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NAME
Tk_RestrictEvents - filter and selectively delay X events
SYNOPSIS
#include <tk.h>
Tk_RestrictProc *
Tk_RestrictEvents(proc, clientData, prevClientDataPtr)
ARGUMENTS
Tk_RestrictProc *proc (in) Predicate
procedure
to call to
filter
incoming X
events.
NULL means
do not
restrict
events at
all.
ClientData clientData (in) Arbitrary
argument
to pass to
proc.
ClientData *prevClientDataPtr (out) Pointer to
place to
save argu-
ment to
previous
restrict
procedure.
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DESCRIPTION
This procedure is useful in certain situations where appli-
cations are only prepared to receive certain X events.
After Tk_RestrictEvents is called, Tk_DoOneEvent (and hence
Tk_MainLoop) will filter X input events through proc. Proc
indicates whether a given event is to be processed immedi-
ately, deferred until some later time (e.g. when the event
restriction is lifted), or discarded. Proc is a procedure
with arguments and result that match the type
Tk_RestrictProc:
typedef Tk_RestrictAction Tk_RestrictProc(
ClientData clientData,
XEvent *eventPtr);
The clientData argument is a copy of the clientData passed
to Tk_RestrictEvents; it may be used to provide proc with
information it needs to filter events. The eventPtr points
to an event under consideration. Proc returns a restrict
action (enumerated type Tk_RestrictAction) that indicates
what Tk_DoOneEvent should do with the event. If the return
value is TK_PROCESS_EVENT, then the event will be handled
immediately. If the return value is TK_DEFER_EVENT, then
the event will be left on the event queue for later process-
ing. If the return value is TK_DISCARD_EVENT, then the
event will be removed from the event queue and discarded
without being processed.
Tk_RestrictEvents uses its return value and prevClientDa-
taPtr to return information about the current event restric-
tion procedure (a NULL return value means there are
currently no restrictions). These values may be used to
restore the previous restriction state when there is no
longer any need for the current restriction.
There are very few places where Tk_RestrictEvents is needed.
In most cases, the best way to restrict events is by chang-
ing the bindings with the bind Tcl command or by calling
Tk_CreateEventHandler and Tk_DeleteEventHandler from C. The
main place where Tk_RestrictEvents must be used is when per-
forming synchronous actions (for example, if you need to
wait for a particular event to occur on a particular window
but you don't want to invoke any handlers for any other
events). The ``obvious'' solution in these situations is to
call XNextEvent or XWindowEvent, but these procedures cannot
be used because Tk keeps its own event queue that is
separate from the X event queue. Instead, call
Tk_RestrictEvents to set up a filter, then call
Tk_DoOneEvent to retrieve the desired event(s).
KEYWORDS
delay, event, filter, restriction
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