- $widget->focusCurrent
- Returns the focus window on the display containing the
$widget, or an empty string if no window in this
application has the focus on that display. - $widget->focus
- If the application currently has the input focus on
$widget's display, this command resets the input focus for $widget's display to $widget and returns an empty string. If the application doesn't currently have the
input focus on $widget's display, $widget will be remembered as the focus for its top-level; the next
time the focus arrives at the top-level, Tk will redi
rect it to $widget. - $widget->focusForce
- Sets the focus of $widget's display to $widget, even
if the application doesn't currently have the input
focus for the display. This command should be used
sparingly, 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. - $widget->focusLast
- Returns the name of the most recent window to have the
input focus among all the windows in the same toplevel as $widget. 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 top-level is
returned. The return value is the window that will
receive the input focus the next time the window man
ager gives the focus to the top-level. - $widget->focusNext
$widget->focusPrev - focusNext is a utility method used for keyboard
traversal, but can be useful in other contexts. It
sets the focus to the ``next'' window after $widget in focus order. The focus order is determined by the
stacking order of windows and the structure of the
window hierarchy. Among siblings, the focus order is
the same as the stacking order, with the lowest window
being first. If a window has children, the window is
visited first, followed by its children (recursively),
followed by its next sibling. Top-level windows other
than $widget are skipped, so that focusNext never returns a window in a different top-level from $wid_
get. - After computing the next window, focusNext examines
the window's -takefocus option to see whether it
should be skipped. If so, focusNext continues on to
the next window in the focus order, until it eventu
ally finds a window that will accept the focus or
returns back to $widget. - focusPrev is similar to focusNext except that it sets
the focus to the window just before $widget in the
focus order. - $widget->focusFollowsMouse
- focusFollowsMouse changes the focus model for the
application to an implicit one where the window under
the mouse gets the focus. After this procedure is
called, whenever the mouse enters a window Tk will
automatically give it the input focus. The focus com
mand may be used to move the focus to a window other
than the one under the mouse, but as soon as the mouse
moves into a new window the focus will jump to that
window. Note: at present there is no built-in support
for returning the application to an explicit focus
model; to do this you'll have to write a script that
deletes the bindings created by focusFollowsMouse.
QUIRKS
When an internal window receives the input focus, Tk
doesn't 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
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.
CAVEATS
- Note that for the Canvas widget, the call to focus has to
be fully qualified. This is because there is already a
focus method for the Canvas widget, which sets the focus
on individual canvas tags. - $canvas->Tk::focus
KEYWORDS
- events, focus, keyboard, top-level, window manager