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, couldn't 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's
       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.

KEYWORDS

       callback, defer, idle callback