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 insen-
       sitive 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 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 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.

       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 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 com-
              mand 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  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.

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(n)