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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）
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
{
}
}
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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原文地址：http://www.cnblogs.com/lenther2002/p/4639580.html
