异步过程调用（asynchronous procedure call）是函数（过程）在特定线程中被异步执行。在Microsoft Windows操作系统中, APC是一种并发机制，用于异步IO或者定时器。
每个方法都提供了一个回调对象，这样就可以在完成远程过程调用后收到异步通知。
Each method provides a callback object so that you are notified asynchronously when the remote procedure call is complete.
如果在异步调用过程中出现错误，则通知用户。
Notify the user if an error occurred during the asynchronous call.
由于此过程是异步调用的，业务状态机将转换到Awaiting Approval状态，在该状态中将等待经理批准或不批准订单。
Since this process is invoked asynchronously, the business state machine transitions to the Awaiting Approval state where it will wait for the manager to approve or disapprove the order.
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