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grab(3tk) Tk Built-In Commands 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.


On macOS a global grab affects all windows created by one Tk
    process. No window in that process other than the grab window can even be
    focused, hence no other window receives key or mouse events. A local grab on
    macOS affects all windows created by one Tcl interpreter. It is possible to
    focus any window belonging to the Tk process during a local grab but the
    grab window is the only window created by its interpreter which receives key
    or mouse events. Windows belonging to the same process but created by
    different interpreters continue to receive key and mouse events
  normally.


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:



  

  
Same as grab set, described below.

  

  
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.

  

  
Releases the grab on window if there is one, otherwise does
      nothing. Returns an empty string.

  

  
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.

  

  
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








SEE ALSO


busy(3tk)






KEYWORDS


grab, keyboard events, pointer events, window







  

    
    Tk