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

Tk                                                                                                     grab(3tk)