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/ApplicationCore/Specifications/BaseSpecification.cs

@@ -14,6 +14,12 @@ namespace Microsoft.eShopWeb.ApplicationCore.Specifications
         public Expression<Func<T, bool>> Criteria { get; }
         public List<Expression<Func<T, object>>> Includes { get; } = new List<Expression<Func<T, object>>>();
         public List<string> IncludeStrings { get; } = new List<string>();
+        public Expression<Func<T, object>> OrderBy { get; private set; }
+        public Expression<Func<T, object>> OrderByDescending { get; private set; }
+
+        public int Take { get; private set; }
+        public int Skip { get; private set; }
+        public bool isPagingEnabled { get; private set; } = false;
 
         protected virtual void AddInclude(Expression<Func<T, object>> includeExpression)
         {
@@ -23,5 +29,19 @@ namespace Microsoft.eShopWeb.ApplicationCore.Specifications
         {
             IncludeStrings.Add(includeString);
         }
+        protected virtual void ApplyPaging(int skip, int take)
+        {
+            Skip = skip;
+            Take = take;
+            isPagingEnabled = true;
+        }
+        protected virtual void ApplyOrderBy(Expression<Func<T, object>> orderByExpression)
+        {
+            OrderBy = orderByExpression;
+        }
+        protected virtual void ApplyOrderByDescending(Expression<Func<T, object>> orderByDescendingExpression)
+        {
+            OrderByDescending = orderByDescendingExpression;
+        }
     }
 }

src/ApplicationCore/Specifications/CatalogFilterPaginatedSpecification.cs

@@ -0,0 +1,14 @@
+using Microsoft.eShopWeb.ApplicationCore.Entities;
+
+namespace Microsoft.eShopWeb.ApplicationCore.Specifications
+{
+    public class CatalogFilterPaginatedSpecification : BaseSpecification<CatalogItem>
+    {
+        public CatalogFilterPaginatedSpecification(int skip, int take, int? brandId, int? typeId)
+            : base(i => (!brandId.HasValue || i.CatalogBrandId == brandId) &&
+                (!typeId.HasValue || i.CatalogTypeId == typeId))
+        {
+            ApplyPaging(skip, take);
+        }
+    }
+}