/// <summary>
/// Schedules new task. The coalesce decision callback will be called for pending and running tasks
/// before returning from this method.
/// </summary>
/// <param name="action">The task to schedule.</param>
/// <param name="arg">The action to schedule. Will be passed as an argument to the coalesce decision callback.</param>
/// <returns>The result returned by the task.</returns>
public Task <TResult> Enqueue(Func <CancellationToken, Task <TResult> > action, TArg arg)
{
CoalescingTaskQueueData <TArg, TResult> work;
CoalescingTaskQueueData <TArg, TResult> newWork;
CoalescingTaskQueueData <TArg, TResult> previousWork;
bool cancelRunningTask;
QueuedTask <TArg, TResult> newTask = null;
do
{
work = _work;
cancelRunningTask = false;
newTask?.Dispose();
if (work.PendingTask != null)
{
// Makes decision on pending task
var decision = _decisionCallback(arg, work.PendingTask.Arg, false);
if (decision == CoalesceDecision.Join)
{
return(_work.PendingTask.Task);
}
}
if (work.RunningTask != null && !work.RunningTask.CancellationRequested)
{
// Makes decision on running task
var decision = _decisionCallback(arg, work.RunningTask.Arg, true);
if (decision == CoalesceDecision.Join)
{
return(_work.RunningTask.Task);
}
// Doesn't cancel running task until successfully scheduling new task, will do that later.
cancelRunningTask = decision == CoalesceDecision.Cancel;
}
newTask = new QueuedTask <TArg, TResult>(action, arg);
if (work.RunningTask == null)
{
// No task is running. Schedule new running task, will start it later.
newWork = new CoalescingTaskQueueData <TArg, TResult>(
null,
newTask);
}
else
{
// A task is running. Schedule new pending task.
newWork = new CoalescingTaskQueueData <TArg, TResult>(
newTask,
work.RunningTask);
}
previousWork = Interlocked.CompareExchange(ref _work, newWork, work);
} while (previousWork != work);
// Scheduling succeeded
// Cancel previous pending task if any
var pendingTask = work.PendingTask;
if (pendingTask != null)
{
pendingTask.Cancel(false);
pendingTask.Dispose();
}
// Cancel running task if requested by decision callback
if (cancelRunningTask)
{
work.RunningTask.Cancel(true);
}
// Start new task if scheduled as running
if (newWork.RunningTask == newTask)
{
Run(newTask);
}
return(newTask.Task);
}