Renamed existing Flatten method to FlattenAsync and added new Flatten method. Also fixed ClientHelper using incorrect guild batch count. (#744)

src/Discord.Net.Core/Extensions/AsyncEnumerableExtensions.cs

@@ -1,14 +1,64 @@
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading;
 using System.Threading.Tasks;
 
 namespace Discord
 {
     public static class AsyncEnumerableExtensions
     {
-        public static async Task<IEnumerable<T>> Flatten<T>(this IAsyncEnumerable<IReadOnlyCollection<T>> source)
+        /// <summary>
+        /// Flattens the specified pages into one <see cref="IEnumerable{T}"/> asynchronously
+        /// </summary>
+        /// <typeparam name="T"></typeparam>
+        /// <param name="source"></param>
+        /// <returns></returns>
+        public static async Task<IEnumerable<T>> FlattenAsync<T>(this IAsyncEnumerable<IEnumerable<T>> source)
         {
-            return (await source.ToArray().ConfigureAwait(false)).SelectMany(x => x);
+            return await source.Flatten().ToArray().ConfigureAwait(false);
+        }
+
+        public static IAsyncEnumerable<T> Flatten<T>(this IAsyncEnumerable<IEnumerable<T>> source)
+        {
+            return new PagedCollectionEnumerator<T>(source);
+        }
+        
+        internal class PagedCollectionEnumerator<T> : IAsyncEnumerator<T>, IAsyncEnumerable<T>
+        {
+            readonly IAsyncEnumerator<IEnumerable<T>> _source;
+            IEnumerator<T> _enumerator;
+
+            public IAsyncEnumerator<T> GetEnumerator() => this;
+
+            internal PagedCollectionEnumerator(IAsyncEnumerable<IEnumerable<T>> source)
+            {
+                _source = source.GetEnumerator();
+            }
+
+            public T Current => _enumerator.Current;
+
+            public void Dispose()
+            {
+                _enumerator?.Dispose();
+                _source.Dispose();
+            }
+
+            public async Task<bool> MoveNext(CancellationToken cancellationToken)
+            {
+                cancellationToken.ThrowIfCancellationRequested();
+                
+                if(!_enumerator?.MoveNext() ?? true)
+                {
+                    if (!await _source.MoveNext(cancellationToken).ConfigureAwait(false))
+                        return false;
+
+                    _enumerator?.Dispose();
+                    _enumerator = _source.Current.GetEnumerator();
+                    return _enumerator.MoveNext();
+                }
+
+                return true;
+            }
         }
     }
 }