Unity 断点下载：使用 UnityWebRequest 实现文件续传

使用 UnityWebRequest 实现断点下载可以通过设置 'range' 头部字段来实现。下面是一个简单的例子：

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

public class DownloadManager : MonoBehaviour {

    private string url = 'http://example.com/myfile.mp4';
    private string savePath = Application.persistentDataPath + "/myfile.mp4";
    private long fileSize;
    private int bufferSize = 1024;
    private int totalDownloaded = 0;

    void Start () {
        StartCoroutine(DownloadFile());
    }

    IEnumerator DownloadFile() {
        UnityWebRequest request = UnityWebRequest.Head(url);
        yield return request.SendWebRequest();

        fileSize = long.Parse(request.GetResponseHeader('Content-Length'));

        if (File.Exists(savePath)) {
            totalDownloaded = (int)new FileInfo(savePath).Length;
            request = UnityWebRequest.Get(url);
            request.SetRequestHeader('Range', 'bytes=' + totalDownloaded + '-');
        } else {
            request = UnityWebRequest.Get(url);
        }

        request.downloadHandler = new DownloadHandlerBuffer();
        yield return request.SendWebRequest();

        if (request.isNetworkError || request.isHttpError) {
            Debug.Log(request.error);
        } else {
            byte[] buffer = new byte[bufferSize];
            int bytesRead;

            using (FileStream file = new FileStream(savePath, FileMode.Append, FileAccess.Write)) {
                file.Seek(totalDownloaded, SeekOrigin.Begin);

                using (Stream stream = request.downloadHandler.data) {
                    while ((bytesRead = stream.Read(buffer, 0, buffer.Length)) > 0) {
                        file.Write(buffer, 0, bytesRead);
                        totalDownloaded += bytesRead;
                    }
                }
            }

            Debug.Log('Download complete. Total downloaded: ' + totalDownloaded);
        }
    }
}

这个例子首先使用 Head 请求获取文件的大小，然后判断本地是否已经存在该文件，如果存在，则设置 'Range' 头部字段，请求文件的剩余部分；如果不存在，则直接请求整个文件。

在下载过程中，使用 FileStream 逐步将下载的数据写入本地文件，并记录已经下载的字节数。如果下载过程中发生错误，可以通过记录已经下载的字节数，重启应用程序后继续下载。