一、Http的Range请求头，结合相应头Accept-Ranges、Content-Range 可以实现如下功能：
1.断点续传。用于下载文件被中断后，继续下载。
2.大文件指定区块下载，如视频、音频拖动播放，直接定位到指定位置下载内容。可以避免每次都读取、传输整个文件，从而提升服务端性能。
3.大文件分包批量下载，再合并完整文件。可以提高下载速度。
二、Http的Range 相关说明：
1.规则要点
请求头Range表示请求的数据起始位置。响应头Accept-Ranges：bytes 表示支持续传。响应头Content-Range表示返回的其实位置、总长度
请求头Range的数字，首尾都包含，长度是： end-begin+1
请求头Range的指定的长度，只是意向下载量，服务端不一定返回请求的长度。比如：bytes=0-, 表示希望下载整个文件，但服务端可以返回有限长度的数据块（有利于性能）。但数据其实位置start需按照请求。
2.在Http 响应请求是 200，表示响应结束，响应成功
Http 响应状态：206，表示响应中，响应部分数据，不会单开Socket链接。
三、在Asp.Net Core中实现自定义 Range 文件响应
1.封装处理的类：
public class DownloadRange
{
public HttpContext context = null;
public HttpRequest request = null;
public HttpResponse response = null;
public DownloadRange(HttpContext ctx)
{
this.context = ctx;
this.request = ctx.Request;
this.response = ctx.Response;
}
private int HttpRangeSize = 1024 * 1024; //最大块长度 1M
public void WriteFile(string file)
{
using (var fs = File.OpenRead(file))
{
WriteStream(fs);
}
}
private void WriteStream(Stream fs)
{
string range = request.Headers["Range"];
range = range ?? "";
range = range.Trim().ToLower();
if (fs.CanSeek)
{
if (range.StartsWith("bytes=") && range.Contains("-"))
{
//分段输出文件
int start = -1, end = -1;
var rgs = range.Substring(6).Split('-');
int.TryParse(rgs[0], out start);
int.TryParse(rgs[1], out end);
if (rgs[0] == "")
{
start = (int)fs.Length - end;
end = (int)fs.Length - 1;
}
if (rgs[1] == "")
{
end = (int)fs.Length - 1;
}
WriteRangeStream(fs, start, end);
}
else
{
//输出整个文件
int l;
byte[] buffer = new byte[40960];
while ((l = fs.Read(buffer, 0, buffer.Length)) > 0)
{
response.Body.Write(buffer, 0, l);
}
}
}
}
private void WriteRangeStream(Stream fs, int start, int end)
{
using (fs)
{
int rangLen = end - start + 1;
if (rangLen > 0)
{
if (rangLen > HttpRangeSize)
{
rangLen = HttpRangeSize;
end = start + HttpRangeSize - 1;
}
}
else
{
throw new Exception("Range error");
}
long size = fs.Length;
//如果是整个文件返回200，否则返回206
if (start == 0 && end + 1 >= size)
{
response.StatusCode = 200;
}
else
{
response.StatusCode = 206;
}
// response.Headers.Add("Accept-Ranges", "bytes");
response.Headers.Add("Content-Range", $"bytes {start}-{end}/{size}");
response.Headers.Add("Content-Length", rangLen.ToString());
int readLen, total = 0;
byte[] buffer = new byte[40960];
//流定位到指定位置
try
{
fs.Seek(start, SeekOrigin.Begin);
while (total < rangLen && (readLen = fs.Read(buffer, 0, buffer.Length)) > 0)
{
total += readLen;
if (total > rangLen)
{
readLen -= (total - rangLen);
total = rangLen;
}
response.Body.Write(buffer, 0, readLen);
}
}
catch (Exception ex)
{
throw ex;
}
}
}
}
2.自定义中间件，处理文件输出
public class OuterImgMiddleware
{
public static string RootPath { get; set; } //配置文件读取绝对位置
private readonly RequestDelegate _next;
public OuterImgMiddleware(RequestDelegate next, Microsoft.AspNetCore.Hosting.IHostingEnvironment env)
{
_next = next;
}
public async Task Invoke(HttpContext context)
{
var path = context.Request.Path.ToString();
var headersDictionary = context.Request.Headers;
if (context.Request.Method == "GET" && !string.IsNullOrEmpty(path))
{
if (
path.Contains("/upload/logo")
|| path.Contains("/upload/image")
|| path.Contains("/upload/ueimage")
)
{
var unauthorizedImagePath = RootPath + path;
FileInfo file = new FileInfo(unauthorizedImagePath);
if (file.Exists)
{
int length = path.LastIndexOf(".") - path.LastIndexOf("/") - 1;
context.Response.Headers["Etag"] = path.Substring(path.LastIndexOf("/") + 1, length);
context.Response.Headers["Last-Modified"] = new DateTime(2018).ToString("r");
context.Response.Headers["Accept-Ranges"] = "bytes";
//context.Response.Headers["Content-Length"] = file.Length.ToString();
if (path.EndsWith(".mp4"))
{
context.Response.ContentType = "video/mp4";
//分段读取内容
DownloadRange download = new DownloadRange(context);
download.WriteFile(unauthorizedImagePath);
}
else
{
context.Response.ContentType = "image/jpeg";
context.Response.Headers["Cache-Control"] = "public"; //指定客户端，服务器都处理缓存
await context.Response.SendFileAsync(unauthorizedImagePath);
}
}
return;
}
}
await _next(context);
}
}
public static class MvcExtensions
{
public static IApplicationBuilder UseOutImg(this IApplicationBuilder builder)
{
return builder.UseMiddleware<OuterImgMiddleware>();
}
}
3. 在服务配置中 ConfigureServices，开启同步读取
//启用允许同步读取
services.Configure<KestrelServerOptions>(x => x.AllowSynchronousIO = true)
.Configure<IISServerOptions>(x => x.AllowSynchronousIO = true);
4.在配置中 Configure，启用中间件
app.UseOutImg();
