Handling transition to state for multiple events

Question

I have a MassTransitStateMachine that orchestrates a process which involves creating multiple events.

Once all of the events are done, I want the state to transition

Answer 1

The solution proposed by Chris won't work in my situation because I have multiple events of the same type arriving. I need to transition only when all of those events have arrived. The CompositeEvent construct doesn't work for this use case.

My solution to this was to raise a new AllDataImported event during the MarkImportCompletedForLocation method. This method now handles determining whether all sub-imports are complete in a thread safe way.

So my state machine definition is:

            During(ImportingData,
            When(DataImported)
                // When we get a data imported event, mark the URI in the locations list as done. 
                .Then(MarkImportCompletedForLocation),

            When(AllDataImported)
                // Once all are done, we can transition to cleaning up...
                .TransitionTo(CleaningUp)
                .Then(CleanUpSources)
        );

The IsAllDataImported method is no longer needed as a filter.

The saga state has a Locations property:

public Dictionary Locations { get; set; }

And the MarkImportCompletedForLocation method is defined as follows:

    private void MarkImportCompletedForLocation(BehaviorContext ctx)
    {
        lock (ctx.Instance.Locations)
        {
            ctx.Instance.Locations[ctx.Data.ImportSource] = true;
            if (ctx.Instance.Locations.Values.All(x => x))
            {
                var allDataImported = new AllDataImportedEvent {CorrelationId = ctx.Instance.CorrelationId};
                this.CreateEventLift(AllDataImported).Raise(ctx.Instance, allDataImported);
            }
        }
    }

(I've just written this so that I understand how the general flow will work; I recognise that the MarkImportCompletedForLocation method needs to be more defensive by verifying that keys exist in the dictionary.)