ASP.NET WebAPi之断点续传下载（中）

本文主要是介绍ASP.NET WebAPi之断点续传下载（中），希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

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
{// Fieldsprivate int bufferSize;private Stream content;private bool contentConsumed;private const int defaultBufferSize = 0x1000;private long start;// Methodspublic 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 Typesprivate 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 PushStreamContentreturn !(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

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