grab(3Tk)

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

SYNOPSIS
     grab ?-global? window
     grab option ?arg arg ...?


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.


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 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 doesn't exist.


KEYWORDS
     grab, keyboard events, pointer events, window