Tk_RestrictEvents(3tk) Tk Library Procedures Tk_RestrictEvents(3tk)
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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.
ClientData arg (in) Arbitrary argument to pass
to proc.
ClientData *prevArgPtr (out) Pointer to place to save ar-
gument to previous restrict
procedure.
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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, 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(
ClientData 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_DIS-
CARD_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 infor-
mation 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
do not 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 Tcl_DoOneEvent to re-
trieve the desired event(s).
KEYWORDS
delay, event, filter, restriction
Tk Tk_RestrictEvents(3tk)
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