Sitecore Multisite Shared Placeholders and Cascading.

GetFromLayoutFieldWithSharedPlaceholders.cs

using Sitecore;
using Sitecore.Data.Fields;
using Sitecore.Data.Items;
using Sitecore.Diagnostics;
using Sitecore.Mvc.Extensions;
using Sitecore.Mvc.Pipelines.Response.GetXmlBasedLayoutDefinition;
using Sitecore.Mvc.Presentation;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Xml.Linq;

namespace YOURNAMESPACE.GetXmlBasedLayoutDefinition
{
    public class GetFromLayoutFieldWithSharedPlaceholders : GetFromLayoutField
    {
        private CascadeRenderingsHelper CascadeRenderings;

        public override void Process(GetXmlBasedLayoutDefinitionArgs args)
        {
            var excludedRenderingsPerItem = (Sitecore.Configuration.Factory.CreateObject("excludeRenderingsPerItem", false) as ExcludeRenderings).ExcludedRenderings;
            this.CascadeRenderings = Sitecore.Configuration.Factory.CreateObject("cascadeRenderings", false) as CascadeRenderingsHelper;

            if (args.Result == null)
            {
                XElement content = GetFromField(args);
                if (content != null)
                {
                    Item item = PageContext.Current.Item;
                    if (item != null && item.Paths.FullPath != Context.Site.StartPath) 
                    {
                        Log.Debug("GetFromLayoutField - Process : Not on homepage");
                        Item homeItem = Context.Database.GetItem(Context.Site.StartPath);

                        if (homeItem != null && ContinueProcess(item, homeItem))
                        {
                            Field homePageLayoutField = homeItem.Fields[FieldIDs.FinalLayoutField];

                            if (homePageLayoutField != null)
                            {
                                string fieldValue = LayoutField.GetFieldValue(homePageLayoutField);

                                if (!fieldValue.IsWhiteSpaceOrNull())
                                {
                                    XElement homePageLayout = XDocument.Parse(fieldValue).Root;
                                    XElement dXElement = content.Element("d");

                                    if (dXElement != null && homePageLayout != null)
                                    {
                                        XElement dXElementInHomePageLayout = homePageLayout.Element("d");

                                        if (dXElementInHomePageLayout != null)
                                        {
                                            var layoutElements = dXElementInHomePageLayout.Elements().ToList();

                                            // remove renderings from list of ItemIds
                                            var itemId = item.ID.Guid;
                                            if (excludedRenderingsPerItem != null && excludedRenderingsPerItem.ContainsKey(itemId))
                                            {
                                                List<XElement> layoutElementsToRemove = layoutElements.Where(x => x.Attribute("id") != null && excludedRenderingsPerItem.Values
                                                                                                                      .Any(y => y.Contains(Guid.Parse(x.Attribute("id").Value))))
                                                                                                      .ToList();

                                                layoutElements = layoutElements.Except(layoutElementsToRemove).ToList();
                                            }

                                            foreach (var placeholder in this.CascadeRenderings.Placeholders)
                                            {
                                                dXElement.Add(ExtractContentsFromLayout(layoutElements, placeholder));
                                            }
                                        }
                                    }
                                }
                            }
                        }
                    }
                }

                args.Result = content;
            }
        }

        private List<XElement> ExtractContentsFromLayout(IEnumerable<XElement> layoutElements, string placeholder)
        {
            List<XElement> elements = new List<XElement>();

            if (layoutElements.Any())
            {
                foreach (XElement element in layoutElements)
                {
                    if (element.HasAttributes)
                    {
                        if (element.Attribute("ph") != null)
                        {
                            string value = element.Attribute("ph").Value.ToLowerInvariant();
                            if (!string.IsNullOrEmpty(value) && (value.StartsWith("/" + placeholder.ToLowerInvariant()) || value.Equals(placeholder.ToLowerInvariant())))
                            {
                                elements.Add(element);
                            }
                        }
                    }
                }
            }

            return elements;
        }

        private bool ContinueProcess(Item item, Item homeItem)
        {
            bool returnValue = false;

            if (item.TemplateID == homeItem.TemplateID)
                return false;

            if (this.CascadeRenderings.Dbs.Contains(item.Database.Name))
                returnValue = true;
            else
                return false;

            //current item under the home item
            if (item.Paths.FullPath.StartsWith(homeItem.Paths.FullPath))
                returnValue = true;
            else
                return false;

            if (this.CascadeRenderings.IncludedTemplates.Contains(item.TemplateID))
                returnValue = true;
            else
                return false;

            return returnValue;
        }
    }
}

MultisiteSharedPlaceholders.config

<?xml version="1.0" encoding="utf-8"?>
<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <pipelines>
      <mvc.getXmlBasedLayoutDefinition>
        <processor type="YOURNAMESPACE.GetXmlBasedLayoutDefinition.GetFromLayoutFieldWithSharedPlaceholders, DLLNAME"
                   patch:instead="processor[@type='Sitecore.Mvc.Pipelines.Response.GetXmlBasedLayoutDefinition.GetFromLayoutField, Sitecore.Mvc']"/>
      </mvc.getXmlBasedLayoutDefinition>

      <getPlaceholderRenderings>
        <processor type="YOURNAMESPACE.GetPlaceholderRenderings.RemoveSharedRenderings, DLLNAME"
                   patch:after="processor[@type='Sitecore.Pipelines.GetPlaceholderRenderings.RemoveNonEditableRenderings, Sitecore.Kernel']"/>
      </getPlaceholderRenderings>
      </pipelines>
    <cascadeRenderings type="YOURNAMESPACE.CascadeRenderingsHelper, DLLNAME">
    <!-- List of placeholders to cascade.  -->
      <Placeholders hint="list">
        <placeholder>siteheader</placeholder>
        <placeholder>sitefooter</placeholder>
      </Placeholders>
      <!-- List of template ids which this cascading code will process.  -->
      <IncludedTemplates hint="list:AddIncludedTemplates">
        <templateId>{122B3DFC-E993-43FB-8487-BFA4623E23C4}</templateId>
        <templateId>{CDE21AC4-57C0-47A4-A431-4229822270DC}</templateId>
        <templateId>{3F92D2F9-EC62-4F2B-8423-74E535565A8B}</templateId>
        <templateId>{7E54B5C1-AD2A-44F7-83DB-E5DDGDD71F0C}</templateId>
      </IncludedTemplates>
      <!-- List of db's this code will run on. You can control it per instance.  -->
      <Dbs hint="list">
        <db>master</db>
        <db>web</db>
      </Dbs>
    </cascadeRenderings>
    <!-- List of exclusions per page/item. List the page item id and the list of renderings under it to exclude -->
    <excludeRenderingsPerItem type="YOURNAMESPACE.ExcludeRenderings, DLLNAME">
      <items hint="raw:AddExcludedRendering">
        <item itemId="{5C6A1126-8918-44FC-B16C-7F8DSDSD8E1F}">
          <renderingId>{033A1329-6B69-4F20-BB6E-D493333D4804}</renderingId>
        </item>
        <item itemId="{385C76DF-68C5-46E2-9450-79A33333F4A7}">
          <renderingId>{25B5FD8D-7E69-4E3B-A29C-F3F3339719DA}</renderingId>
        </item>
      </items>
    </excludeRenderingsPerItem>
  </sitecore>
</configuration>

RemoveSharedRenderings.cs

using Sitecore;
using Sitecore.Data.Items;
using Sitecore.Diagnostics;
using Sitecore.Pipelines.GetPlaceholderRenderings;

namespace YOURNAMESPACE.GetPlaceholderRenderings
{
    public class RemoveSharedRenderings
    {
        private CascadeRenderingsHelper CascadeRenderings;

        public void Process(GetPlaceholderRenderingsArgs args)
        {
            Assert.IsNotNull(args, "args");

            this.CascadeRenderings = Sitecore.Configuration.Factory.CreateObject("cascadeRenderings", false) as CascadeRenderingsHelper;

            Item item = Context.Item;
            if (item != null && item.Paths.FullPath != Context.Site.StartPath)
            {
                Item homeItem = Context.Database.GetItem(Context.Site.StartPath);
                if (item != null && ContinueProcess(item, homeItem))
                {
                    foreach (var placeholder in this.CascadeRenderings.Placeholders)
                    {
                        string value = args.PlaceholderKey;
                        if (!string.IsNullOrEmpty(value) && (value.StartsWith("/" + placeholder.ToLowerInvariant()) || value.Equals(placeholder.ToLowerInvariant())))
                        {
                            args.HasPlaceholderSettings = false;
                            args.PlaceholderRenderings.Clear();
                        }
                    }
                }
            }
        }


        private bool ContinueProcess(Item item, Item homeItem)
        {
            bool returnValue = false;

            if (homeItem != null && item != null && item.TemplateID == homeItem.TemplateID)
                return false;

            if (item != null && this.CascadeRenderings.Dbs.Contains(item.Database.Name))
                returnValue = true;
            else
                return false;

            //current item under the home item
            if (homeItem != null && item != null && item.Paths.FullPath.StartsWith(homeItem.Paths.FullPath))
                returnValue = true;
            else
                return false;

            if (item != null && this.CascadeRenderings.IncludedTemplates.Contains(item.TemplateID))
                returnValue = true;
            else
                return false;

            return returnValue;
        }
    }
}