Tk_RestrictEvents (3)
filter and selectively delay X events
SYNOPSIS
#include <tk.h>
Tk_RestrictProc *
Tk_RestrictEvents(proc, clientData, prevClientDataPtr)
ARGUMENTS
Tk_RestrictProc **prevClientDataPtr
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 previous restrict procedure.
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 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).
KEYWORDS
delay, event, filter, restriction
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