会在非同步回传的处理开始之前，以及回传要求传送到伺服器之前引发。
Raised before the processing of an asynchronous postback starts and the postback request is sent to the server.
如果资源可用，则从快取满足资源要求，否则会将资源要求传送至伺服器。
Satisfies a request for a resource from the cache, if the resource is available; otherwise, sends a request for a resource to the server.
快取原则会决定是否可以从快取取得所要求的资源，而不需要将要求传送至资源主机电脑。
The cache policy determines whether the requested resource can be taken from a cache instead of sending the request to the resource host computer.
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