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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18 This command implements simple pointer and keyboard grabs for Tk. Tk's
19 grabs are different than the grabs described in the Xlib documentation.
20 When a grab is set for a particular window, Tk restricts all pointer
21 events to the grab window and its descendants in Tk's window hierarchy.
22 Whenever the pointer is within the grab window's subtree, the pointer
23 will behave exactly the same as if there had been no grab at all and
24 all events will be reported in the normal fashion. When the pointer is
25 outside window's tree, button presses and releases and mouse motion
26 events are reported to window, and window entry and window exit events
27 are ignored. The grab subtree “owns” the pointer: windows outside the
28 grab subtree will be visible on the screen but they will be insensitive
29 until the grab is released. The tree of windows underneath the grab
30 window can include top-level windows, in which case all of those top-
31 level windows and their descendants will continue to receive mouse
32 events during the grab.
33 Two forms of grabs are possible: local and global. A local grab
35 affects only the grabbing application: events will be reported to
36 other applications as if the grab had never occurred. Grabs are local
37 by default. A global grab locks out all applications on the screen, so
38 that only the given subtree of the grabbing application will be sensi‐
39 tive to pointer events (mouse button presses, mouse button releases,
40 pointer motions, window entries, and window exits). During global
41 grabs the window manager will not receive pointer events either.
42 During local grabs, keyboard events (key presses and key releases) are
44 delivered as usual: the window manager controls which application
45 receives keyboard events, and if they are sent to any window in the
46 grabbing application then they are redirected to the focus window.
47 During a global grab Tk grabs the keyboard so that all keyboard events
48 are always sent to the grabbing application. The focus command is
49 still used to determine which window in the application receives the
50 keyboard events. The keyboard grab is released when the grab is
51 released.
52 Grabs apply to particular displays. If an application has windows on
54 multiple displays then it can establish a separate grab on each dis‐
55 play. The grab on a particular display affects only the windows on
56 that display. It is possible for different applications on a single
57 display to have simultaneous local grabs, but only one application can
58 have a global grab on a given display at once.
59 The grab command can take any of the following forms:
61 grab ?-global? window
63 Same as grab set, described below.
64 grab current ?window?
66 If window is specified, returns the name of the current grab
67 window in this application for window's display, or an empty
68 string if there is no such window. If window is omitted, the
69 command returns a list whose elements are all of the windows
70 grabbed by this application for all displays, or an empty string
71 if the application has no grabs.
72 grab release window
74 Releases the grab on window if there is one, otherwise does
75 nothing. Returns an empty string.
76 grab set ?-global? window
78 Sets a grab on window. If -global is specified then the grab is
79 global, otherwise it is local. If a grab was already in effect
80 for this application on window's display then it is automati‐
81 cally released. If there is already a grab on window and it has
82 the same global/local form as the requested grab, then the com‐
83 mand does nothing. Returns an empty string.
84 grab status window
86 Returns none if no grab is currently set on window, local if a
87 local grab is set on window, and global if a global grab is set.
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90 It is very easy to use global grabs to render a display completely
91 unusable (e.g. by setting a grab on a widget which does not respond to
92 events and not providing any mechanism for releasing the grab). Take
93 extreme care when using them!
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96 It took an incredibly complex and gross implementation to produce the
97 simple grab effect described above. Given the current implementation,
98 it is not safe for applications to use the Xlib grab facilities at all
99 except through the Tk grab procedures. If applications try to manipu‐
100 late X's grab mechanisms directly, things will probably break.
101 If a single process is managing several different Tk applications, only
103 one of those applications can have a local grab for a given display at
104 any given time. If the applications are in different processes, this
105 restriction does not exist.
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108 Set a grab so that only one button may be clicked out of a group. The
109 other buttons are unresponsive to the mouse until the middle button is
110 clicked.
111 pack [button .b1 -text "Click me! #1" -command {destroy .b1}]
112 pack [button .b2 -text "Click me! #2" -command {destroy .b2}]
113 pack [button .b3 -text "Click me! #3" -command {destroy .b3}]
114 grab .b2
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118 grab, keyboard events, pointer events, window
119Tk grab(n)