WebApi2官网学习记录--HttpClient Message Handlers

在客户端，HttpClient使用message handle处理request。默认的handler是HttpClientHandler,用来发送请求和获取response从服务端。可以在client pipline中插入自定义的message handler：

自定义的message handler:

class MessageHandler1 : DelegatingHandler
{
    private int _count = 0;

    protected override Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, System.Threading.CancellationToken cancellationToken)
    {
        _count++;
        request.Headers.Add("X-Custom-Header", _count.ToString());
        return base.SendAsync(request, cancellationToken);
    }
}

class LoggingHandler : DelegatingHandler
{
    StreamWriter _writer;

    public LoggingHandler(Stream stream)
    {
        _writer = new StreamWriter(stream);
    }

    protected override async Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, System.Threading.CancellationToken cancellationToken)
    {
        var response = await base.SendAsync(request, cancellationToken);

        if (!response.IsSuccessStatusCode)
        {
            _writer.WriteLine("{0}	{1}	{2}", request.RequestUri, 
                (int)response.StatusCode, response.Headers.Date);
        }
        return response;
    }

    protected override void Dispose(bool disposing)
    {
        if (disposing)
        {
            _writer.Dispose();
        }
        base.Dispose(disposing);
    }
}

将Message Handler添加到Client Pipeline

HttpClient client = HttpClientFactory.Create(new Handler1(), new Handler2(), new Handler3());

Message Handlers调用顺序根据创建时传入的顺序一致，而response消息的传播正好与此相反。