Tk_RestrictEvents
Tk_RestrictEvents(3) Tk Library Procedures Tk_RestrictEvents(3)
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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 argument to pre-
vious restrict proce-
dure.
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DESCRIPTION
This procedure is useful in certain situations where applications 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 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(
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 consid-
eration. Proc returns a restrict action (enumerated type Tk_Restric-
tAction) 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_Dele-
teEventHandler 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 ``obvi-
ous'' solution in these situations is to call XNextEvent or XWindow-
Event, 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
Tk Tk_RestrictEvents(3)
