reactive/Ix.NET/Source/System.Interactive.Async/Select.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.Tasks;

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

            return CreateEnumerable(
                () =>
                {
                    var e = source.GetEnumerator();
                    var current = default(TResult);

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

                    return CreateEnumerator(
                        async ct =>
                        {
                            if (await e.MoveNext(cts.Token)
                                       .ConfigureAwait(false))
                            {
                                current = selector(e.Current);
                                return true;
                            }
                            return false;
                        },
                        () => current,
                        d.Dispose,
                        e
                    );
                });
        }

        public static IAsyncEnumerable<TResult> Select<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, int, TResult> selector)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (selector == null)
                throw new ArgumentNullException(nameof(selector));

            return CreateEnumerable(
                () =>
                {
                    var e = source.GetEnumerator();
                    var current = default(TResult);
                    var index = 0;

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

                    return CreateEnumerator(
                        async ct =>
                        {
                            if (await e.MoveNext(cts.Token)
                                       .ConfigureAwait(false))
                            {
                                current = selector(e.Current, checked(index++));
                                return true;
                            }
                            return false;
                        },
                        () => current,
                        d.Dispose,
                        e
                    );
                });
        }
    }
}