focus(3) User Contributed Perl Documentation 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 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
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 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 method focusFollowsMouse may be invoked to create an implicit focus
model: it reconfigures Tk so that the focus is set to a window when‐
ever the mouse enters it. The methods focusNext and focusPrev imple‐
ment 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 dis‐
play, 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 redirect it to $widget.
$widget->focusForce
Sets the focus of $widget's display to $widget, even if the appli‐
cation 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 top-level as $widget. If no win‐
dow 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.
$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 $widget.
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 eventually 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 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 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 Can‐
vas widget, which sets the focus on individual canvas tags.
$canvas->Tk::focus
KEYWORDS
events, focus, keyboard, top-level, window manager
perl v5.8.8 2004-02-28 focus(3)
