grab(3pm) User Contributed Perl Documentation grab(3pm)
NAME
Tk::grab - Confine pointer and keyboard events to a window sub-tree
SYNOPSIS
$widget->grab
$widget->grabOption
DESCRIPTION
This set of methods implement 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 $widget's tree, button presses and releases and mouse
motion events are reported to $widget, 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 method is still
used to determine which window in the application receives the keyboard
events. The keyboard grab is released when the grab is released.
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 methods take any of the following forms:
$widget->grabCurrent
Returns the current grab window in this application for $widget's
display, or an empty string if there is no such window.
$widget->grabs
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.
Not implemented yet!
$widget->grabRelease
Releases the grab on $widget if there is one, otherwise does
nothing. Returns an empty string.
$widget->grab
Sets a local grab on $widget. If a grab was already in effect for
this application on $widget's display then it is automatically
released. If there is already a local grab on $widget, then the
command does nothing. Returns an empty string.
$widget->grabGlobal
Sets a global grab on $widget. If a grab was already in effect for
this application on $widget's display then it is automatically
released. If there is already a global grab on $widget, then the
command does nothing. Returns an empty string.
$widget->grabStatus
Returns none if no grab is currently set on $widget, local if a
local grab is set on $widget, and global if a global grab is set.
BUGS
It took an incredibly complex and gross implementation to produce the
simple grab effect described above. Given the current implementation,
it isn't 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 doesn't exist.
KEYWORDS
grab, keyboard events, pointer events, window
perl v5.40.1 2025-04-13 grab(3pm)
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