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 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 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
Tk_DoOneEvent to retrieve the desired event(s).
KEYWORDS¶
delay, event, filter, restriction