This exception is only applicable in the caller's thread while polling for a response.
2
服务请求者发出请求消息，并随后向服务轮询响应，或者向请求者发出回调。
The service requestor dispatches the request message and subsequently polls the service for the response, or a callback is dispatched to the requestor.
3
回调方法将句柄返回应用程序，这样，应用程序能继续处理其他消息，不必轮询响应消息。
The callback method returns the handle back to the application so that it can keep processing other messages without having to poll for the response message.
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