基于HTTP的断点传输

最近项目需要做一个类似于迅雷的文件下载功能，这类需求可能比较常见，希望可以帮助到有需要的同学

要求：

1.支持断点传输

2. 多文件同时下载

3. 由于是客户内部试用，服务器只支持HTTP文件下载不支持FTP文件（并没有用户权限的要求）

由于根据服务器传来的Json字符串解析后进行下载，首先构建Json对象，推荐一个库Newtonsoft.Json

 public class DownloadFile
    {
        [JsonProperty("police_num")]
        public string PoliceNumber { get; set; }

        [JsonProperty("equipment_num")]
        public string EquipmentNumber { get; set; }

        [JsonProperty("upload_date")]
        public DateTime UploadDateTime { get; set; }

        [JsonProperty("type")]
        public string FileType { get; set; }

        [JsonProperty("file_name")]
        public string FileName { get; set; }

        [JsonProperty("file_path")]
        public string FilePath { get; set; }
    }

JsonProperty不区分大小写

public class DownloadTask : ObservableObject
    {
        /// <summary>
        /// </summary>
        public static readonly int BufferSize = 1024;

        /// <summary>
        ///     一次请求大小,每次请求100kb
        /// </summary>
        public static long Step = 1024*100;

        private int _process;

        /// <summary>
        ///     下载文件参数
        /// </summary>
        public DownloadFile DownloadFile { get; set; }

        /// <summary>
        ///     下载文件状态
        /// </summary>
        public SegmentState SegmentState { get; set; }

        /// <summary>
        ///     默认存储路径
        /// </summary>
        public string DefaultDirectory { get; set; }

        /// <summary>
        ///     当前进度
        /// </summary>
        public long CurrentSize { get; set; }

        /// <summary>
        ///     文件流对象，用于生成文件
        /// </summary>
        public FileStream FileStream { get; set; }

        /// <summary>
        ///     下载起始位置
        /// </summary>
        public long RangeFrom { get; set; }

        /// <summary>
        ///     总大小
        /// </summary>
        public long TotalSize { get; set; }

        /// <summary>
        ///     链接地址
        /// </summary>
        public string Url { get; set; }

        /// <summary>
        ///     界面显示进度
        /// </summary>
        public int Process
        {
            get { return _process; }
            set
            {
                _process = value;
                RaisePropertyChanged("Process");
            }
        }
    }

下载服务类，DownloaderService.cs，功能包括启动下载，暂停，针对文件是否存在进行初始化，获取文件大小方便界面显示

 public class DownloaderService
    {

        public void OnStart(DownloadTask downloadTask)
        {
            if (!InitService(downloadTask)) return;
            var downloadThread = new Thread(BeginDownloadFile) { IsBackground = true };
            downloadThread.Start(downloadTask);
        }

        public void OnStop(DownloadTask downloadTask)
        {
            downloadTask.SegmentState = SegmentState.Paused;
        }

        private bool InitService(DownloadTask downloadTask)
        {
            try
            {
                if (string.IsNullOrEmpty(downloadTask.Url))
                {
                    return false;
                }

                downloadTask.DownloadFile.FileName = downloadTask.Url.Substring(downloadTask.Url.LastIndexOf('/') + 1);

                var fullName = Path.Combine(downloadTask.DefaultDirectory, downloadTask.DownloadFile.FileName);

                if (File.Exists(fullName))
                {
                    if (downloadTask.TotalSize == 0)
                    {
                        GetTotalSize(downloadTask);
                    }
                    if (downloadTask.CurrentSize == downloadTask.TotalSize)
                    {
                        if (MessageBox.Show("文件已经存在是否覆盖此文件", "TCL", MessageBoxButton.YesNo, MessageBoxImage.Information) !=
                        MessageBoxResult.Yes)
                            return false;
                        File.Delete(fullName);
                        downloadTask.CurrentSize = 0;
                    }
                    downloadTask.FileStream = new FileStream(fullName, FileMode.OpenOrCreate, FileAccess.ReadWrite);
                    downloadTask.SegmentState = SegmentState.Downloading;
                    return true;
                }
                downloadTask.SegmentState = SegmentState.Downloading;
                downloadTask.FileStream = new FileStream(fullName, FileMode.OpenOrCreate, FileAccess.ReadWrite);
                return true;
            }
            catch (Exception ex)
            {
                LogLightConsole.WriteLog(ex.ToString(), EunmMsgType.Error);
                return false;
            }
        }

        private void BeginDownloadFile(object obj)
        {
            try
            {
                var downloadTask = (DownloadTask)obj;
                while (true)
                {
                    if (downloadTask.SegmentState == SegmentState.Paused)
                    {
                        downloadTask.FileStream.Close();
                        break;
                    }
                    //从0计数，需要减一
                    var from = downloadTask.CurrentSize;
                    if (from < 0)
                    {
                        from = 0;
                    }

                    var to = downloadTask.CurrentSize + DownloadTask.Step - 1;
                    if (to >= downloadTask.TotalSize && downloadTask.TotalSize > 0)
                    {
                        to = downloadTask.TotalSize - 1;
                    }
                    if (downloadTask.TotalSize == 0)
                    {
                        GetTotalSize(downloadTask);
                    }
                    if (from >= downloadTask.TotalSize || downloadTask.CurrentSize >= downloadTask.TotalSize)
                    {
                        downloadTask.SegmentState = SegmentState.Finished;
                        downloadTask.FileStream.Close();
                        downloadTask.Process = 100;
                        return;
                    }

                    var request = (HttpWebRequest)WebRequest.Create(downloadTask.Url);
                    //request.Method = "GET";            
                    request.AddRange("bytes", from, to);
                    request.Timeout = 1000 * 20;
                    var response = (HttpWebResponse)request.GetResponse();
                    var buffer = new byte[1024];
                    using (var stream = response.GetResponseStream())
                    {
                        if (stream != null)
                        {
                            var size = stream.Read(buffer, 0, buffer.Length);
                            while (size > 0)
                            {
                                //只将读出的字节写入文件
                                downloadTask.FileStream.Write(buffer, 0, size);
                                downloadTask.CurrentSize += size;
                                size = stream.Read(buffer, 0, buffer.Length);
                                downloadTask.Process = (int)(downloadTask.CurrentSize * 100 / downloadTask.TotalSize);
                                downloadTask.FileStream.Flush();
                            }
                        }
                        //如果返回的response头中Content-Range值为空，说明服务器不支持Range属性，不支持断点续传,返回的是所有数据
                        if (response.Headers["Content-Range"] == null)
                        {
                            downloadTask.SegmentState = SegmentState.Finished;
                        }
                    }
                }

            }
            catch (Exception ex)
            {
                LogLightConsole.WriteLog("BeginDownloadFile" + ex, EunmMsgType.Error);
            }
        }

        /// <summary>
        ///     获取文件总大小
        /// </summary>
        public void GetTotalSize(DownloadTask downloadTask)
        {
            var request = (HttpWebRequest)WebRequest.Create(downloadTask.Url);
            //request.Method = "POST";
            //request.Headers.Add("charset:" + "utf-8");
            request.Timeout = 1000 * 20;
            var response = (HttpWebResponse)request.GetResponse();
            downloadTask.TotalSize = response.ContentLength;
        }
    }

HTTP 1.1版本本身已经扩展了断点传输功能，但是这需要服务器同时也支持，这只是提供了一个简单的例子，具体的针对数据上下文以及成功失败的callback函数，就是做一下委托封装用就好，如果具体使用有什么问题可以私信我，谢谢大家支持！

response.Headers["Content-Range"] == null 说明服务器不支持断点传输