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/*
* Copyright (c) 2023 Vaughn Nugent
*
* Library: VNLib
* Package: VNLib.Plugins.Extensions.Loading
* File: IAsyncLazy.cs
*
* IAsyncLazy.cs is part of VNLib.Plugins.Extensions.Loading which is part of the larger
* VNLib collection of libraries and utilities.
*
* VNLib.Plugins.Extensions.Loading is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License as
* published by the Free Software Foundation, either version 3 of the
* License, or (at your option) any later version.
*
* VNLib.Plugins.Extensions.Loading is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see https://www.gnu.org/licenses/.
*/
using System;
using System.Threading.Tasks;
using System.Runtime.CompilerServices;
namespace VNLib.Plugins.Extensions.Loading
{
/// <summary>
/// Represents an asynchronous lazy operation. with non-blocking access to the target value.
/// </summary>
/// <typeparam name="T">The result type</typeparam>
public interface IAsyncLazy<T>
{
/// <summary>
/// Gets a value indicating whether the asynchronous operation has completed.
/// </summary>
bool Completed { get; }
/// <summary>
/// Gets a task that represents the asynchronous operation.
/// </summary>
/// <returns></returns>
TaskAwaiter<T> GetAwaiter();
/// <summary>
/// Gets the target value of the asynchronous operation without blocking.
/// If the operation failed, throws an exception that caused the failure.
/// If the operation has not completed, throws an exception.
/// </summary>
T Value { get; }
}
/// <summary>
/// Extension methods for <see cref="IAsyncLazy{T}"/>
/// </summary>
public static class AsyncLazyExtensions
{
/// <summary>
/// Gets an <see cref="IAsyncLazy{T}"/> wrapper for the specified <see cref="Task{T}"/>
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="task"></param>
/// <returns>The async operation task wrapper</returns>
public static IAsyncLazy<T> AsLazy<T>(this Task<T> task) => new AsyncLazy<T>(task);
/// <summary>
/// Tranforms one lazy operation into another using the specified handler
/// </summary>
/// <typeparam name="T"></typeparam>
/// <typeparam name="TResult">The resultant type</typeparam>
/// <param name="lazy"></param>
/// <param name="handler">The function that will peform the transformation of the lazy result</param>
/// <returns>A new <see cref="IAsyncLazy{T}"/> that returns the transformed type</returns>
public static IAsyncLazy<TResult> Transform<T, TResult>(this IAsyncLazy<T> lazy, Func<T, TResult> handler)
{
_ = lazy ?? throw new ArgumentNullException(nameof(lazy));
_ = handler ?? throw new ArgumentNullException(nameof(handler));
//Await the lazy task, then pass the result to the handler
static async Task<TResult> OnResult(IAsyncLazy<T> lazy, Func<T, TResult> cb)
{
T result = await lazy;
return cb(result);
}
return OnResult(lazy, handler).AsLazy();
}
#nullable disable
private sealed class AsyncLazy<T> : IAsyncLazy<T>
{
private readonly Task<T> _task;
private T _result;
public AsyncLazy(Task<T> task)
{
_task = task ?? throw new ArgumentNullException(nameof(task));
_ = task.ContinueWith(SetResult, TaskScheduler.Default);
}
///<inheritdoc/>
public bool Completed => _task.IsCompleted;
///<inheritdoc/>
public T Value
{
get
{
if (_task.IsCompletedSuccessfully)
{
return _result;
}
else if(_task.IsFaulted)
{
//Compress and raise exception from result
return _task.GetAwaiter().GetResult();
}
else
{
throw new InvalidOperationException("The asynchronous operation has not completed.");
}
}
}
/*
* Only set the result if the task completed successfully.
*/
private void SetResult(Task<T> task)
{
if (task.IsCompletedSuccessfully)
{
_result = task.Result;
}
}
///<inheritdoc/>
public TaskAwaiter<T> GetAwaiter() => _task.GetAwaiter();
}
#nullable enable
}
}
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