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