为NLog自定义LayoutRenderer

长话短说

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

现在来完成课后的小作业： 将重点日志字段显示到Nlog的LayoutRenderer上。

本文实现一个简单流畅的LoggingHttpMessageHandler，记录该HttpClient请求耗时，并显示到 NLog LayoutRenderer。

什么是Layout Renderer？

nlog日志上输出的特定字段，便于检索和分类。

配置nlog显示elapse Renderer

# 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"/> 

// :: [Info].[System.Net.Http.HttpClient.bce-request.LogicalHandler].[}].[415.2504]
HTTP request http://localhost:5000/v1/eqid/e741e8d600151edc000000035decf3bf after 415.2504ms end -OK
// :: [Info].[System.Net.Http.HttpClient.bce-request.LogicalHandler].[}].[80.2951]
HTTP request http://localhost:5000/v1/eqid/2a41e8d600151edc000000028decf3bf after 80.2951ms end -OK
// :: [Info].[System.Net.Http.HttpClient.bce-request.LogicalHandler].[}].[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 都来自日志写入的信息， 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。

---------------------------------1-----------------------------

写入日志时，为Message传入参数{Url}, {Elapse}, {StatusCode}：

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(, "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

　　有简单的lambda方式，这里我们采用稍微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());
        }
    }
# 以上三个参数Url，Elapse，StatusCode都在LogEventInfo.Prpperties键值对中

--------------------------------------------------------

按照文档的要求，尽量早点注册自定义Nlog LayoutRenderer

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

关于将该HttpMessgaeHandler 应用到HttpClientFactory，请参阅 《解剖HttpClientFactory，自由扩展HttpMessageHandler》思路，

本文主要目的为nlog 实现并添加自定义LayoutRenderer.