Raised before the processing of an asynchronous postback starts and the postback request is sent to the server.
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。
The next phase after a postback is apply request values.
后视图的下一个阶段是应用请求值。
The course object is only injected on a postback, which is triggered by the user submitting the course editor form, not by every request that involves the courseAction component.
课程对象仅在回发时注入，而回发是用户提交课程编辑器表单触发的，不是由每个涉及courseAction 组件的请求触发的。
It is important to note that if the request is a postback, the values of the controls have not yet been restored from view state.
需要注意的是，如果请求需要进行回调，那么这些控件的值是不能够在viewstate中被缓存的。
Determine whether the request comes as a postback.
判断请求是否是postback。
Gets the request object that represents the current postback.
获取表示当前回发的请求对象。
Gets the request object that represents the current postback.
获取表示当前回发的请求对象。
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