grab(3Tk) grab(3Tk)
grab - Confine pointer and keyboard events to a window sub-tree
grab ?-global? window
grab option ?arg arg ...?
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 toplevel
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 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.
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grab(3Tk) grab(3Tk)
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.
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 doesn't exist.
grab, keyboard events, pointer events, window
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