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【】。
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>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);