NextSortOrderReplacer.cs

using System.Linq;
using Sitecore.Configuration;
using Sitecore.Data.Items;
using Sitecore.Diagnostics;
using Sitecore.Pipelines.ExpandInitialFieldValue;

namespace CustomExpandTokenProcessors.Pipelines
{
  public class NextSortOrderReplacer : ExpandInitialFieldValueProcessor
  {
    private const string Token = "$nextsortorder";

    public override void Process(ExpandInitialFieldValueArgs args)
    {
      Assert.ArgumentNotNull(args, "args");

      if (args.SourceField.Value.Contains(Token))
      {
        if (args.TargetItem != null && args.TargetItem.Parent != null)
        {
          int nextSortOrder = GetNextSortOrder(args);
          args.Result = args.Result.Replace(Token, nextSortOrder.ToString());
        }
        else
        {
          args.Result = args.Result.Replace(Token, Settings.DefaultSortOrder.ToString());
        }
      }
    }

    private static int GetNextSortOrder(ExpandInitialFieldValueArgs args)
    {
      int nextSortOrder = Settings.DefaultSortOrder;

      Item parent = args.TargetItem.Parent;
      if (parent.HasChildren)
      {
        Item itemWithHighestSortOrder = parent.Children.OrderByDescending(x => x.Appearance.Sortorder).First();
        nextSortOrder = nextSortOrder + itemWithHighestSortOrder.Appearance.Sortorder;
      }
      
      return nextSortOrder;
    }
  }
}