Ubuntu Manpage: grab - Confine pointer and keyboard events to a window sub-tree

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 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.

WARNING

       It is very easy to use global grabs to render a display completely 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 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 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

KEYWORDS

       grab, keyboard events, pointer events, window