/// <summary>
/// Enqueues absolute time scheduled work in the timer queue or the short term work list.
/// </summary>
/// <param name="state">State to pass to the action.</param>
/// <param name="dueTime">Absolute time to run the work on. The timer queue is responsible to execute the work close to the specified time, also accounting for system clock changes.</param>
/// <param name="action">Action to run, potentially recursing into the scheduler.</param>
/// <returns>Disposable object to prevent the work from running.</returns>
private IDisposable Enqueue <TState>(TState state, DateTimeOffset dueTime, Func <IScheduler, TState, IDisposable> action)
{
//
// Work that's due in the past is sent to the underlying scheduler through the Schedule
// overload for execution at TimeSpan.Zero. We don't go to the overload for immediate
// scheduling in order to:
//
// - Preserve the time-based nature of the call as surfaced to the underlying scheduler,
// as it may use different queuing strategies.
//
// - Optimize for the default behavior of LocalScheduler where a virtual call to Schedule
// for immediate execution calls into the abstract Schedule method with TimeSpan.Zero.
//
var due = Scheduler.Normalize(dueTime - Now);
if (due == TimeSpan.Zero)
{
return(Schedule <TState>(state, TimeSpan.Zero, action));
}
//
// We're going down the path of queueing up work or scheduling it, so we need to make
// sure we can get system clock change notifications. If not, the call below is expected
// to throw NotSupportedException. WorkItem.Invoke decreases the ref count again to allow
// the system clock monitor to stop if there's no work left. Notice work items always
// reach an execution stage since we don't dequeue items but merely mark them as cancelled
// through WorkItem.Dispose. Double execution is also prevented, so the ref count should
// correctly balance out.
//
SystemClock.AddRef();
var workItem = new WorkItem <TState>(this, state, dueTime, action);
if (due <= SHORTTERM)
{
ScheduleShortTermWork(workItem);
}
else
{
ScheduleLongTermWork(workItem);
}
return(workItem);
}