NAME

       Tk_RestrictEvents - filter and selectively delay X events

SYNOPSIS

       #include <tk.h>

       Tk_RestrictProc *
       Tk_RestrictEvents(proc, arg, prevArgPtr)

ARGUMENTS

       Tk_RestrictProc *proc (in)                 Predicate procedure to call to filter incoming X events.  NULL
                                                  means do not restrict events at all.

       void *arg (in)                             Arbitrary argument to pass to proc.

       void **prevArgPtr (out)                    Pointer  to  place  to  save  argument  to  previous  restrict
                                                  procedure.
________________________________________________________________________________________________________________

DESCRIPTION

       This procedure is useful in certain situations where applications are only prepared to receive certain  X
       events.   After  Tk_RestrictEvents  is called, Tcl_DoOneEvent (and hence Tk_MainLoop) will filter X input
       events through proc.  Proc indicates whether a given event is to be processed immediately, 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(
                      void *arg,
                      XEvent *eventPtr);
       The  arg  argument  is a copy of the arg 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 Tcl_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
       processing.  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 prevArgPtr to return  information  about  the  current  event
       restriction  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 changing 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 performing
       synchronous actions (for example, if you need to wait for a particular event to  occur  on  a  particular
       window but you do not 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 Tcl_DoOneEvent to retrieve the desired event(s).

KEYWORDS

       delay, event, filter, restriction

Tk                                                                                        Tk_RestrictEvents(3tk)