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前文《解剖HttpClientFactory，自由扩展HttpMessageHandler》主要讲如何为HttpClientFactory自定义HttpMessageHandler组件,  

现在完成课后的小作业： 将重点日志字段显示到Nlog的Layout Renderer上

本文自定义一个NLog Layout Renderer（显示HttpClient请求的耗时）

什么是Layout Renderer？

nlog日志上输出的特定字段，便于检索和分类。

# 截取自nlog.config配置文件
<target name="bce-request"

        xsi:type="File"layout="${date:format=yy/MM/dd HH\:mm\:ss} [${level}].[${logger}].[${threadid}}].[${elapse}]${newline}${message} ${exception:format=tostring}"fileName="${logDir}/bce-request.log" encoding="utf-8"/>

以上配置输出如下日志:

19/12/08 22:46:29 [Info].[System.Net.Http.HttpClient.bce-request.LogicalHandler].[6}].[415.2504]
HTTP request http://localhost:5000/v1/eqid/e741e8d600151edc000000035decf3bf after 415.2504ms end -OK 
19/12/08 22:47:15 [Info].[System.Net.Http.HttpClient.bce-request.LogicalHandler].[40}].[80.2951]
HTTP request http://localhost:5000/v1/eqid/2a41e8d600151edc000000028decf3bf after 80.2951ms end -OK 
19/12/08 22:48:06 [Info].[System.Net.Http.HttpClient.bce-request.LogicalHandler].[43}].[36.8624]
HTTP request http://localhost:5000/v1/eqid/1a41e8d600151edc000000028decf3bf after 36.8624ms end -OK

头脑风暴

nlog所有的日志Render依赖日志写入时的信息， 因此我们在写入日志时附带该Renderer值， 然后配置nlog显示日志时提取该Renderer值。

1

写入日志时，为Message传入参数{Url}, {Elapse}, {StatusCode}, 这三个参数值可被提取作为 Renderer

public class CustomHttpMessageHandler : DelegatingHandler{private readonly ILogger _logger;public AttachTraceIdScopeHttpMessageHandler(ILogger logger){_logger = logger ?? throw new ArgumentNullException(nameof(logger));}protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,CancellationToken cancellationToken){if (request == null){throw new ArgumentNullException(nameof(request));}var stopwatch = Stopwatch.StartNew();var response = await base.SendAsync(request, cancellationToken);stopwatch.Stop();_logger.Log(LogLevel.Information, new EventId(101, "Request End"),"HTTP request {Url} after {Elapse}ms end -{StatusCode}",request.RequestUri, stopwatch.Elapsed.TotalMilliseconds, response.StatusCode);return response;}}

2

添加自定义LayOutRenderer

　　① 简单的lambda方式，

      ② 我们采用稍灵活的自定义类方式：

关键点是实现LayoutRenderer的抽象方法Append,  从LogEventInfo中提出Renderer值：

[LayoutRenderer("elapse")]public class ElapseLayoutRenderer : LayoutRenderer{protected override void Append(StringBuilder builder, LogEventInfo logEvent){builder.Append(logEvent.Properties["Elapse"].ToString());}}
# 参数Url、Elapse、StatusCode均可在LogEventInfo.Prpperties键值对提取

3

按照文档的要求，尽早注册自定义Nlog Layout Renderer：

public static void Main(string[] args)
{LayoutRenderer.Register<ElapseLayoutRenderer>("elapse");......
}

END

关于将该HttpMessgaeHandler应用到HttpClientFactory，请参阅《解剖HttpClientFactory，自由扩展HttpMessageHandler》思路。本文演示为nlog添加自定义LayoutRenderer。

+ https://github.com/NLog/NLog/wiki/How-to-write-a-custom-layout-renderer 

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