Extending Umbraco Content Delivery API

AuthorFilter.cs

using Umbraco.Cms.Core.DeliveryApi;

namespace UmbracoPlayground.UI.Application.Filters
{
    public class AuthorFilter : IFilterHandler
    {
        private const string FilterType = "author:";

        public FilterOption BuildFilterOption(string filter)
        {
            var value = filter.Substring(FilterType.Length);

            return new FilterOption
            {
                FieldName = "author",
                Operator = FilterOperation.Is,
                Values = new[] { value }
            };
        }

        public bool CanHandle(string query)
        {
            return string.IsNullOrWhiteSpace(query)
                ? false
                : query.StartsWith(FilterType, StringComparison.OrdinalIgnoreCase);
        }
    }
}

AuthorIndexer.cs

using System;
using Umbraco.Cms.Core.DeliveryApi;
using Umbraco.Cms.Core.Models;
using Umbraco.Cms.Core.Web;

namespace UmbracoPlayground.UI.Application.Filters
{
    public class AuthorIndexer : IContentIndexHandler
    {
        private readonly IUmbracoContextFactory _umbracoContextFactory;

        public AuthorIndexer(IUmbracoContextFactory umbracoContextFactory)
        {
            _umbracoContextFactory = umbracoContextFactory;
        }

        public IEnumerable<IndexField> GetFields()
        {
            return new[]
            {
                new IndexField
                {
                    FieldName = "author",
                    FieldType = FieldType.StringSortable,
                    VariesByCulture = false
                }
            };
        }

        public IEnumerable<IndexFieldValue> GetFieldValues(IContent content, string? culture)
        {
            var authorId = content.GetValue<GuidUdi>("author");

            if (authorId == null)
            {
                return Enumerable.Empty<IndexFieldValue>();
            }

            var context = _umbracoContextFactory.EnsureUmbracoContext();

            if (context == null) return Enumerable.Empty<IndexFieldValue>();
            
            var node = context?.UmbracoContext?.Content?.GetById(authorId);
            
            if(node == null) return Enumerable.Empty<IndexFieldValue>();
            
            return new[]
            {
                new IndexFieldValue
                {
                    FieldName = "author",
                    Values = new object[] { node.Name }
                }
            };
        }
    }
}