grab - Confine pointer and keyboard events to a window sub-tree
grab ?
-global?
window
grab option ?
arg arg ...?
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:
-
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.
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!
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.
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
busy(3tk)
grab, keyboard events, pointer events, window