在mvc中有一個相對比較獨立的類OutputCacheAttribute，一看它的名字我們就知道應該與什么緩存有關了吧。
public class OutputCacheAttribute : ActionFilterAttribute, IExceptionFilter 在這個類中涉及到兩個重要的東西OutputCacheParameters緩存配置、ObjectCache緩存的管理方式，這兩個主要是用來干什么的我們后面再說吧。
OutputCacheAttribute繼承於ActionFilterAttribute特性，那么我們就來看看它那4個方法是怎么實現的吧：
public override void OnActionExecuting(ActionExecutingContext filterContext) {
if (filterContext == null) {
throw new ArgumentNullException("filterContext");
}
if (filterContext.IsChildAction) {
ValidateChildActionConfiguration();
// Already actively being captured? (i.e., cached child action inside of cached child action)
// Realistically, this needs write substitution to do properly (including things like authentication)
if (GetChildActionFilterFinishCallback(filterContext) != null) {
throw new InvalidOperationException(MvcResources.OutputCacheAttribute_CannotNestChildCache);
}
// Already cached?
string uniqueId = GetChildActionUniqueId(filterContext);
string cachedValue = ChildActionCacheInternal.Get(uniqueId) as string;
if (cachedValue != null) {
filterContext.Result = new ContentResult() { Content = cachedValue };
return;
}
// Swap in a new TextWriter so we can capture the output
StringWriter cachingWriter = new StringWriter(CultureInfo.InvariantCulture);
TextWriter originalWriter = filterContext.HttpContext.Response.Output;
filterContext.HttpContext.Response.Output = cachingWriter;
// Set a finish callback to clean up
SetChildActionFilterFinishCallback(filterContext, wasException => {
// Restore original writer
filterContext.HttpContext.Response.Output = originalWriter;
// Grab output and write it
string capturedText = cachingWriter.ToString();
filterContext.HttpContext.Response.Write(capturedText);
// Only cache output if this wasn't an error
if (!wasException) {
ChildActionCacheInternal.Add(uniqueId, capturedText, DateTimeOffset.UtcNow.AddSeconds(Duration));
}
});
}
}
public override void OnActionExecuted(ActionExecutedContext filterContext) {
if (filterContext == null) {
throw new ArgumentNullException("filterContext");
}
// Complete the request if the child action threw an exception
if (filterContext.IsChildAction && filterContext.Exception != null) {
CompleteChildAction(filterContext, wasException: true);
}
}
public override void OnResultExecuting(ResultExecutingContext filterContext) {
if (filterContext == null) {
throw new ArgumentNullException("filterContext");
}
if (!filterContext.IsChildAction) {
// we need to call ProcessRequest() since there's no other way to set the Page.Response intrinsic
using (OutputCachedPage page = new OutputCachedPage(_cacheSettings)) {
page.ProcessRequest(HttpContext.Current);
}
}
從這4個方法我們可以知道一個普通的Action和一個子Action的處理方式是不同的。
首先我們來看看一個主Action的緩存處理方式：OnActionExecuting、OnActionExecuted、 OnResultExecuted都沒做什么處理，唯一處理的是OnResultExecuting方法，同時該方法對子Action也沒做什么處理。 OnResultExecuting的處理很簡單
if (!filterContext.IsChildAction) {
// we need to call ProcessRequest() since there's no other way to set the Page.Response intrinsic
using (OutputCachedPage page = new OutputCachedPage(_cacheSettings)) {
page.ProcessRequest(HttpContext.Current);
}
}
OutputCachedPage是一個普通的Page類
private sealed class OutputCachedPage : Page {
private OutputCacheParameters _cacheSettings;
public OutputCachedPage(OutputCacheParameters cacheSettings) {
// Tracing requires Page IDs to be unique.
ID = Guid.NewGuid().ToString();
_cacheSettings = cacheSettings;
}
protected override void FrameworkInitialize() {
// when you put the <%@ OutputCache %> directive on a page, the generated code calls InitOutputCache() from here
base.FrameworkInitialize();
InitOutputCache(_cacheSettings);
}
}
}
public override void OnResultExecuted(ResultExecutedContext filterContext) {
if (filterContext == null) {
throw new ArgumentNullException("filterContext");
}
if (filterContext.IsChildAction) {
CompleteChildAction(filterContext, wasException: filterContext.Exception != null);
}
}
從這里可以看出來主Action是完成一次標准的Http請求，它的處理方式和傳統的asp.net的緩存處理方式是一樣的，由OutputCacheModule來處理，所以OutputCacheAttribute中有一個OutputCacheParameters東東就是用在這個時候的。
為什么我們的子Action要區別對待了？
RouteData routeData = CreateRouteData(vpd.Route, routeValues, vpd.DataTokens, htmlHelper.ViewContext);
HttpContextBase httpContext = htmlHelper.ViewContext.HttpContext;
RequestContext requestContext = new RequestContext(httpContext, routeData);
ChildActionMvcHandler handler = new ChildActionMvcHandler(requestContext);
httpContext.Server.Execute(HttpHandlerUtil.WrapForServerExecute(handler), textWriter, true /* preserveForm */);
這里是調用子Action的關鍵代碼，從這里我們可以知道子Action的調用並沒有創建新的HttpContext，它還是沿用主Action的 HttpContext，也就是說它並不是一次完整、標准的http請求，這里只是主Action調用了一個handler而已，只是這個handler 把mvc該做的工作差不多都做了而已。
現在我們來看子Action的處理方式，在OnActionExecuting方法處理相對要多一點，首先調用ValidateChildActionConfiguration方法來驗證緩存配置
private void ValidateChildActionConfiguration() {
if (Duration <= 0) {
throw new InvalidOperationException(MvcResources.OutputCacheAttribute_InvalidDuration);
}
if (String.IsNullOrWhiteSpace(VaryByParam)) {
throw new InvalidOperationException(MvcResources.OutputCacheAttribute_InvalidVaryByParam);
}
if (!String.IsNullOrWhiteSpace(CacheProfile) ||
!String.IsNullOrWhiteSpace(SqlDependency) ||
!String.IsNullOrWhiteSpace(VaryByContentEncoding) ||
!String.IsNullOrWhiteSpace(VaryByHeader) ||
_locationWasSet || _noStoreWasSet) {
throw new InvalidOperationException(MvcResources.OutputCacheAttribute_ChildAction_UnsupportedSetting);
}
}
public OutputCacheLocation Location {
get {
return _cacheSettings.Location;
}
set {
_cacheSettings.Location = value;
_locationWasSet = true;
}
}
public bool NoStore {
get {
return _cacheSettings.NoStore;
}
set {
_cacheSettings.NoStore = value;
_noStoreWasSet = true;
}
}
大家一定要注意這個檢查方法啊：Location、NoStore屬性石完全不能設置 的，CacheProfile 、SqlDependency 、VaryByContentEncoding、 VaryByHeader、 VaryByHeader這幾個值差不多也不能設置，要設置頁只能設置空字符那和不設置也沒什么區別因為默認就是null。
接下來就是通過當前的ActionExecutingContext來獲取緩存key進而獲取緩存對象。
string uniqueId = GetChildActionUniqueId(filterContext);
string cachedValue = ChildActionCacheInternal.Get(uniqueId) as string;
ChildActionCacheInternal 相關代碼如下：
private Func<ObjectCache> _childActionCacheThunk = () => ChildActionCache;
internal OutputCacheAttribute(ObjectCache childActionCache) {
_childActionCacheThunk = () => childActionCache;
}
public static ObjectCache ChildActionCache {
get {
return _childActionCache ?? MemoryCache.Default;
}
set {
_childActionCache = value;
}
}
private ObjectCache ChildActionCacheInternal {
get {
return _childActionCacheThunk();
}
}
我想現在大家應該對OutputCacheAttribute中的ObjectCache有個了解了吧，它就是用來緩存子Action的處理。一旦我們獲取到緩存結果我們就返回一個ContentResult給 filterContext.Result 屬性並退出該方法。
if (cachedValue != null) {
filterContext.Result = new ContentResult() { Content = cachedValue };
return;
}
從前面的文章我們知道Filter和Action的調用時在ControllerActionInvoker的InvokeActionMethodFilter方法，這個方法的主要邏輯如下：
internal static ActionExecutedContext InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func<ActionExecutedContext> continuation) {
filter.OnActionExecuting(preContext);
if (preContext.Result != null) {
return new ActionExecutedContext(preContext, preContext.ActionDescriptor, true /* canceled */, null /* exception */) {
Result = preContext.Result
};
}
bool wasError = false;
ActionExecutedContext postContext = null;
try {
postContext = continuation();
}
catch (ThreadAbortException) {
..
}
catch (Exception ex) {
...
}
if (!wasError) {
filter.OnActionExecuted(postContext);
}
return postContext;
}
一旦ActionExecutingContext的Result有值我們就退出該方法，這里的退出就保證了Action的不執行。
如果我們沒有找到緩存對象，那么我們就創建一個臨時StringWriter實例，讓它替換當前HttpContext.Response.Output實例，還需要把HttpContext.Response.Output保存起來，已備后面還原。
StringWriter cachingWriter = new StringWriter(CultureInfo.InvariantCulture);
TextWriter originalWriter = filterContext.HttpContext.Response.Output;
filterContext.HttpContext.Response.Output = cachingWriter; 后面調用SetChildActionFilterFinishCallback注冊一個回調方法，在當前HttpContext.Items中插 入暫存數據，於此方法相關的還有幾個方法GetChildActionFilterFinishCallback 、CompleteChildAction 、ClearChildActionFilterFinishCallback
private static void SetChildActionFilterFinishCallback(ControllerContext controllerContext, Action<bool> callback) {
controllerContext.HttpContext.Items[_childActionFilterFinishCallbackKey] = callback;
}
private static Action<bool> GetChildActionFilterFinishCallback(ControllerContext controllerContext) {
return controllerContext.HttpContext.Items[_childActionFilterFinishCallbackKey] as Action<bool>;
}
private static void CompleteChildAction(ControllerContext filterContext, bool wasException) {
Action<bool> callback = GetChildActionFilterFinishCallback(filterContext);
if (callback != null) {
ClearChildActionFilterFinishCallback(filterContext);
callback(wasException);
}
}
private static void ClearChildActionFilterFinishCallback(ControllerContext controllerContext) {
controllerContext.HttpContext.Items.Remove(_childActionFilterFinishCallbackKey);
}
現在OnActionExecuting方法結束了，OnActionExecuted方法也沒什么特殊的處理，主要就是看看有沒有異常出現，代碼如下：
if (filterContext.IsChildAction && filterContext.Exception != null) {
CompleteChildAction(filterContext, wasException: true);
}
而OnResultExecuting方法也沒有什么處理，最后剩下的就是OnResultExecuted方法，
if (filterContext.IsChildAction) {
CompleteChildAction(filterContext, wasException: filterContext.Exception != null);
}
說白了就是調用我們先前注冊的回調方法。方法的具體類容是：
filterContext.HttpContext.Response.Output = originalWriter;
// Grab output and write it
string capturedText = cachingWriter.ToString();
filterContext.HttpContext.Response.Write(capturedText);
// Only cache output if this wasn't an error
if (!wasException) {
ChildActionCacheInternal.Add(uniqueId, capturedText, DateTimeOffset.UtcNow.AddSeconds(Duration));
}
這里的capturedText就是我們子Action所對應view的文本結果，originalWriter是主Action的 Response.Output對象，即是把子Action的返回結果寫到主Action的輸出流中。最后看看是否有錯誤發生，如果沒有就把此次緩存類容 放到緩存ObjectCache中。
在這里我補充一下，前面提到一個GetChildActionUniqueId是根據ActionExecutingContext來創建緩存key的，那么這個key與哪些東西相關了。
internal string GetChildActionUniqueId(ActionExecutingContext filterContext) {
StringBuilder uniqueIdBuilder = new StringBuilder();
// Start with a prefix, presuming that we share the cache with other users
uniqueIdBuilder.Append(_cacheKeyPrefix);
// Unique ID of the action description
uniqueIdBuilder.Append(filterContext.ActionDescriptor.UniqueId);
// Unique ID from the VaryByCustom settings, if any
uniqueIdBuilder.Append(DescriptorUtil.CreateUniqueId(VaryByCustom));
if (!String.IsNullOrEmpty(VaryByCustom)) {
string varyByCustomResult = filterContext.HttpContext.ApplicationInstance.GetVaryByCustomString(HttpContext.Current, VaryByCustom);
uniqueIdBuilder.Append(varyByCustomResult);
}
// Unique ID from the VaryByParam settings, if any
uniqueIdBuilder.Append(GetUniqueIdFromActionParameters(filterContext, SplitVaryByParam(VaryByParam)));
// The key is typically too long to be useful, so we use a cryptographic hash
// as the actual key (better randomization and key distribution, so small vary
// values will generate dramtically different keys).
using (SHA256 sha = SHA256.Create()) {
return Convert.ToBase64String(sha.ComputeHash(Encoding.UTF8.GetBytes(uniqueIdBuilder.ToString())));
}
}
從GetChildActionUniqueId的方法我們知道這個key與Action本生 有關，不同的Action其key不同，與緩存的VaryByCustom屬性有關，即使是同一個VaryByCustom取值不同其可以也不同，這里調 用了HttpApplication的GetVaryByCustomString方法
總結一下吧：mvc的緩存分類2部分 一部分是主Action的緩存，主Action是一個完整的http請，它是借助OutputCacheModule來緩存的，而子Action並不是一個完整的http請求它只是一個簡單的數據緩存，借助於ObjectCache。