Easy to use generic event handler add-on for Marten.
Install the Ermines Nuget package:
> dotnet add package Ermines
Let's create an event handler for MyEvent:
using Ermine;
public class MyEventHandler : IAsyncEventHandler<MyEvent>
{
public Task Handle(MyEvent @event, IDocumentSession session, CancellationtToken ct)
{
// store a read model, call another service, do the fandago
return Task.CompletedTask;
}
}
if you prefer a synchronous event handler use the interface ISyncEventHandler<TEvent>
Configure your Marten document store to call Ermines event handlers when events are saved to the docuemnt store:
var docStore = DocumentStore.For(x =>
{
x.Events.InlineProjections.Add(new EventDispatcher(t =>
{
if (t == typeof(MyEvent))
return new [] { new MyEventHandler() };
return null;
}));
// other document store config
});
This registers the Ermines EventDispatcher with Marten and sets up a factory for creating event handlers based on event types. If you are using a dependency injection container you can use that to resolve event handlers:
var container = ... // create DI container
var docStore = DocumentStore.For(x =>
{
x.Events.InlineProjections.Add(new EventDispatcher(t => container.GetServices(t)));
});
Now whenever an event of type MyEvent is saved the event handler MyEventHanlder is called. That is when:
using (var sesion = docStore.LightweightSession())
{
session.Events.Append(streamId, new MyEvent());
await sessoin.SaveChangAsync();
}
the Handle metthod on an instance of MyEventHandler is called with the new event and the session as arguments.
Beware that the event handlers are called during the SaveChangesAsync call.