reactive/Ix.NET/Source/System.Interactive.Async/Expand.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the Apache 2.0 License.
// See the LICENSE file in the project root for more information. 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

namespace System.Linq
{
    public static partial class AsyncEnumerable
    {
        public static IAsyncEnumerable<TSource> Expand<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, IAsyncEnumerable<TSource>> selector)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (selector == null)
                throw new ArgumentNullException(nameof(selector));

            return CreateEnumerable(
                () =>
                {
                    var e = default(IAsyncEnumerator<TSource>);

                    var cts = new CancellationTokenDisposable();
                    var a = new AssignableDisposable();
                    var d = Disposable.Create(cts, a);

                    var queue = new Queue<IAsyncEnumerable<TSource>>();
                    queue.Enqueue(source);

                    var current = default(TSource);

                    var f = default(Func<CancellationToken, Task<bool>>);
                    f = async ct =>
                        {
                            if (e == null)
                            {
                                if (queue.Count > 0)
                                {
                                    var src = queue.Dequeue();

                                    e = src.GetEnumerator();

                                    a.Disposable = e;
                                    return await f(ct)
                                               .ConfigureAwait(false);
                                }
                                return false;
                            }
                            if (await e.MoveNext(ct)
                                       .ConfigureAwait(false))
                            {
                                var item = e.Current;
                                var next = selector(item);

                                queue.Enqueue(next);
                                current = item;
                                return true;
                            }
                            e = null;
                            return await f(ct)
                                       .ConfigureAwait(false);
                        };

                    return CreateEnumerator(
                        f,
                        () => current,
                        d.Dispose,
                        e
                    );
                });
        }
    }
}