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 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 prevClientDataPtr 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 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).