Bring back ProjectionFactory

[agehrke]

Bring back ProjectionFactory but with a slightly different interface. This factory would allow consumers to make additional "initialization" of projections:

public interface IProjectionFactory
{
    public Task<IProjection> CreateAsync<TProjection>(EventStreamId streamId, CancellationToken cancellationToken)
        where TProjection : IProjection;
}

A few things to note:

• The method is async to allow implementations to do I/O
• Returns IProjection rather than TProjection - the ProjectionProcessor does not need to actual type. Returning IProjection allows factory implementations to return a decorated TProjection - however it may be cumbersome to decorate an IProjection implementation when they consume a lot of event types.
• Takes an EventStreamId

The default implementation DefaultProjectionFactory just resolves TProjection through IServiceProvider. To allow a IProjectionFactory to create instances that consumes scoped resources, the type is registered as transient.

One could argue that the IProjection.InitializeAsync method could be invoked by the DefaultProjectionFactory as that method is somewhat an async constructor.

Other changes

I have also changed ProjectionProcessor to use IServiceScopeFactory instead of IServiceProvider which, IMO, make it more clear that we are only creating scopes.

[agehrke]

If my PR #66 gets merged I could create some functional tests to show how the IProjectionFactory could be used.

[Copilot]

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

[christianhelle]

@agehrke Please update the change log

[christianhelle]

I can see this as something slightly more costly than just passing in the events.Key, but I don't think it matters

[agehrke]

Actually no. Previously there was an implicit conversion from StreamId to EventStreamId. Now I'm just explicit about that conversion.

[agehrke]

@agehrke Please update the change log

I have updated changelog.md