Can ValueGenerator be applied in add and update situation?

The issue centers on ValueGenerator, HasValueGenerator, ChangeTracker, and SaveChanges/SaveChangesAsync, using the supplied CreatedBy and UpdatedBy configuration. Start by examining how value generation is triggered during add and update operations, then reproduce the example and compare both paths. Done means establishing whether update-time generation is supported and documenting the appropriate behavior or limitation.

Steps to reproduce

Summary: I am trying to understand the ValueGenerator | .HasValueGenerator feature. To do so I
am trying to create a simple tracking example where I saved the username of the person that created, and the username of person that updated a class.

The issue

It looks like the ValueGenerator is only called on an add. Is that correct?

I can of course override SaveChanges\Async and get at the ChangeTracker to do this (which is what I do in EF6), but I wondered if there was a better way in EF Core.

For your laughter here is my code:

internal class SimpleTrackingConfig<T> where T : class, ISimpleTracking
{
    private readonly UserNameValueGenerator _valueGenerator;
    public SimpleTrackingConfig(Func<string> getCurrentUser)
    {
        _valueGenerator = new UserNameValueGenerator(getCurrentUser);
    }

    public void Configure(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<T>()
            .Property(x => x.CreatedOn)
            .HasDefaultValueSql("getutcdate()");

        modelBuilder.Entity<T>()
            .Property(x => x.CreatedBy)
            .HasMaxLength(256)
            .HasValueGenerator((p, e) => UserNameFactory());

        modelBuilder.Entity<T>()
            .Property(x => x.UpdatedOn)
            .HasComputedColumnSql("getutcdate()");

        modelBuilder.Entity<T>()
            .Property(x => x.UpdatedBy)
            .HasMaxLength(256)
            .HasValueGenerator((p, e) => UserNameFactory());
    }

    private ValueGenerator<string> UserNameFactory()
    {
        return _valueGenerator;
    }

    private class UserNameValueGenerator : ValueGenerator<string>
    {
        private readonly Func<string> _getCurrentUser;

        public UserNameValueGenerator(Func<string> getCurrentUser)
        {
            _getCurrentUser = getCurrentUser;
        }

        public override bool GeneratesTemporaryValues => false;

        public override string Next(EntityEntry entry)
        {
            return _getCurrentUser == null ? "" : _getCurrentUser();
        }
    }
}

Further technical details

EF Core version: "Microsoft.EntityFrameworkCore.SqlServer": "1.1.0-preview1-final"
Operating system: Windows 10
Visual Studio version: VS2015 update 3

Other details about my project setup: