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


       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 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 af-
       fects only the grabbing application:  events will be reported to	 other
       applications as if the grab had never occurred.	Grabs are local	by de-
       fault.  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 re-
       ceives keyboard events, and if they are sent to any window in the grab-
       bing  application then they are redirected to the focus window.	During
       a global	grab Tk	grabs the keyboard so that all keyboard	events are al-
       ways 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.

       It is very easy to use global grabs to render a display completely  un-
       usable  (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!

       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  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 does	not exist.

       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
	      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

       grab, keyboard events, pointer events, window

Tk								       grab(n)


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