MqttApplicationMessage 属性

有效载荷包含将被发布的应用消息。数据的内容和格式是应用特定的。
The payload contains application messages to be published. The content and format of the data is application specific.

这个字段表示应用消息分发的服务质量等级保证。分为，最多一次，最少一次，正好一次，只发不推送。
This field indicates the quality of service level guarantee for application message distribution. Divided into, at most once, at least once, exactly once

该消息是否在服务器端进行保留，详细的说明参照文档的备注
Whether the message is retained on the server. For details, refer to the remarks of the document.

主题名（Topic Name）用于识别有效载荷数据应该被发布到哪一个信息通道。
The Topic Name is used to identify which information channel the payload data should be published to.