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WebApi系列~在WebApi中實現Cors訪(fang)問(wen)

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說在前

Cors是(shi)個比較(jiao)熱的(de)技術，這(zhe)在(zai)蔣(jiang)金楠的(de)博客里也有體現(xian)，Cors簡單來說(shuo)就是(shi)“跨域資源訪問”的(de)意(yi)思(si)，這(zhe)種訪問我們指的(de)是(shi)Ajax實現(xian)的(de)異(yi)(yi)步訪問，形象(xiang)點說(shuo)就是(shi)，一個A網(wang)站(zhan)公開一些接口方法，對于B網(wang)站(zhan)和C網(wang)站(zhan)可(ke)以通過發(fa)Xmlhttprequest請求(qiu)(qiu)來調用A網(wang)站(zhan)的(de)方法，對于xmlhttprequest封裝比較(jiao)好的(de)插(cha)件如jquery的(de)$.ajax，它可(ke)以讓開發(fa)者很容易(yi)的(de)編寫AJAX異(yi)(yi)步請求(qiu)(qiu)，無論是(shi)Get,Post,Put,Delete請求(qiu)(qiu)都(dou)可(ke)以發(fa)送。

Cors并(bing)不(bu)是什么新的(de)技術，它(ta)只是對HTTP請(qing)求頭進行了一個加工(gong)，還有(you)我們的(de)Cors架構(gou)里(li)，對jsonp也有(you)封(feng)裝(zhuang)，讓開發者在(zai)使用jsonp訪問里(li)，編寫(xie)的(de)代碼量更少(shao)，更直觀，呵呵。（Jsonp和(he)Json沒什么關系，它(ta)是從一個URI返(fan)回一個Script響應(ying)塊，所以，JSONP本身是和(he)域名沒關系的(de)，而傳統上(shang)的(de)JSON是走xmlhttprequest的(de)，它(ta)在(zai)默認情(qing)況下，是不(bu)能跨(kua)域訪問的(de)）

做在后

一　　下面先說一下，對jsonp的封裝

1 注冊jsonp類型，在global.asax里Application_Start方法中

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

2 編(bian)寫JsonpMediaTypeFormatter這(zhe)個(ge)類型中實現了對jsonp請求的(de)響(xiang)應，并在響(xiang)應流中添加指定信息，如callback方(fang)法名。

 /// <summary>
    /// 對jsonp響應(ying)流的封裝
    /// </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　　對指定域名實現友好的跨域資源訪問

1 在global.asax中注冊這個(ge)(ge)HttpHandler，使它對HTTP的處理進行二(er)次加工，它可以有同步和異(yi)步兩個(ge)(ge)版本，本例中實(shi)現異(yi)步方式實(shi)現

 //對指(zhi)定URI的網站進行跨(kua)域資源的共享
 GlobalConfiguration.Configuration.MessageHandlers.Add(new EntityFrameworks.Web.Core.Handlers.CorsMessageHandler());

下面是MessageHandlers原代(dai)碼，實現(xian)對HTTP請(qing)求的二(er)次處理(li)

    /// <summary>
    /// 跨域資源(yuan)訪(fang)問的(de)HTTP處理程序
    /// </summary>
    public class CorsMessageHandler : DelegatingHandler
    {
        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
        {
            //得到描述目標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);
            //根據(ju)HttpActionDescriptor得到應用的CorsAttribute特性(xing)
            CorsAttribute corsAttribute = actionDescriptor.GetCustomAttributes<CorsAttribute>().FirstOrDefault() ??
                controllerDescriptor.GetCustomAttributes<CorsAttribute>().FirstOrDefault();
            if (null == corsAttribute)
            {
                return base.SendAsync(request, cancellationToken);
            }
            //利用(yong)CorsAttribute實施(shi)授(shou)權并生成響應報頭
            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(tian)加響(xiang)應報頭
            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(yi)便處理可以(yi)跨域訪問(wen)的(de)域名，如(ru)B網站和C網站

    /// <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)
        {

            //設置響應頭"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())
            {
                //設置響應頭"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擴(kuo)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 下面是為指(zhi)定(ding)的API類型添(tian)加指(zhi)定(ding)域名訪問的特性(xing)

    [CorsAttribute("//localhost:11879/", "//localhost:5008/")]/*需(xu)要(yao)加在類上*/
    public class ValuesController : ApiController
  {
    //代碼(ma)省(sheng)略
  }

下面看一下實例的結果：

上(shang)圖中分別使用(yong)了jsonp和json兩種方法，看一下它們的響應結果(guo)

CORS實際上(shang)是在服務端(duan)的(de)響(xiang)應頭上(shang)添(tian)加的(de)標(biao)準的(de)Access-Control-Allow-Origin的(de)信息，它是一(yi)種跨域(yu)資源訪問(wen)的(de)標(biao)準

可(ke)以看到，jsonp實現上是(shi)一(yi)種(zhong)遠程JS方法的(de)(de)調(diao)用，客(ke)戶(hu)端(duan)(duan)(duan)發起(qi)一(yi)個HTTP請求(qiu)，這(zhe)通(tong)過callback參數(shu)（一(yi)串隨(sui)機數(shu)）來(lai)區別多個客(ke)戶(hu)端(duan)(duan)(duan)，每個客(ke)戶(hu)端(duan)(duan)(duan)的(de)(de)請求(qiu)callback都是(shi)不同的(de)(de)，它(ta)們由服務器端(duan)(duan)(duan)處理(li)數(shu)據(ju)，再通(tong)過callback隨(sui)機數(shu)去為指(zhi)定客(ke)戶(hu)端(duan)(duan)(duan)返回(hui)數(shu)據(ju)。

感謝您的閱讀！

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posted @ 2014-02-20 10:32  張占嶺  閱讀(15434)  評論(10)    收藏  舉報