1grab(n)                      Tk Built-In Commands                      grab(n)
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NAME

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

11       grab ?-global? window
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13       grab option ?arg arg ...?
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DESCRIPTION

17       This command implements simple pointer and keyboard grabs for Tk.  Tk's
18       grabs are different than the grabs described in the Xlib documentation.
19       When  a  grab  is set for a particular window, Tk restricts all pointer
20       events to the grab window and its descendants in Tk's window hierarchy.
21       Whenever  the  pointer is within the grab window's subtree, the pointer
22       will behave exactly the same as if there had been no grab  at  all  and
23       all events will be reported in the normal fashion.  When the pointer is
24       outside window's tree, button presses and  releases  and  mouse  motion
25       events  are reported to window, and window entry and window exit events
26       are ignored. The grab subtree “owns” the pointer: windows  outside  the
27       grab subtree will be visible on the screen but they will be insensitive
28       until the grab is released.  The tree of windows  underneath  the  grab
29       window  can  include top-level windows, in which case all of those top-
30       level windows and their descendants  will  continue  to  receive  mouse
31       events during the grab.
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33       Two  forms  of  grabs  are  possible:   local and global.  A local grab
34       affects only the grabbing application:   events  will  be  reported  to
35       other  applications as if the grab had never occurred.  Grabs are local
36       by default.  A global grab locks out all applications on the screen, so
37       that  only the given subtree of the grabbing application will be sensi‐
38       tive to pointer events (mouse button presses,  mouse  button  releases,
39       pointer  motions,  window  entries,  and  window exits).  During global
40       grabs the window manager will not receive pointer events either.
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42       During local grabs, keyboard events (key presses and key releases)  are
43       delivered  as  usual:   the  window  manager controls which application
44       receives keyboard events, and if they are sent to  any  window  in  the
45       grabbing  application  then  they  are  redirected to the focus window.
46       During a global grab Tk grabs the keyboard so that all keyboard  events
47       are  always  sent  to  the  grabbing application.  The focus command is
48       still used to determine which window in the  application  receives  the
49       keyboard  events.   The  keyboard  grab  is  released  when the grab is
50       released.
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52       On macOS a global grab affects all windows created by one  Tk  process.
53       No  window  in  that  process  other  than  the grab window can even be
54       focused, hence no other window receives key or mouse events.   A  local
55       grab  on  macOS affects all windows created by one Tcl interpreter.  It
56       is possible to focus any window belonging to the Tk  process  during  a
57       local grab but the grab window is the only window created by its inter‐
58       preter which receives key or mouse events.  Windows  belonging  to  the
59       same  process but created by different interpreters continue to receive
60       key and mouse events normally.
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62       Grabs apply to particular displays.  If an application has  windows  on
63       multiple  displays  then  it can establish a separate grab on each dis‐
64       play.  The grab on a particular display affects  only  the  windows  on
65       that  display.   It  is possible for different applications on a single
66       display to have simultaneous local grabs, but only one application  can
67       have a global grab on a given display at once.
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69       The grab command can take any of the following forms:
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71       grab ?-global? window
72              Same as grab set, described below.
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74       grab current ?window?
75              If  window  is  specified,  returns the name of the current grab
76              window in this application for window's  display,  or  an  empty
77              string  if  there  is no such window.  If window is omitted, the
78              command returns a list whose elements are  all  of  the  windows
79              grabbed by this application for all displays, or an empty string
80              if the application has no grabs.
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82       grab release window
83              Releases the grab on window if  there  is  one,  otherwise  does
84              nothing.  Returns an empty string.
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86       grab set ?-global? window
87              Sets a grab on window.  If -global is specified then the grab is
88              global, otherwise it is local.  If a grab was already in  effect
89              for  this  application  on window's display then it is automati‐
90              cally released.  If there is already a grab on window and it has
91              the  same global/local form as the requested grab, then the com‐
92              mand does nothing.  Returns an empty string.
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94       grab status window
95              Returns none if no grab is currently set on window, local  if  a
96              local grab is set on window, and global if a global grab is set.
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WARNING

99       It  is  very  easy  to  use global grabs to render a display completely
100       unusable (e.g. by setting a grab on a widget which does not respond  to
101       events  and  not providing any mechanism for releasing the grab).  Take
102       extreme care when using them!
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BUGS

105       It took an incredibly complex and gross implementation to  produce  the
106       simple  grab effect described above.  Given the current implementation,
107       it is not safe for applications to use the Xlib grab facilities at  all
108       except  through the Tk grab procedures.  If applications try to manipu‐
109       late X's grab mechanisms directly, things will probably break.
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111       If a single process is managing several different Tk applications, only
112       one  of those applications can have a local grab for a given display at
113       any given time.  If the applications are in different  processes,  this
114       restriction does not exist.
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EXAMPLE

117       Set  a grab so that only one button may be clicked out of a group.  The
118       other buttons are unresponsive to the mouse until the middle button  is
119       clicked.
120              pack [button .b1 -text "Click me! #1" -command {destroy .b1}]
121              pack [button .b2 -text "Click me! #2" -command {destroy .b2}]
122              pack [button .b3 -text "Click me! #3" -command {destroy .b3}]
123              grab .b2
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SEE ALSO

126       busy(n)
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KEYWORDS

129       grab, keyboard events, pointer events, window
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133Tk                                                                     grab(n)
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