swagger文档转换为WebApiClient声明式代码

1 swagger简介

Swagger是一个规范且完整的框架，提供描述、生产、消费和可视化RESTful Web Service。其核心是使用json来规范描述RESTful接口，另外有提供UI来查看接口说明，并有一套生成不同语言的客户端调用代码生成器。

1.1 对Api提供者

自顶向下

使用Swagger编辑器创建Swagger定义，然后使用Swagger代码生成工具生成服务器实现。

自底向上

为已有的REST API创建Swagger定义。一般的，Api提供者都会选择这种方式，比如在asp.net里集成swagger的支持，在写好接口代码之后，访问对应的swagger的访问Uri地址，就可以得到swagger.json。例如：http://petstore.swagger.io/v2/swagger.json

{
  "swagger": "2.0",
  "info": {
    "tags": null,
    "description": "This is a sample server Petstore server.  You can find out more about Swagger at [http://swagger.io](http://swagger.io) or on [irc.freenode.net, #swagger](http://swagger.io/irc/).  For this sample, you can use the api key `special-key` to test the authorization filters.",
    "version": "1.0.0",
    "title": "Swagger Petstore",
    "termsOfService": "http://swagger.io/terms/",
    "contact": {
      "email": "apiteam@swagger.io"
    },
    "license": {
      "name": "Apache 2.0",
      "url": "http://www.apache.org/licenses/LICENSE-2.0.html"
    }
  },
  "host": "petstore.swagger.io",
  "basePath": "/v2",
  "tags": [
  ...

1.2 对Api使用者

使用swagger UI

一些提供者的站点会提供swagger ui来查看其swagger.json，例如：http://petstore.swagger.io/ 有了这些UI，自己手工编写客户端调用代码也非常简单了。

使用Swagger Codegen

可以Swagger Codegen的将swagger.json逆向生成你需要的客户端调用接口代码，本质上是使用了代码模板结合swagger.json描述来生成代码。在.net里，有一个Nswag项目，可以将swagger.json生成使用HttpClient来请求接口的c#代码。但是这些代码的质量也比较差，比如以下代码的HttpClient的生命周期也就无法很好的维护。

/// <summary>Add a new pet to the store</summary>
/// <param name="body">Pet object that needs to be added to the store</param>
/// <exception cref="SwaggerException">A server side error occurred.</exception>
/// <param name="cancellationToken">A cancellation token that can be used by other objects or threads to receive notice of cancellation.</param>
public async System.Threading.Tasks.Task AddPetAsync(Pet body, System.Threading.CancellationToken cancellationToken)
{
    var urlBuilder_ = new System.Text.StringBuilder();
    urlBuilder_.Append(BaseUrl != null ? BaseUrl.TrimEnd('/') : "").Append("/pet");

    var client_ = new System.Net.Http.HttpClient();
    try
    {
        using (var request_ = new System.Net.Http.HttpRequestMessage())
        {
            var content_ = new System.Net.Http.StringContent(Newtonsoft.Json.JsonConvert.SerializeObject(body, _settings.Value));
            content_.Headers.ContentType = System.Net.Http.Headers.MediaTypeHeaderValue.Parse("application/json");
            request_.Content = content_;
            request_.Method = new System.Net.Http.HttpMethod("POST");

            PrepareRequest(client_, request_, urlBuilder_);
            var url_ = urlBuilder_.ToString();
            request_.RequestUri = new System.Uri(url_, System.UriKind.RelativeOrAbsolute);
            PrepareRequest(client_, request_, url_);

            var response_ = await client_.SendAsync(request_, System.Net.Http.HttpCompletionOption.ResponseHeadersRead, cancellationToken).ConfigureAwait(false);
            try
            {
                var headers_ = System.Linq.Enumerable.ToDictionary(response_.Headers, h_ => h_.Key, h_ => h_.Value);
                if (response_.Content != null && response_.Content.Headers != null)
                {
                    foreach (var item_ in response_.Content.Headers)
                        headers_[item_.Key] = item_.Value;
                }

                ProcessResponse(client_, response_);

                var status_ = ((int)response_.StatusCode).ToString();
                if (status_ == "405")
                {
                    var responseData_ = response_.Content == null ? null : await response_.Content.ReadAsStringAsync().ConfigureAwait(false);
                    throw new SwaggerException("Invalid input", (int)response_.StatusCode, responseData_, headers_, null);
                }
                else
                if (status_ != "200" && status_ != "204")
                {
                    var responseData_ = response_.Content == null ? null : await response_.Content.ReadAsStringAsync().ConfigureAwait(false);
                    throw new SwaggerException("The HTTP status code of the response was not expected (" + (int)response_.StatusCode + ").", (int)response_.StatusCode, responseData_, headers_, null);
                }
            }
            finally
            {
                if (response_ != null)
                    response_.Dispose();
            }
        }
    }
    finally
    {
        if (client_ != null)
            client_.Dispose();
    }
}

2 WebApiClient.tools简介

WebApiClient是.net平台的一款RESTful声明式的面向切面客户端，其几乎100%实现了swagger定义的规范，WebApiClient.tools.swagger旨在将swagger.json逆向生成符合WebApiClient的声明式c#代码。

2.1 作用

使用原生HttpClient，你可能需要20行代码包装调用一个接口；使用WebApiClient，你可能只需要一行代码来定义接口方法；使用WebApiClient + WebApiClient.tools.swagger，你一行代码都不用写。

2.2 工作原理

1. 使用NSwag解析json得到SwaggerDocument
2. 使用RazorEngine将SwaggerDocument传入cshtml模板编译得到html
3. 使用AngleSharp将html的文本代码提取，得到WebApiClient的声明式代码
4. 代码美化，输出到本地文件

2.3 样例效果

接口代码

    /// <summary>
    /// Everything about your Pets
    /// </summary>
    [TraceFilter]
    [HttpHost("https://petstore.swagger.io/v2/")]
    public interface IPetApi : IHttpApi
    {
        /// <summary>
        /// Add a new pet to the store
        /// </summary>
        /// <param name="body">Pet object that needs to be added to the store</param>
        [HttpPost("pet")]
        ITask<HttpResponseMessage> AddPetAsync( [Required] [JsonContent] Pet body );

        /// <summary>
        /// Update an existing pet
        /// </summary>
        /// <param name="body">Pet object that needs to be added to the store</param>
        [HttpPut("pet")]
        ITask<HttpResponseMessage> UpdatePetAsync( [Required] [JsonContent] Pet body );

    ...

模型代码

    public class Pet
    {
        [AliasAs("id")]
        public long? Id { get; set; }

        [AliasAs("category")]
        public Category Category { get; set; }

        [AliasAs("name")]
        [Required(AllowEmptyStrings = true)]
        public string Name { get; set; }

        [AliasAs("photoUrls")]
        [Required]
        public List<string> PhotoUrls { get; set; } = new List<string>();

    ...

3 相关资源

WebApiClient

github: https://github.com/dotnetcore/WebApiClient

WebApiClient.tools

github: https://github.com/xljiulang/WebApiClient.Tools

NSwag

github: https://github.com/RSuter/NSwag

RazorEngine

github: https://github.com/Antaris/RazorEngine