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, 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
argument 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_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(3)