Subcontent Computed Field

SubcontentField.cs

using System.Collections.Generic;
using System.Linq;
using Blade.Utility;
using Sitecore;
using Sitecore.ContentSearch;
using Sitecore.ContentSearch.ComputedFields;
using Sitecore.Data.Fields;
using Sitecore.Data.Items;
using Sitecore.Layouts;

namespace Foo.ContentSearch.ComputedFields
{
	/// <summary>
	/// Computed field that contains all textual content of items that are rendering data sources on the current item's layout details
	/// </summary>
	public class SubcontentField : IComputedIndexField
	{
		public object ComputeFieldValue(IIndexable indexable)
		{
			var sitecoreIndexable = indexable as SitecoreIndexableItem;

			if (sitecoreIndexable == null) return null;

			// find renderings with datasources set
			var customDataSources = ExtractRenderingDataSourceItems(sitecoreIndexable.Item);

			// extract text from data sources
			var contentToAdd = customDataSources.SelectMany(GetItemContent).ToList();

			if (contentToAdd.Count == 0) return null;

			return string.Join(" ", contentToAdd);
		}

		/// <summary>
		/// Finds all renderings on an item's layout details with valid custom data sources set and returns the data source items.
		/// </summary>
		protected virtual IEnumerable<Item> ExtractRenderingDataSourceItems(Item baseItem)
		{
			string currentLayoutXml = LayoutField.GetFieldValue(baseItem.Fields[FieldIDs.LayoutField]);
			if (string.IsNullOrEmpty(currentLayoutXml)) yield break;

			LayoutDefinition layout = LayoutDefinition.Parse(currentLayoutXml);

			// loop over devices in the rendering
			for (int deviceIndex = layout.Devices.Count - 1; deviceIndex >= 0; deviceIndex--)
			{
				var device = layout.Devices[deviceIndex] as DeviceDefinition;

				if (device == null) continue;

				// loop over renderings within the device
				for (int renderingIndex = device.Renderings.Count - 1; renderingIndex >= 0; renderingIndex--)
				{
					var rendering = device.Renderings[renderingIndex] as RenderingDefinition;

					if (rendering == null) continue;

					// if the rendering has a custom data source, we resolve the data source item and place its text fields into the content to add
					if (!string.IsNullOrWhiteSpace(rendering.Datasource))
					{
						// DataSourceHelper is a component of Blade
						var dataSource = DataSourceHelper.ResolveDataSource(rendering.Datasource, baseItem);

						if (dataSource != baseItem)
						{
							yield return dataSource;
						}
					}
				}
			}
			
		} 

		/// <summary>
		/// Extracts textual content from an item's fields
		/// </summary>
		protected virtual IEnumerable<string> GetItemContent(Item dataSource)
		{
			foreach (Field field in dataSource.Fields)
			{
				// this check is what Sitecore uses to determine if a field belongs in _content (see LuceneDocumentBuilder.AddField())
				if (!IndexOperationsHelper.IsTextField(new SitecoreItemDataField(field))) continue;

				string fieldValue = (field.Value ?? string.Empty).StripHtml();

				if (!string.IsNullOrWhiteSpace(fieldValue)) yield return fieldValue;
			}
		}

		public string FieldName { get; set; }
		public string ReturnType { get; set; }
	}
}

you rock. thanks.
not sure there's a better way to propose a gist update, but in interest of extending lessons learned back to the blog (also updated a fork if my lousy note-patch is unclear):

40- var currentLayoutXml = LayoutField.GetFieldValue(baseItem.Fields[FieldIDs.LayoutField]);
40+var currentLayoutXml = LayoutField.GetFieldValue(baseItem.Fields[FieldIDs.FinalLayoutField]);
-> Updates currentLayoutXml to use FinalLayoutField (where the datasources are!)

80+ dataSource.Fields.ReadAll();
-> ensures dataSource FieldCollection._fields is populated with all fields

Doesn't iterating over all devices run the risk of duplicating some/most of the text? Would it be better to choose the one that gives the best text for indexing. Which one would be implementation dependent.