WebApi系列~在WebApi中实现Cors访问

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说在前

Cors是个比较热的技术，这在蒋金楠的博客里也有体现，Cors简单来说就是“跨域资源访问”的意思，这种访问我们指的是Ajax实现的异步访问，形象点说就是，一个A网站公开一些接口方法，对于B网站和C网站可以通过发Xmlhttprequest请求来调用A网站的方法，对于xmlhttprequest封装比较好的插件如jquery的$.ajax，它可以让开发者很容易的编写AJAX异步请求，无论是Get,Post,Put,Delete请求都可以发送。

Cors并不是什么新的技术，它只是对HTTP请求头进行了一个加工，还有我们的Cors架构里，对jsonp也有封装，让开发者在使用jsonp访问里，编写的代码量更少，更直观，呵呵。（Jsonp和Json没什么关系，它是从一个URI返回一个Script响应块，所以，JSONP本身是和域名没关系的，而传统上的JSON是走xmlhttprequest的，它在默认情况下，是不能跨域访问的）

做在后

一　　下面先说一下，对jsonp的封装

1 注册jsonp类型，在global.asax里Application_Start方法中

 GlobalConfiguration.Configuration.Formatters.Insert(0, new EntityFrameworks.Web.Core.JsonpMediaTypeFormatter());

2 编写JsonpMediaTypeFormatter这个类型中实现了对jsonp请求的响应，并在响应流中添加指定信息，如callback方法名。

 /// <summary>
    /// 对jsonp响应流的封装
    /// </summary>
    public class JsonpMediaTypeFormatter : JsonMediaTypeFormatter
    {
        public string Callback { get; private set; }
        public JsonpMediaTypeFormatter(string callback = null)
        {
            this.Callback = callback;
        }
        public override Task WriteToStreamAsync(
            Type type,
            object value,
            Stream writeStream,
            HttpContent content,
            TransportContext transportContext)
        {
            if (string.IsNullOrEmpty(this.Callback))
            {
                return base.WriteToStreamAsync(type, value, writeStream, content, transportContext);
            }
            try
            {
                this.WriteToStream(type, value, writeStream, content);
                return Task.FromResult<AsyncVoid>(new AsyncVoid());
            }
            catch (Exception exception)
            {
                TaskCompletionSource<AsyncVoid> source = new TaskCompletionSource<AsyncVoid>();
                source.SetException(exception);
                return source.Task;
            }
        }
        private void WriteToStream(
            Type type,
            object value,
            Stream writeStream,
            HttpContent content)
        {
            JsonSerializer serializer = JsonSerializer.Create(this.SerializerSettings);
            using (StreamWriter streamWriter = new StreamWriter(writeStream, this.SupportedEncodings.First()))
            using (JsonTextWriter jsonTextWriter = new JsonTextWriter(streamWriter) { CloseOutput = false })
            {
                jsonTextWriter.WriteRaw(this.Callback + "(");
                serializer.Serialize(jsonTextWriter, value);
                jsonTextWriter.WriteRaw(")");
            }
        }
        public override MediaTypeFormatter GetPerRequestFormatterInstance(
            Type type,
            HttpRequestMessage request,
            MediaTypeHeaderValue mediaType)
        {
            if (request.Method != HttpMethod.Get)
            {
                return this;
            }
            string callback;
            if (request.GetQueryNameValuePairs().ToDictionary(pair => pair.Key,
                 pair => pair.Value).TryGetValue("callback", out callback))
            {
                return new JsonpMediaTypeFormatter(callback);
            }
            return this;
        }
        [StructLayout(LayoutKind.Sequential, Size = 1)]
        private struct AsyncVoid
        {
        }

    }

 

二　　对指定域名实现友好的跨域资源访问

1 在global.asax中注册这个HttpHandler，使它对HTTP的处理进行二次加工，它可以有同步和异步两个版本，本例中实现异步方式实现

 //对指定URI的网站进行跨域资源的共享
 GlobalConfiguration.Configuration.MessageHandlers.Add(new EntityFrameworks.Web.Core.Handlers.CorsMessageHandler());

下面是MessageHandlers原代码，实现对HTTP请求的二次处理

    /// <summary>
    /// 跨域资源访问的HTTP处理程序
    /// </summary>
    public class CorsMessageHandler : DelegatingHandler
    {
        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
        {
            //得到描述目标Action的HttpActionDescriptor
            HttpMethod originalMethod = request.Method;
            bool isPreflightRequest = request.IsPreflightRequest();
            if (isPreflightRequest)
            {
                string method = request.Headers.GetValues("Access-Control-Request-Method").First();
                request.Method = new HttpMethod(method);
            }

            HttpConfiguration configuration = request.GetConfiguration();
            HttpControllerDescriptor controllerDescriptor = configuration.Services.GetHttpControllerSelector().SelectController(request);
            HttpControllerContext controllerContext = new HttpControllerContext(request.GetConfiguration(), request.GetRouteData(), request)
            {
                ControllerDescriptor = controllerDescriptor
            };
            HttpActionDescriptor actionDescriptor = configuration.Services.GetActionSelector().SelectAction(controllerContext);
            //根据HttpActionDescriptor得到应用的CorsAttribute特性
            CorsAttribute corsAttribute = actionDescriptor.GetCustomAttributes<CorsAttribute>().FirstOrDefault() ??
                controllerDescriptor.GetCustomAttributes<CorsAttribute>().FirstOrDefault();
            if (null == corsAttribute)
            {
                return base.SendAsync(request, cancellationToken);
            }
            //利用CorsAttribute实施授权并生成响应报头
            IDictionary<string, string> headers;
            request.Method = originalMethod;
            bool authorized = corsAttribute.TryEvaluate(request, out headers);
            HttpResponseMessage response;
            if (isPreflightRequest)
            {
                if (authorized)
                {
                    response = new HttpResponseMessage(HttpStatusCode.OK);
                }
                else
                {
                    response = request.CreateErrorResponse(HttpStatusCode.BadRequest, corsAttribute.ErrorMessage);
                }
            }
            else
            {
                response = base.SendAsync(request, cancellationToken).Result;
            }

            //添加响应报头
            if (headers != null && headers.Any())
                foreach (var item in headers)
                    response.Headers.Add(item.Key, item.Value);

            return Task.FromResult<HttpResponseMessage>(response);
        }
    }

2 添加Cors特性，以便处理可以跨域访问的域名，如B网站和C网站

    /// <summary>
    /// Cors特性
    /// </summary>
   [AttributeUsage(AttributeTargets.Class, Inherited = true, AllowMultiple = false)]   
    public class CorsAttribute : Attribute
    {
        public Uri[] AllowOrigins { get; private set; }
        public string ErrorMessage { get; private set; }
        public CorsAttribute(params string[] allowOrigins)
        {
            this.AllowOrigins = (allowOrigins ?? new string[0]).Select(origin => new Uri(origin)).ToArray();
        }
        public bool TryEvaluate(HttpRequestMessage request, out IDictionary<string, string> headers)
        {
            headers = null;
            string origin = null;
            try
            {
                origin = request.Headers.GetValues("Origin").FirstOrDefault();
            }
            catch (Exception)
            {
                this.ErrorMessage = "Cross-origin request denied";
                return false;
            }
            Uri originUri = new Uri(origin);
            if (this.AllowOrigins.Contains(originUri))
            {
                headers = this.GenerateResponseHeaders(request);
                return true;
            }

            this.ErrorMessage = "Cross-origin request denied";
            return false;
        }

        private IDictionary<string, string> GenerateResponseHeaders(HttpRequestMessage request)
        {

            //设置响应头"Access-Control-Allow-Methods"

            string origin = request.Headers.GetValues("Origin").First();

            Dictionary<string, string> headers = new Dictionary<string, string>();

            headers.Add("Access-Control-Allow-Origin", origin);

            if (request.IsPreflightRequest())
            {
                //设置响应头"Access-Control-Request-Headers"
                //和"Access-Control-Allow-Headers"
                headers.Add("Access-Control-Allow-Methods", "*");

                string requestHeaders = request.Headers.GetValues("Access-Control-Request-Headers").FirstOrDefault();

                if (!string.IsNullOrEmpty(requestHeaders))
                {
                    headers.Add("Access-Control-Allow-Headers", requestHeaders);
                }
            }
            return headers;
        }
    }

    /// <summary>
    /// HttpRequestMessage扩展方法
    /// </summary>
    public static class HttpRequestMessageExtensions
    {
        public static bool IsPreflightRequest(this HttpRequestMessage request)
        {
            return request.Method == HttpMethod.Options
                && request.Headers.GetValues("Origin").Any()
                && request.Headers.GetValues("Access-Control-Request-Method").Any();
        }
    }

3 下面是为指定的API类型添加指定域名访问的特性

    [CorsAttribute("http://localhost:11879/", "http://localhost:5008/")]/*需要加在类上*/
    public class ValuesController : ApiController
  {
    //代码省略
  }

下面看一下實例的結果：

上图中分别使用了jsonp和json两种方法，看一下它们的响应结果

CORS实际上是在服务端的响应头上添加的标准的Access-Control-Allow-Origin的信息，它是一种跨域资源访问的标准

可以看到，jsonp实现上是一种远程JS方法的调用，客户端发起一个HTTP请求，这通过callback参数（一串随机数）来区别多个客户端，每个客户端的请求callback都是不同的，它们由服务器端处理数据，再通过callback随机数去为指定客户端返回数据。

感谢您的阅读！

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