Tk Built-In Commands grab(1T)
NAME
grab - Confine pointer and keyboard events to a window sub-
tree
SYNOPSIS
grab ?-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 partic-
ular window, Tk restricts all pointer events to the grab
window and its descendants in Tk's window hierarchy. When-
ever 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 descen-
dants 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 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 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 keyboard grab is released
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Tk Built-In Commands grab(1T)
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 application for window's display,
or an empty string if there is no such window. If win-
dow is omitted, the command returns a list whose ele-
ments are all of the windows grabbed by this applica-
tion for all displays, or an empty string if the appli-
cation 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
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 com-
mand 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 com-
pletely unusable (e.g. by setting 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 isn't safe for applications to
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Tk Built-In Commands grab(1T)
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 appli-
cations, 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 doesn't
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
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWTk
Interface Stability Uncommitted
NOTES
Source for Tk is available on http:/opensolaris.org.
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