#  Copyright (c) 1990-1994 The Regents of the University of California.
#  Copyright (c) 1994-1996 Sun Microsystems, Inc.
#  See the file "license.terms" for information on usage and redistribution
#  of this file, and for a DISCLAIMER OF ALL WARRANTIES.
#
#

=head1 NAME

Tk::focus - Manage the input focus

=for category User Interaction

=head1 SYNOPSIS

S<   >I<$widget>-E<gt>B<focus>

S<   >I<$widget>-E<gt>B<focus>I<Option>

S<   >I<$widget>-E<gt>B<focusNext>

S<   >I<$widget>-E<gt>B<focusPrev>

S<   >I<$widget>-E<gt>B<focusFollowsMouse>

=head1 DESCRIPTION

The B<focus> methods are used to manage the Tk input focus.
At any given time, one window on each display is designated as
the I<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 I<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 B<focusFollowsMouse> may be invoked to
create an I<implicit> focus model:  it reconfigures Tk so that
the focus is set to a window whenever the mouse enters it.
The methods B<focusNext> and B<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 B<focus> methods can take any of the following forms:

=over 4

=item I<$widget>-E<gt>B<focusCurrent>

Returns the focus window on the display containing
the I<$widget>,  or an empty string if no window in
this application has the focus on that display.

=item I<$widget>-E<gt>B<focus>

If the application currently has the input focus on I<$widget>'s
display, this command resets the input focus for I<$widget>'s display
to I<$widget> and returns an empty string.
If the application doesn't currently have the  input focus on
I<$widget>'s display, I<$widget> will be remembered as the focus
for its top-level;  the next time the focus arrives at the top-level,
Tk will redirect it to I<$widget>.

=item I<$widget>-E<gt>B<focusForce>

Sets the focus of I<$widget>'s display to I<$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.

=item I<$widget>-E<gt>B<focusLastfor>

Returns the name of the most recent window to have the input focus
among all the windows in the same top-level as I<$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 manager gives
the focus to the top-level.

=item I<$widget>-E<gt>B<focusNext>

=item I<$widget>-E<gt>B<focusPrev>

B<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 I<$widget> in focus order.
The focus order is determined by
the stacking order of windows and the structure of the window hierarchy.
Among siblings, the focus order is the same as the stacking order, with the
lowest window being first.
If a window has children, the window is visited first, followed by
its children (recursively), followed by its next sibling.
Top-level windows other than I<$widget> are skipped, so that
B<focusNext> never returns a window in a different top-level
from I<$widget>.

After computing the next window, B<focusNext> examines the
window's B<-takefocus> option to see whether it should be skipped.
If so, B<focusNext> continues on to the next window in the focus
order, until it eventually finds a window that will accept the focus
or returns back to I<$widget>.

B<focusPrev> is similar to B<focusNext> except that it
sets the focus to the window just before I<$widget> in the focus order.

=item I<$widget>-E<gt>B<focusFollowsMouse>

B<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 B<focus> command 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
B<focusFollowsMouse>.

=back

=head1 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.

=head1 CAVEATS

Note that for the B<Canvas> widget, the call to B<focus> has to be
fully qualified. This is because there is already a focus method for
the B<Canvas> widget, which sets the focus on individual canvas tags.

S<    >I<$canvas>-E<gt>B<Tk::focus>


=head1 KEYWORDS

events, focus, keyboard, top-level, window manager

=cut