Tcl_DoWhenIdle(3)           Tcl Library Procedures           Tcl_DoWhenIdle(3)

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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(n), Tcl_CreateFileHandler(3), Tcl_CreateTimerHandler(3)

KEYWORDS
       callback, defer, idle callback

Tcl                                   7.5                    Tcl_DoWhenIdle(3)