客户机代码将轮询输出队列以获得响应，或等待响应消息交付时的回调通知。
The client code would either poll the output queue for the response or wait for some kind of call-back notification when the response message has been delivered.
在解析过程中，通过回调事件通知客户事件处理程序。
Client event handlers are informed through callbacks about the events during this parsing process.
要接收通知，高级回调接口的实现必须要用MQTT客户端注册。
To receive notifications, an implementation of the advanced callback interface must be registered with the MQTT client.
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