1grab(n) Tk Built-In Commands grab(n)
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8 grab - Confine pointer and keyboard events to a window sub-tree
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11 grab ?-global? window
12 grab option ?arg arg ...?
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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.
32 Two forms of grabs are possible: local and global. A local grab af‐
34 fects only the grabbing application: events will be reported to other
35 applications as if the grab had never occurred. Grabs are local by de‐
36 fault. A global grab locks out all applications on the screen, so that
37 only the given subtree of the grabbing application will be sensitive to
38 pointer events (mouse button presses, mouse button releases, pointer
39 motions, window entries, and window exits). During global grabs the
40 window manager will not receive pointer events either.
41 During local grabs, keyboard events (key presses and key releases) are
43 delivered as usual: the window manager controls which application re‐
44 ceives keyboard events, and if they are sent to any window in the grab‐
45 bing application then they are redirected to the focus window. During
46 a global grab Tk grabs the keyboard so that all keyboard events are al‐
47 ways sent to the grabbing application. The focus command is still used
48 to determine which window in the application receives the keyboard
49 events. The keyboard grab is released when the grab is released.
50 On macOS a global grab affects all windows created by one Tk process.
52 No window in that process other than the grab window can even be fo‐
53 cused, hence no other window receives key or mouse events. A local
54 grab on macOS affects all windows created by one Tcl interpreter. It
55 is possible to focus any window belonging to the Tk process during a
56 local grab but the grab window is the only window created by its inter‐
57 preter which receives key or mouse events. Windows belonging to the
58 same process but created by different interpreters continue to receive
59 key and mouse events normally.
60 Grabs apply to particular displays. If an application has windows on
62 multiple displays then it can establish a separate grab on each dis‐
63 play. The grab on a particular display affects only the windows on
64 that display. It is possible for different applications on a single
65 display to have simultaneous local grabs, but only one application can
66 have a global grab on a given display at once.
67 The grab command can take any of the following forms:
69 grab ?-global? window
71 Same as grab set, described below.
72 grab current ?window?
74 If window is specified, returns the name of the current grab
75 window in this application for window's display, or an empty
76 string if there is no such window. If window is omitted, the
77 command returns a list whose elements are all of the windows
78 grabbed by this application for all displays, or an empty string
79 if the application has no grabs.
80 grab release window
82 Releases the grab on window if there is one, otherwise does
83 nothing. Returns an empty string.
84 grab set ?-global? window
86 Sets a grab on window. If -global is specified then the grab is
87 global, otherwise it is local. If a grab was already in effect
88 for this application on window's display then it is automati‐
89 cally released. If there is already a grab on window and it has
90 the same global/local form as the requested grab, then the com‐
91 mand does nothing. Returns an empty string.
92 grab status window
94 Returns none if no grab is currently set on window, local if a
95 local grab is set on window, and global if a global grab is set.
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98 It is very easy to use global grabs to render a display completely un‐
99 usable (e.g. by setting a grab on a widget which does not respond to
100 events and not providing any mechanism for releasing the grab). Take
101 extreme care when using them!
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104 It took an incredibly complex and gross implementation to produce the
105 simple grab effect described above. Given the current implementation,
106 it is not safe for applications to use the Xlib grab facilities at all
107 except through the Tk grab procedures. If applications try to manipu‐
108 late X's grab mechanisms directly, things will probably break.
109 If a single process is managing several different Tk applications, only
111 one of those applications can have a local grab for a given display at
112 any given time. If the applications are in different processes, this
113 restriction does not exist.
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116 Set a grab so that only one button may be clicked out of a group. The
117 other buttons are unresponsive to the mouse until the middle button is
118 clicked.
119 pack [button .b1 -text "Click me! #1" -command {destroy .b1}]
120 pack [button .b2 -text "Click me! #2" -command {destroy .b2}]
121 pack [button .b3 -text "Click me! #3" -command {destroy .b3}]
122 grab .b2
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125 busy(n)
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128 grab, keyboard events, pointer events, window
129Tk grab(n)