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 previous
restrict procedure.
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DESCRIPTION
This procedure is useful in certain situations where applications are only prepared to receive cer-tain certain
tain 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 con-sideration. consideration.
sideration. 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
Tk Tk_RestrictEvents(3)
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