Merge pull request #622 from Microsoft/IncrementalLoadingCollection-Mod

Remove TSource default constructor requirement for IncrementalLoadingCollection

Author: deltakosh

Microsoft.Toolkit.Uwp/IncrementalLoadingCollection/IncrementalLoadingCollection.cs

@@ -15,6 +15,7 @@
 using System.Collections.ObjectModel;
 using System.ComponentModel;
 using System.Linq;
+using System.Reflection;
 using System.Runtime.InteropServices.WindowsRuntime;
 using System.Threading;
 using System.Threading.Tasks;
@@ -38,7 +39,7 @@ namespace Microsoft.Toolkit.Uwp
     /// <seealso cref="ISupportIncrementalLoading"/>
     public class IncrementalLoadingCollection<TSource, IType> : ObservableCollection<IType>,
          ISupportIncrementalLoading
-         where TSource : IIncrementalSource<IType>, new()
+         where TSource : IIncrementalSource<IType>
     {
         /// <summary>
         /// Gets a value indicating the source of incremental loading.
@@ -134,7 +135,7 @@ private set
         /// </param>
         /// <seealso cref="IIncrementalSource{TSource}"/>
         public IncrementalLoadingCollection(int itemsPerPage = 20, Action onStartLoading = null, Action onEndLoading = null, Action<Exception> onError = null)
-            : this(new TSource(), itemsPerPage, onStartLoading, onEndLoading, onError)
+            : this(InstantiateSourceByReflection(), itemsPerPage, onStartLoading, onEndLoading, onError)
         {
         }
 
@@ -159,6 +160,11 @@ public IncrementalLoadingCollection(int itemsPerPage = 20, Action onStartLoading
         /// <seealso cref="IIncrementalSource{TSource}"/>
         public IncrementalLoadingCollection(TSource source, int itemsPerPage = 20, Action onStartLoading = null, Action onEndLoading = null, Action<Exception> onError = null)
         {
+            if (source == null)
+            {
+                throw new ArgumentNullException(nameof(source));
+            }
+
             Source = source;
 
             _onStartLoading = onStartLoading;
@@ -196,6 +202,18 @@ protected virtual async Task<IEnumerable<IType>> LoadDataAsync(CancellationToken
             return result;
         }
 
+        private static TSource InstantiateSourceByReflection()
+        {
+            var type = typeof(TSource);
+            ConstructorInfo constructor = type.GetConstructor(new Type[0]);
+            if (constructor == null)
+            {
+                throw new InvalidOperationException("TSource must have a parameterless constructor");
+            }
+
+            return (TSource)constructor.Invoke(null);
+        }
+
         private async Task<LoadMoreItemsResult> LoadMoreItemsAsync(uint count, CancellationToken cancellationToken)
         {
             uint resultCount = 0;