NAME

       Tcl_DoWhenIdle, Tcl_CancelIdleCall - invoke a procedure when there are no pending events

SYNOPSIS

       #include <tcl.h>

       Tcl_DoWhenIdle(proc, clientData)

       Tcl_CancelIdleCall(proc, clientData)

ARGUMENTS

       Tcl_IdleProc *proc (in)               Procedure to invoke.

       ClientData clientData (in)            Arbitrary one-word value to pass to proc.
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DESCRIPTION

       Tcl_DoWhenIdle  arranges  for  proc  to be invoked when the application becomes idle.  The
       application is considered to be idle when Tcl_DoOneEvent has been called, could  not  find
       any  events to handle, and is about to go to sleep waiting for an event to occur.  At this
       point all pending Tcl_DoWhenIdle handlers are invoked.  For each  call  to  Tcl_DoWhenIdle
       there  will  be a single call to proc;  after proc is invoked the handler is automatically
       removed.  Tcl_DoWhenIdle is only usable in programs that use  Tcl_DoOneEvent  to  dispatch
       events.

       Proc should have arguments and result that match the type Tcl_IdleProc:

              typedef void Tcl_IdleProc(
                      ClientData clientData);

       The  clientData  parameter  to  proc  is  a  copy  of  the  clientData  argument  given to
       Tcl_DoWhenIdle.  Typically, clientData points to a data structure containing  application-
       specific information about what proc should do.

       Tcl_CancelIdleCall may be used to cancel one or more previous calls to Tcl_DoWhenIdle:  if
       there is a Tcl_DoWhenIdle handler registered for proc and clientData, then it  is  removed
       without  invoking  it.   If there is more than one handler on the idle list that refers to
       proc and clientData, all of the handlers are removed.  If no existing handlers match  proc
       and clientData then nothing happens.

       Tcl_DoWhenIdle is most useful in situations where (a) a piece of work will have to be done
       but (b) it is possible that something will happen in the near future that will change what
       has  to  be  done  or  require  something different to be done.  Tcl_DoWhenIdle allows the
       actual work to be deferred until all pending events have been processed.   At  this  point
       the exact work to be done will presumably be known and it can be done exactly once.

       For  example, Tcl_DoWhenIdle might be used by an editor to defer display updates until all
       pending commands have been processed.  Without this feature,  redundant  redisplays  might
       occur in some situations, such as the processing of a command file.

BUGS

       At  present  it  is not safe for an idle callback to reschedule itself continuously.  This
       will interact badly with certain features  of  Tk  that  attempt  to  wait  for  all  idle
       callbacks  to  complete.   If  you  would  like  for an idle callback to reschedule itself
       continuously, it is better to use a timer handler with a zero timeout period.

SEE ALSO

       after(3tcl), Tcl_CreateFileHandler(3tcl), Tcl_CreateTimerHandler(3tcl)

KEYWORDS

       callback, defer, idle callback