unity2021下载文件，支持断点续传，支持MD5，支持进度条显示

Unity 2021中可以使用UnityWebRequest来下载文件并实现断点续传和MD5校验。以下是一个简单的示例代码：

using UnityEngine;
using UnityEngine.Networking;
using System.IO;
using System;

public class DownloadManager : MonoBehaviour
{
    private UnityWebRequest request;
    private string downloadUrl;
    private string savePath;
    private string md5Hash;
    private bool isDownloading = false;

    public void StartDownload(string url, string path, string md5)
    {
        downloadUrl = url;
        savePath = path;
        md5Hash = md5;

        if (File.Exists(savePath))
        {
            // 如果文件已存在，则计算文件的md5值，与给定的md5值进行比较
            string fileMd5 = GetMD5HashFromFile(savePath);
            if (fileMd5.Equals(md5Hash))
            {
                // 文件已下载完毕，直接返回
                Debug.Log("File already downloaded.");
                return;
            }
            else
            {
                // 文件未下载完毕，删除已下载的部分，重新下载
                File.Delete(savePath);
            }
        }

        isDownloading = true;
        request = UnityWebRequest.Get(downloadUrl);
        long fileLength = GetDownloadFileSize(downloadUrl);
        request.SetRequestHeader("Range", "bytes=" + FileSize(savePath) + "-");
        request.downloadHandler = new DownloadHandlerFile(savePath);
        StartCoroutine(DownloadFile(fileLength));
    }

    private IEnumerator DownloadFile(long fileLength)
    {
        request.SendWebRequest();

        while (!request.isDone)
        {
            float progress = (float)request.downloadedBytes / fileLength;
            Debug.Log("Download progress: " + progress);
            yield return null;
        }

        isDownloading = false;

        if (!request.isNetworkError && !request.isHttpError)
        {
            // 下载完成后，计算文件的md5值，与给定的md5值进行比较
            string fileMd5 = GetMD5HashFromFile(savePath);
            if (fileMd5.Equals(md5Hash))
            {
                Debug.Log("File downloaded successfully.");
            }
            else
            {
                Debug.LogError("MD5 check failed.");
                File.Delete(savePath);
            }
        }
        else
        {
            Debug.LogError("Download failed: " + request.error);
            File.Delete(savePath);
        }
    }

    private long GetDownloadFileSize(string url)
    {
        request = UnityWebRequest.Head(url);
        request.SendWebRequest();
        while (!request.isDone) { }

        if (!request.isNetworkError && !request.isHttpError)
        {
            return long.Parse(request.GetResponseHeader("Content-Length"));
        }
        else
        {
            Debug.LogError("Get file size failed: " + request.error);
            return -1;
        }
    }

    private long FileSize(string path)
    {
        if (File.Exists(path))
        {
            FileInfo fileInfo = new FileInfo(path);
            return fileInfo.Length;
        }
        else
        {
            return 0;
        }
    }

    private string GetMD5HashFromFile(string path)
    {
        if (File.Exists(path))
        {
            using (var md5 = System.Security.Cryptography.MD5.Create())
            {
                using (var stream = File.OpenRead(path))
                {
                    byte[] hash = md5.ComputeHash(stream);
                    return BitConverter.ToString(hash).Replace("-", "").ToLowerInvariant();
                }
            }
        }
        else
        {
            return "";
        }
    }
}

您可以在调用StartDownload方法时，传入下载文件的url、保存路径以及md5值。如果文件已经存在并且md5值与给定的md5值相同，则不会重新下载；否则会删除已下载的部分，重新下载。在下载过程中，可以通过协程获取下载进度并显示进度条。下载完成后，会计算文件的md5值并与给定的md5值进行比较，确保文件的完整性。