Paging infrastructure implemented through specifications. (#151)

* Paging criterias added in BaseSpecification. New paginated specification added (CatalogFilterPaginatedSpecification) . EFRepository cleaned up and paging implementation added. Usage of the new paging infrastructure in CatalogService (Web and WebRazor).

* Use IReadOnlyList<T> instead of List<T> as return type from repositories.

* - Ordering possibility added in the specification.
- Evaluation of the specification placed in separate class.

src/Infrastructure/Data/SpecificationEvaluator.cs

@@ -0,0 +1,44 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.eShopWeb.ApplicationCore.Entities;
+using Microsoft.eShopWeb.ApplicationCore.Interfaces;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace Microsoft.eShopWeb.Infrastructure.Data
+{
+    public class SpecificationEvaluator<T> where T : BaseEntity
+    {
+        public static IQueryable<T> GetQuery(IQueryable<T> inputQuery, ISpecification<T> specification)
+        {
+            var query = inputQuery;
+
+            // modify the IQueryable using the specification's criteria expression
+            if (specification.Criteria != null)
+                query = query.Where(specification.Criteria);
+
+            // Includes all expression-based includes
+            query = specification.Includes.Aggregate(query,
+                                    (current, include) => current.Include(include));
+
+            // Include any string-based include statements
+            query = specification.IncludeStrings.Aggregate(query,
+                                    (current, include) => current.Include(include));
+
+            // Apply ordering if expressions are set
+            if (specification.OrderBy != null)
+                query = query.OrderBy(specification.OrderBy);
+            else if (specification.OrderByDescending != null)
+                query = query.OrderByDescending(specification.OrderByDescending);
+
+            // Apply paging if enabled
+            if (specification.isPagingEnabled)
+            {
+                query = query.Skip(specification.Skip)
+                             .Take(specification.Take);
+            }
+            return query;
+        }
+    }
+}