ParallelDownload.cs

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Channels;
using System.Threading.Tasks;
using Cysharp.Threading.Tasks;
using UniRx;
using UnityEngine;
using UnityEngine.Networking;
using Channel = System.Threading.Channels.Channel;

public class ParallelDownload
{
    private void ParallelTry(IEnumerable<DownloadTask> tasks, CancellationToken token)
    {
        var parallelOptions = new ParallelOptions
        {
            MaxDegreeOfParallelism = 4,
            CancellationToken = token,
        };
        Parallel.ForEach(tasks, parallelOptions, Body);
    }

    private async void ChannelTry(IEnumerable<DownloadTask> tasks, CancellationToken token)
    {
        var channel = Channel.CreateUnbounded<DownloadTask>(new UnboundedChannelOptions()
        {
            SingleWriter = true,
        });

        var channelWriter = channel.Writer;
        foreach (var downloadTask in tasks)
        {
            await channelWriter.WaitToWriteAsync(token).ConfigureAwait(false);
            await channelWriter.WriteAsync(downloadTask, token).ConfigureAwait(false);
        }

        channelWriter.TryComplete();

        var parallelCount = 4;
        for (var i = 0; i < parallelCount; i++)
        {
            Loop(channel.Reader, token).Forget();
        }
    }

    private static async UniTaskVoid Loop(
        System.Threading.Channels.ChannelReader<DownloadTask> reader, CancellationToken token)
    {
        while (true)
        {
            var waitToRead = await reader.WaitToReadAsync(token);
            if (!waitToRead)
            {
                break;
            }

            if (reader.TryRead(out var task))
            {
                await task.Download();
            }
        }
    }

    private static async void Body(DownloadTask task)
    {
        await task.Download();
    }
}

class DownloadTask
{
    private readonly UniTaskCompletionSource _completionSource = new();

    public UniTask Result => _completionSource.Task;

    public Uri Uri;

    public string Path;

    public readonly FloatReactiveProperty Progress = new();

    public async UniTask Download()
    {
        try
        {
            var unityWebRequest = UnityWebRequest.Get(Uri);
            unityWebRequest.downloadHandler = new DownloadHandlerFile(Path, false);
            await unityWebRequest.SendWebRequest();
            _completionSource.TrySetResult();
        }
        catch (Exception e)
        {
            _completionSource.TrySetException(e);
        }
    }
}