


grab(n)                Tk Built-In Commands               grab(n)


NAME
       grab - Confine pointer and keyboard events to a window sub-tree

SYNOPSIS
       (grab window)

       (grab :global window)

       (grab option ?arg arg ...?)


DESCRIPTION
       This  procedure  implements  simple  pointer and keyboard grabs for Tk.
       Tk's grabs are different than the grabs described in the Xlib  documen-
       tation.   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 sensi-
       tive 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 procedure 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 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.




STk                            3.1                              1





grab(n)                Tk Built-In Commands               grab(n)


       The grab procedure can take any of the following forms:

       (grab  window)

       (grab :global window)
              Same as grab set, described below.

       (grab 'current )

       (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
              procedure 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  window)

       (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 pro-
              cedure 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 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 doesn't exist.


SEE ALSO
       focus





STk                            3.1                              2


