grab(3)
NAME
grab - Confine pointer and keyboard events to a window
sub-tree
SYNOPSIS
$widget->grab $widget->grabOption
DESCRIPTION
This set of methods implement simple pointer and keyboard
grabs for Tk. Tk's grabs are different than the grabs
described in the Xlib documentation. When a grab is set
for a particular window, Tk restricts all pointer events
to the grab window and its descendants in Tk's window
hierarchy. Whenever the pointer is within the grab win
dow's subtree, the pointer will behave exactly the same as
if there had been no grab at all and all events will be
reported in the normal fashion. When the pointer is out
side $widget's tree, button presses and releases and mouse
motion events are reported to $widget, and window entry
and window exit events are ignored. The grab subtree
``owns'' the pointer: windows outside the grab subtree
will be visible on the screen but they will be insensitive
until the grab is released. The tree of windows under
neath the grab window can include top-level windows, in
which case all of those top-level windows and their
descendants will continue to receive mouse events during
the grab.
Two forms of grabs are possible: local and global. A
local grab affects only the grabbing application: events
will be reported to other applications as if the grab had
never occurred. Grabs are local by default. A global
grab locks out all applications on the screen, so that
only the given subtree of the grabbing application will be
sensitive to pointer events (mouse button presses, mouse
button releases, pointer motions, window entries, and win
dow exits). During global grabs the window manager will
not receive pointer events either.
During local grabs, keyboard events (key presses and key
releases) are delivered as usual: the window manager con
trols which application receives keyboard events, and if
they are sent to any window in the grabbing application
then they are redirected to the focus window. During a
global grab Tk grabs the keyboard so that all keyboard
events are always sent to the grabbing application. The
focus method is still used to determine which window in
the application receives the keyboard events. The key
board grab is released when the grab is released.
Grabs apply to particular displays. If an application has
windows on multiple displays then it can establish a sepa
rate grab on each display. The grab on a particular dis
play affects only the windows on that display. It is pos
sible for different applications on a single display to
have simultaneous local grabs, but only one application
can have a global grab on a given display at once.
The grab methods take any of the following forms:
- $widget->grabCurrent
- Returns the current grab window in this application
for $widget's display, or an empty string if there is no such window. - $widget->grabs
- Returns a list whose elements are all of the windows
grabbed by this application for all displays, or an
empty string if the application has no grabs. - Not implemented yet!
- $widget->grabRelease
- Releases the grab on $widget if there is one, other
wise does nothing. Returns an empty string. - $widget->grab
- Sets a local grab on $widget. If a grab was already
in effect for this application on $widget's display
then it is automatically released. If there is
already a local grab on $widget, then the command does nothing. Returns an empty string. - $widget->grabGlobal
- Sets a global grab on $widget. If a grab was already
in effect for this application on $widget's display
then it is automatically released. If there is
already a global grab on $widget, then the command
does nothing. Returns an empty string. - $widget->grabStatus
- Returns none if no grab is currently set on $widget, local if a local grab is set on $widget, and global if a global grab is set.
BUGS
It took an incredibly complex and gross implementation to
produce the simple grab effect described above. Given the
current implementation, it isn't safe for applications to
use the Xlib grab facilities at all except through the Tk
grab procedures. If applications try to manipulate X's
grab mechanisms directly, things will probably break.
If a single process is managing several different Tk
applications, only one of those applications can have a
local grab for a given display at any given time. If the
applications are in different processes, this restriction
doesn't exist.
KEYWORDS
- grab, keyboard events, pointer events, window