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grab(n) Tk Built-In Commands grab(n)
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NAME
grab - Confine pointer and keyboard events to a window sub-tree
SYNOPSIS
grab ?-global? window
grab option ?arg arg ...?
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DESCRIPTION
This command implements simple pointer and keyboard grabs for Tk. Tk's grabs are differ-
ent than the grabs described in the Xlib documentation. When a grab is set for a particu-
lar 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 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 grab-
bing 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 window 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 controls which application receives keyboard events, and if they are
sent to any window in the grabbing application then they are redirected to the focus win-
dow. 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 win-
dow in the application receives the keyboard events. The keyboard 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 separate grab on each display. The grab on a particular display
affects only the windows on that display. It is possible 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 appli-
cation for window'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 windows
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, otherwise 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 dis-
play 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.
WARNING
It is very easy to use global grabs to render a display completely unusable (e.g. by set-
ting a grab on a widget which does not respond to events and not providing any mechanism
for releasing the grab). Take extreme care when using them!
BUGS
It took an incredibly complex and gross implementation to produce the simple grab effect
described above. Given the current implementation, it is not 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 applica-
tions are in different processes, this restriction does not exist.
EXAMPLE
Set a grab so that only one button may be clicked out of a group. The other buttons are
unresponsive to the mouse until the middle button is clicked.
pack [button .b1 -text "Click me! #1" -command {destroy .b1}]
pack [button .b2 -text "Click me! #2" -command {destroy .b2}]
pack [button .b3 -text "Click me! #3" -command {destroy .b3}]
grab .b2
KEYWORDS
grab, keyboard events, pointer events, window
Tk grab(n)
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On Apache
Under GNU General Public License
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