grab(n) Tk Built-In Commands grab(n)_________________________________________________________________NAMEgrab - Confine pointer and keyboard events to a window
sub-tree
SYNOPSISgrab ?-global? window
grab option ?arg arg ...?
_________________________________________________________________DESCRIPTION
This command implements 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 par-
ticular 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 window'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 outside window's
tree, button presses and releases and mouse motion events
are reported to window, 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 underneath the grab win-
dow can include top-level windows, in which case all of
those top-level windows and their descendants will con-
tinue 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 command 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.
Tk 1
grab(n) Tk Built-In Commands grab(n)
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 command can take any of the following forms:
grab ?-global? window
Same as grab set, described below.
grab current ?window?
If window is specified, returns the name of the
current grab window in this application for win-
dow's display, or an empty string if there is no
such window. If window is omitted, the command
returns a list whose elements are all of the win-
dows grabbed by this application for all displays,
or an empty string if the application has no grabs.
grab release window
Releases the grab on window if there is one, other-
wise does nothing. Returns an empty string.
grab set ?-global? window
Sets a grab on window. If -global is specified
then the grab is global, otherwise it is local. If
a grab was already in effect for this application
on window's display then it is automatically
released. If there is already a grab on window and
it has the same global/local form as the requested
grab, then the command does nothing. Returns an
empty string.
grab status window
Returns none if no grab is currently set on window,
local if a local grab is set on window, 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
Tk 2
grab(n) Tk Built-In Commands grab(n)
doesn't exist.
KEYWORDS
grab, keyboard events, pointer events, window
Tk 3
