ASP.NET WebAPi之断点续传下载（中）

前言

前情回顾：上一篇我们遗留了两个问题，一个是未完全实现断点续传，另外则是在响应时是返回StreamContent还是PushStreamContent呢？这一节我们重点来解决这两个问题，同时就在此过程中需要注意的地方一并指出，若有错误之处，请指出。

StreamContent compare to PushStreamContent

我们来看看StreamContent代码，如下：

public class StreamContent : HttpContent{    // Fields    private int bufferSize;    private Stream content;    private bool contentConsumed;    private const int defaultBufferSize = 0x1000;    private long start;    // Methods    public StreamContent(Stream content);]    public StreamContent(Stream content, int bufferSize);     protected override Task<Stream> CreateContentReadStreamAsync();    protected override void Dispose(bool disposing);    private void PrepareContent();    protected override Task SerializeToStreamAsync(Stream stream, TransportContext context);    protected internal override bool TryComputeLength(out long length);    // Nested Types    private class ReadOnlyStream : DelegatingStream    {......}}

似乎没有什么可看的，但是有一句话我们需要注意，如下：

 private const int defaultBufferSize = 0x1000;

在StreamContent的第二个构造函数为

 public StreamContent(Stream content, int bufferSize);

上述给定的默认一次性输入到缓冲区大小为4k，这对我们有何意义呢？当我们写入到响应中时，一般我们直接利用的是第一个构造函数，如下：

  var response = new HttpResponseMessage();  response.Content = new StreamContent(fileStream);

到这里我们明白了这么做是有问题的，当下载时默认读取的是4k，如果文件比较大下载的时间则有延长，所以我们在返回时一定要给定缓冲大小，那么给定多少呢？为达到更好的性能最多是80k，如下：

 private const int BufferSize = 80 * 1024; response.Content = new StreamContent(fileStream, BufferSize);

此时下载的速度则有很大的改善，有人就说了为何是80k呢？这个问题我也不知道，老外验证过的，这是链接【.NET Asynchronous stream read/write】。

好了说完StreamContent，接下来我们来看看PushStreamContent，从字面意思来为推送流内容，难道是充分利用了缓冲区吗，猜测可以有，就怕没有任何想法，我们用源码来证明看看。

我们只需看看WebHost模式下对于缓冲策略是怎么选择的，我们看看此类 WebHostBufferPolicySelector  实现，代码如下：

     /// <summary>    /// Provides an implementation of <see cref="IHostBufferPolicySelector"/> suited for use    /// in an ASP.NET environment which provides direct support for input and output buffering.    /// </summary>    public class WebHostBufferPolicySelector : IHostBufferPolicySelector    {        ....../// <summary>        /// Determines whether the host should buffer the <see cref="HttpResponseMessage"/> entity body.        /// </summary>        /// <param name="response">The <see cref="HttpResponseMessage"/>response for which to determine        /// whether host output buffering should be used for the response entity body.</param>        /// <returns><c>true</c> if buffering should be used; otherwise a streamed response should be used.</returns>        public virtual bool UseBufferedOutputStream(HttpResponseMessage response)        {            if (response == null)            {                throw Error.ArgumentNull("response");            }            // Any HttpContent that knows its length is presumably already buffered internally.            HttpContent content = response.Content;            if (content != null)            {                long? contentLength = content.Headers.ContentLength;                if (contentLength.HasValue && contentLength.Value >= 0)                {                    return false;                }                // Content length is null or -1 (meaning not known).                  // Buffer any HttpContent except StreamContent and PushStreamContent                return !(content is StreamContent || content is PushStreamContent);            }            return false;        }    }

从上述如下一句可以很明显的知道：

 return !(content is StreamContent || content is PushStreamContent);

除了StreamContent和PushStreamContent的HttpContent之外，其余都进行缓冲，所以二者的区别不在于缓冲，那到底是什么呢？好了我们还未查看PushStreamContent的源码，我们继续往下走，查看其源代码如下，我们仅仅只看关于这个类的描述以及第一个构造函数即可，如下：

  /// <summary>    /// Provides an <see cref="HttpContent"/> implementation that exposes an output <see cref="Stream"/>    /// which can be written to directly. The ability to push data to the output stream differs from the     /// <see cref="StreamContent"/> where data is pulled and not pushed.    /// </summary>    public class PushStreamContent : HttpContent    {        private readonly Func<Stream, HttpContent, TransportContext, Task> _onStreamAvailable;        /// <summary>        /// Initializes a new instance of the <see cref="PushStreamContent"/> class. The        /// <paramref name="onStreamAvailable"/> action is called when an output stream        /// has become available allowing the action to write to it directly. When the         /// stream is closed, it will signal to the content that is has completed and the         /// HTTP request or response will be completed.        /// </summary>        /// <param name="onStreamAvailable">The action to call when an output stream is available.</param>        public PushStreamContent(Action<Stream, HttpContent, TransportContext> onStreamAvailable)            : this(Taskify(onStreamAvailable), (MediaTypeHeaderValue)null)        {        }
        ......   }

对于此类的描述大意是：PushStreamContent与StreamContent的不同在于，PushStreamContent在于将数据push【推送】到输出流中，而StreamContent则是将数据从流中【拉取】。 

貌似有点晦涩，我们来举个例子，在webapi中我们常常这样做，读取文件流并返回到响应流中，若是StreamContent，我们会如下这样做：

response.Content = new StreamContent(File.OpenRead(filePath));

上面的释义我用大括号着重括起，StreamContent着重于【拉取】，当响应时此时将从文件流写到输出流，通俗一点说则是我们需要从文件流中去获取数据并写入到输出流中。我们再来看看PushStreamContent的用法，如下：

XDocument xDoc = XDocument.Load("cnblogs_backup.xml", LoadOptions.None);PushStreamContent xDocContent = new PushStreamContent((stream, content, context) =>{     xDoc.Save(stream);     stream.Close();},"application/xml");

PushStreamContent着重于【推送】，当我们加载xml文件时，当我们一旦进行保存时此时则会将数据推送到输出流中。

二者区别在于：StreamContent从流中【拉取】数据，而PushStreamContent则是将数据【推送】到流中。

那么此二者应用的场景是什么呢？

（1）对于下载文件我们则可以通过StreamContent来实现直接从流中拉取，若下载视频流此时则应该利用PushStreamContent来实现，因为未知服务器视频资源的长度，此视频资源来源于别的地方。

（2）数据量巨大，发送请求到webapi时利用PushStreamContent。

当发送请求时，常常序列化数据并请求webapi，我们可能这样做：

    var client = new HttpClient();    string json = JsonConvert.SerializeObject(data);    var response = await client.PostAsync(uri, new StringContent(json));

当数据量比较小时没问题，若数据比较大时进行序列化此时则将序列化的字符串加载到内存中，鉴于此这么做不可行，此时我们应该利用PushStreamContent来实现。

    var client = new HttpClient();    var content = new PushStreamContent((stream, httpContent, transportContext) =>    {        var serializer = new JsonSerializer();        using (var writer = new StreamWriter(stream))        {            serializer.Serialize(writer, data);        }    });    var response = await client.PostAsync(uri, content);

为什么要这样做呢？我们再来看看源码，里面存在这样一个方法。

  protected override Task SerializeToStreamAsync(Stream stream, TransportContext context);

其内部实现利用异步状态机实现，所以当数据量巨大时利用PushStreamContent来返回将会有很大的改善，至此，关于二者的区别以及常见的应用场景已经叙述完毕，接下来我们继续断点续传问题。

断点续传改进 

上一篇我们讲过获取Range属性中的集合通过如下：

request.Headers.Range

我们只取该集合中的第一个范围元素，通过如下

 RangeItemHeaderValue range = rangeHeader.Ranges.First();

此时我们忽略了返回的该范围对象中有当前下载的进度

range.From.HasValue  range.To.HasValue

我们获取二者的值然后进行重写Stream实时读取剩余部分，下面我们一步一步来看。

定义文件操作接口

    public interface IFileProvider    {        bool Exists(string name);        FileStream Open(string name);        long GetLength(string name);    }

实现该操作文件接口

    public class FileProvider : IFileProvider    {        private readonly string _filesDirectory;        private const string AppSettingsKey = "DownloadDir";        public FileProvider()        {            var fileLocation = ConfigurationManager.AppSettings[AppSettingsKey];            if (!String.IsNullOrWhiteSpace(fileLocation))            {                _filesDirectory = fileLocation;            }        }        /// <summary>        /// 判断文件是否存在        /// </summary>        /// <param name="name"></param>        /// <returns></returns>        public bool Exists(string name)        {            string file = Directory.GetFiles(_filesDirectory, name, SearchOption.TopDirectoryOnly)                    .FirstOrDefault();            return true;        }        /// <summary>        /// 打开文件        /// </summary>        /// <param name="name"></param>        /// <returns></returns>        public FileStream Open(string name)        {            var fullFilePath = Path.Combine(_filesDirectory, name);            return File.Open(fullFilePath,                FileMode.Open, FileAccess.Read, FileShare.Read);        }        /// <summary>        /// 获取文件长度        /// </summary>        /// <param name="name"></param>        /// <returns></returns>        public long GetLength(string name)        {            var fullFilePath = Path.Combine(_filesDirectory, name);            return new FileInfo(fullFilePath).Length;        }    }

获取范围对象中的值进行赋值给封装的对象

    public class FileInfo    {        public long From;        public long To;        public bool IsPartial;        public long Length;    }

下载控制器，对文件操作进行初始化

    public class FileDownloadController : ApiController    {        private const int BufferSize = 80 * 1024;        private const string MimeType = "application/octet-stream";        public IFileProvider FileProvider { get; set; }        public FileDownloadController()        {            FileProvider = new FileProvider();        }        ......    }

接下来则是文件下载的逻辑，首先判断请求文件是否存在，然后获取文件的长度

            if (!FileProvider.Exists(fileName))            {                throw new HttpResponseException(HttpStatusCode.NotFound);            }            long fileLength = FileProvider.GetLength(fileName);

将请求中的范围对象From和To的值并判断当前已经下载进度以及剩余进度

        private FileInfo GetFileInfoFromRequest(HttpRequestMessage request, long entityLength)        {            var fileInfo = new FileInfo            {                From = 0,                To = entityLength - 1,                IsPartial = false,                Length = entityLength            };            var rangeHeader = request.Headers.Range;            if (rangeHeader != null && rangeHeader.Ranges.Count != 0)            {                if (rangeHeader.Ranges.Count > 1)                {                    throw new HttpResponseException(HttpStatusCode.RequestedRangeNotSatisfiable);                }                RangeItemHeaderValue range = rangeHeader.Ranges.First();                if (range.From.HasValue && range.From < 0 || range.To.HasValue && range.To > entityLength - 1)                {                    throw new HttpResponseException(HttpStatusCode.RequestedRangeNotSatisfiable);                }                fileInfo.From = range.From ?? 0;                fileInfo.To = range.To ?? entityLength - 1;                fileInfo.IsPartial = true;                fileInfo.Length = entityLength;                if (range.From.HasValue && range.To.HasValue)                {                    fileInfo.Length = range.To.Value - range.From.Value + 1;                }                else if (range.From.HasValue)                {                    fileInfo.Length = entityLength - range.From.Value + 1;                }                else if (range.To.HasValue)                {                    fileInfo.Length = range.To.Value + 1;                }            }            return fileInfo;        }

在响应头信息中的对象ContentRangeHeaderValue设置当前下载进度以及其他响应信息

         private void SetResponseHeaders(HttpResponseMessage response, FileInfo fileInfo,                                      long fileLength, string fileName)        {            response.Headers.AcceptRanges.Add("bytes");            response.StatusCode = fileInfo.IsPartial ? HttpStatusCode.PartialContent                                      : HttpStatusCode.OK;            response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment");            response.Content.Headers.ContentDisposition.FileName = fileName;            response.Content.Headers.ContentType = new MediaTypeHeaderValue(MimeType);            response.Content.Headers.ContentLength = fileInfo.Length;            if (fileInfo.IsPartial)            {                response.Content.Headers.ContentRange                    = new ContentRangeHeaderValue(fileInfo.From, fileInfo.To, fileLength);            }        }

最重要的一步则是将FileInfo对象的值传递给我们自定义实现的流监控当前下载进度。

    public class PartialContentFileStream : Stream    {        private readonly long _start;        private readonly long _end;        private long _position;        private FileStream _fileStream;        public PartialContentFileStream(FileStream fileStream, long start, long end)        {            _start = start;            _position = start;            _end = end;            _fileStream = fileStream;            if (start > 0)            {                _fileStream.Seek(start, SeekOrigin.Begin);            }        }        /// <summary>        /// 将缓冲区数据写到文件        /// </summary>        public override void Flush()        {            _fileStream.Flush();        }        /// <summary>        /// 设置当前下载位置        /// </summary>        /// <param name="offset"></param>        /// <param name="origin"></param>        /// <returns></returns>        public override long Seek(long offset, SeekOrigin origin)        {            if (origin == SeekOrigin.Begin)            {                _position = _start + offset;                return _fileStream.Seek(_start + offset, origin);            }            else if (origin == SeekOrigin.Current)            {                _position += offset;                return _fileStream.Seek(_position + offset, origin);            }            else            {                throw new NotImplementedException("SeekOrigin.End未实现");            }        }        /// <summary>        /// 依据偏离位置读取        /// </summary>        /// <param name="buffer"></param>        /// <param name="offset"></param>        /// <param name="count"></param>        /// <returns></returns>        public override int Read(byte[] buffer, int offset, int count)        {            int byteCountToRead = count;            if (_position + count > _end)            {                byteCountToRead = (int)(_end - _position) + 1;            }            var result = _fileStream.Read(buffer, offset, byteCountToRead);            _position += byteCountToRead;            return result;        }        public override IAsyncResult BeginRead(byte[] buffer, int offset, int count, AsyncCallback callback, object state)        {            int byteCountToRead = count;            if (_position + count > _end)            {                byteCountToRead = (int)(_end - _position);            }            var result = _fileStream.BeginRead(buffer, offset,                                               count, (s) =>                                               {                                                   _position += byteCountToRead;                                                   callback(s);                                               }, state);            return result;        }        ......    }

更新上述下载的完整逻辑 

        public HttpResponseMessage GetFile(string fileName)        {            fileName = "HBuilder.windows.5.2.6.zip";            if (!FileProvider.Exists(fileName))            {                throw new HttpResponseException(HttpStatusCode.NotFound);            }            long fileLength = FileProvider.GetLength(fileName);            var fileInfo = GetFileInfoFromRequest(this.Request, fileLength);            var stream = new PartialContentFileStream(FileProvider.Open(fileName),                                                 fileInfo.From, fileInfo.To);            var response = new HttpResponseMessage();            response.Content = new StreamContent(stream, BufferSize);            SetResponseHeaders(response, fileInfo, fileLength, fileName);            return response;        }

下面我们来看看演示结果：

好了，到了这里我们也得到了我们想要的结果。

总结 

本节我们将上节遗留的问题一一进行比较详细的叙述并最终解决，是不是就这么完全结束了呢？那本节定义为中篇岂不是不对头了，本节是在web端进行下载，下节我们利用webclient来进行断点续传。想了想无论是mvc上传下载，还是利用webapi来上传下载又或者是将mvc和webapi结合来上传下载基本都已经囊括，这都算是在项目中比较常用的吧，所以也就花了很多时间去研究。对于webapi的断点续传关键它本身就提供了比较多的api来给我们调用，所以还是很不错，webapi一个很轻量的服务框架，你值得拥有see u，反正周末，哟，不早了，休息休息。