DragonFly On-Line Manual Pages
Tk_RestrictEvents(3) Tk Library Procedures Tk_RestrictEvents(3)
Tk_RestrictEvents - filter and selectively delay X events
Tk_RestrictEvents(proc, arg, prevArgPtr)
Tk_RestrictProc *proc (in) Predicate procedure to call
to filter incoming X events.
NULL means do not restrict
events at all.
ClientData arg (in) Arbitrary argument to pass
ClientData *prevArgPtr (out) Pointer to place to save
argument to previous
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(
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).
delay, event, filter, restriction