grab(3tk) Tk Built-In Commands grab(3tk)
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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 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 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 af-
fects only the grabbing application: events will be reported to other
applications as if the grab had never occurred. Grabs are local by de-
fault. 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 re-
ceives keyboard events, and if they are sent to any window in the grab-
bing application then they are redirected to the focus window. During
a global grab Tk grabs the keyboard so that all keyboard events are al-
ways 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 when the grab is released.
On macOS a global grab affects all windows created by one Tk process.
No window in that process other than the grab window can even be fo-
cused, hence no other window receives key or mouse events. A local
grab on macOS affects all windows created by one Tcl interpreter. It
is possible to focus any window belonging to the Tk process during a
local grab but the grab window is the only window created by its inter-
preter which receives key or mouse events. Windows belonging to the
same process but created by different interpreters continue to receive
key and mouse events normally.
Grabs apply to particular displays. If an application has windows on
multiple displays then it can establish a separate grab on each dis-
play. 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 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 display then it is automati-
cally 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 completely un-
usable (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 is not safe for applications to use the Xlib grab facilities at all
except through the Tk grab procedures. If applications try to manipu-
late 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 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
SEE ALSO
busy(3tk)
KEYWORDS
grab, keyboard events, pointer events, window
Tk grab(3tk)
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