grab(n) Tk 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 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 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
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 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
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grab(n) Tk grab(n)
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 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 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.
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grab(n) Tk grab(n)
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
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