reactive/Ix.NET/Source/System.Interactive.Async/Generate.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> Generate<TState, TResult>(TState initialState, Func<TState, bool> condition, Func<TState, TState> iterate, Func<TState, TResult> resultSelector)
        {
            if (condition == null)
                throw new ArgumentNullException(nameof(condition));
            if (iterate == null)
                throw new ArgumentNullException(nameof(iterate));
            if (resultSelector == null)
                throw new ArgumentNullException(nameof(resultSelector));

            return CreateEnumerable(
                () =>
                {
                    var i = initialState;
                    var started = false;
                    var current = default(TResult);

                    return CreateEnumerator(
                        ct =>
                        {
                            var b = false;
                            try
                            {
                                if (started)
                                    i = iterate(i);

                                b = condition(i);

                                if (b)
                                    current = resultSelector(i);
                            }
                            catch (Exception ex)
                            {
                                return TaskExt.Throw<bool>(ex);
                            }

                            if (!b)
                                return TaskExt.False;

                            if (!started)
                                started = true;

                            return TaskExt.True;
                        },
                        () => current,
                        () => { }
                    );
                });
        }
    }
}