緊接着上文Asp.net web Api源碼分析-Action的執行 我們的Action已經執行完畢，現在需要把Action的返回結果轉化為HttpResponseMessage 實例，我們也知道轉化工作主要在HttpRequestMessage的CreateResponse附加方法中，
public static HttpResponseMessage CreateResponse<T>(this HttpRequestMessage request, HttpStatusCode statusCode, T value, HttpConfiguration configuration)
{
if (request == null)
{
throw Error.ArgumentNull("request");
}
configuration = configuration ?? request.GetConfiguration();
if (configuration == null)
{
throw Error.InvalidOperation(SRResources.HttpRequestMessageExtensions_NoConfiguration);
}
IContentNegotiator contentNegotiator = configuration.Services.GetContentNegotiator();
if (contentNegotiator == null)
{
throw Error.InvalidOperation(SRResources.HttpRequestMessageExtensions_NoContentNegotiator, typeof(IContentNegotiator).FullName);
}
IEnumerable<MediaTypeFormatter> formatters = configuration.Formatters;
// Run content negotiation
ContentNegotiationResult result = contentNegotiator.Negotiate(typeof(T), request, formatters);
if (result == null)
{
// no result from content negotiation indicates that 406 should be sent.
return new HttpResponseMessage
{
StatusCode = HttpStatusCode.NotAcceptable,
RequestMessage = request,
};
}
else
{
MediaTypeHeaderValue mediaType = result.MediaType;
return new HttpResponseMessage
{
// At this point mediaType should be a cloned value (the content negotiator is responsible for returning a new copy)
Content = new ObjectContent<T>(value, result.Formatter, mediaType),
StatusCode = statusCode,
RequestMessage = request
};
}
}
首先這里需要一個IContentNegotiator實例，這里有這么一句代碼：
IContentNegotiator contentNegotiator = configuration.Services.GetContentNegotiator();在DefaultServices中有 SetSingle<IContentNegotiator>(new DefaultContentNegotiator());這句，我們知道默認的contentNegotiator是DefaultContentNegotiator實例。DefaultContentNegotiator構造函數也比較普通，這里我們還需要一個數據的格式化Formatters，這里有這么一句 IEnumerable<MediaTypeFormatter> formatters = configuration.Formatters;，在Asp.net web Api源碼分析-HttpServer的創建
曾提到Formatters主要有JsonMediaTypeFormatter,XmlMediaTypeFormatter, FormUrlEncodedMediaTypeFormatter,JQueryMvcFormUrlEncodedFormatter這4個。接着我們知道要干什么了，需要把我們的value轉換為需要的格式，這里創建一個 ContentNegotiationResult result = contentNegotiator.Negotiate(typeof(T), request, formatters);實例，其中Negotiate的主要實現如下：
public virtual ContentNegotiationResult Negotiate(Type type, HttpRequestMessage request, IEnumerable<MediaTypeFormatter> formatters)
{
....。。。。。。
// Go through each formatter to compute how well it matches.
Collection<MediaTypeFormatterMatch> matches = ComputeFormatterMatches(type, request, formatters);
// Select best formatter match among the matches
MediaTypeFormatterMatch bestFormatterMatch = SelectResponseMediaTypeFormatter(matches);
// We found a best formatter
if (bestFormatterMatch != null)
{
// Find the best character encoding for the selected formatter
Encoding bestEncodingMatch = SelectResponseCharacterEncoding(request, bestFormatterMatch.Formatter);
if (bestEncodingMatch != null)
{
bestFormatterMatch.MediaType.CharSet = bestEncodingMatch.WebName;
}
MediaTypeHeaderValue bestMediaType = bestFormatterMatch.MediaType;
MediaTypeFormatter bestFormatter = bestFormatterMatch.Formatter.GetPerRequestFormatterInstance(type, request, bestMediaType);
return new ContentNegotiationResult(bestFormatter, bestMediaType);
}
return null;
}
這里的Negotiate方法實現比較復雜，首先找到當前請求支持的一個MediaTypeFormatterMatch集合，然后再從這個集合中找到找最合適一個bestFormatterMatch （在實際開發中這個實例往往都是JsonMediaTypeFormatter），如果找到那么我們繼續找一個bestEncodingMatch （最合適的一個編碼很多時候這里是utf-8），這里最后調用GetPerRequestFormatterInstance方法得到一個MediaTypeFormatter實例，然后用這么實例創建一個ContentNegotiationResult實例。
現在我們回到CreateResponse方法中來，最后直接返回一個 HttpResponseMessage實例。
MediaTypeHeaderValue mediaType = result.MediaType;
return new HttpResponseMessage
{
// At this point mediaType should be a cloned value (the content negotiator is responsible for returning a new copy)
Content = new ObjectContent<T>(value, result.Formatter, mediaType),
StatusCode = statusCode,
RequestMessage = request
};
這里的ObjectContent的創建也比較一般，我們也就忽略它吧。
現在我們回到HttpControllerHandler的BeginProcessRequest方法中來，
Task responseBodyTask = _server.Value.SendAsync(request, CancellationToken.None)
.Then(response => ConvertResponse(httpContextBase, response, request));
SendAsync已經執行完畢了，我們再來看看ConvertResponse吧，它主要就是把HttpResponseMessage的信息寫到HttpResponseBase信息做。
internal static Task ConvertResponse(HttpContextBase httpContextBase, HttpResponseMessage response, HttpRequestMessage request)
{
Contract.Assert(httpContextBase != null);
Contract.Assert(request != null);
// A null response creates a 500 with no content
if (response == null)
{
CreateEmptyErrorResponse(httpContextBase.Response);
return TaskHelpers.Completed();
}
CopyResponseStatusAndHeaders(httpContextBase, response);
CacheControlHeaderValue cacheControl = response.Headers.CacheControl;
// TODO 335085: Consider this when coming up with our caching story
if (cacheControl == null)
{
// DevDiv2 #332323. ASP.NET by default always emits a cache-control: private header.
// However, we don't want requests to be cached by default.
// If nobody set an explicit CacheControl then explicitly set to no-cache to override the
// default behavior. This will cause the following response headers to be emitted:
// Cache-Control: no-cache
// Pragma: no-cache
// Expires: -1
httpContextBase.Response.Cache.SetCacheability(HttpCacheability.NoCache);
}
// Asynchronously write the response body. If there is no body, we use
// a completed task to share the Finally() below.
// The response-writing task will not fault -- it handles errors internally.
Task writeResponseContentTask = (response.Content == null)
? TaskHelpers.Completed()
: WriteResponseContentAsync(httpContextBase, response, request);
return writeResponseContentTask.Finally(() =>
{
request.DisposeRequestResources();
request.Dispose();
response.Dispose();
});
}
這里的CopyResponseStatusAndHeaders方法比較簡單我也不多說了，就是把HttpResponseMessage中的StatusAndHeaders信息寫到 httpContextBase.Response中去，
我們還是關注一下WriteResponseContentAsync的實現吧：
internal static Task WriteResponseContentAsync(HttpContextBase httpContextBase, HttpResponseMessage response, HttpRequestMessage request)
{
HttpResponseBase httpResponseBase = httpContextBase.Response;
HttpContent responseContent = response.Content;
var unused = response.Content.Headers.ContentLength;
CopyHeaders(response.Content.Headers, httpContextBase);
// Select output buffering based on the user-controlled buffering policy
bool isBuffered = _bufferPolicySelector.Value != null ? _bufferPolicySelector.Value.UseBufferedOutputStream(response) : true;
httpResponseBase.BufferOutput = isBuffered;
return isBuffered
? WriteBufferedResponseContentAsync(httpContextBase, responseContent, request)
: WriteStreamedResponseContentAsync(httpResponseBase, responseContent);
}
前面CopyResponseStatusAndHeaders方法把HttpResponseMessage中的Headers信息寫到 httpContextBase.Response中去了，這里繼續把response.Content.Headers信息寫到 httpContextBase.Response中去。
private static readonly Lazy<IHostBufferPolicySelector> _bufferPolicySelector =
new Lazy<IHostBufferPolicySelector>(() => GlobalConfiguration.Configuration.Services.GetHostBufferPolicySelector());
在GlobalConfiguration中有這么一句 config.Services.Replace(typeof(IHostBufferPolicySelector), new WebHostBufferPolicySelector());所以我們知道_bufferPolicySelector這里其實是一個WebHostBufferPolicySelector實例，這里調用WebHostBufferPolicySelector的UseBufferedOutputStream方法來獲取當前輸出信息是否采用輸出緩存，一般情況下這個個方法返回true，主要實現如下：
public virtual bool UseBufferedOutputStream(HttpResponseMessage response)
{
HttpContent content = response.Content;
if (content != null)
{
long? contentLength = content.Headers.ContentLength;
if (contentLength.HasValue && contentLength.Value >= 0)
{
return false;
}
return !(content is StreamContent || content is PushStreamContent);
}
return false;
}
}
一般情況下response.Content.Headers.ContentLength為null。所以后面調用的是WriteBufferedResponseContentAsync方法，而該方法的主要實現就一句代碼
Task writeResponseContentTask = HttpResponseMessage.Content.CopyToAsync(HttpContextBase.Response.OutputStream);把當前HttpResponseMessage.Content中的數據全部寫到HttpContextBase.Response.OutputStream中去。到這里我們的BeginProcessRequest就差不多執行完了，后面就是調用回調函數的事情了。
到本章為止，整個web api的主要流程就說完了，這個系列中有關參數的具體綁定和返回值的格式化我是忽略了的，他們的實現都相對比較復雜，后面再抽時間來看看他們是如何實現 吧，個人對web api也不是很熟悉，發現它和mvc 中的很多代碼相似，相比之下mvc的使用比web api要廣泛得多，所以這里建議大家多讀讀mvc的源碼，讀了之后再來讀web api源碼相對要輕松很多了。