vfp MqttClient

MqttClient 是一个 Visual FoxPro 中的类库，用于连接 MQTT 服务器并进行消息发布和订阅。以下是 MqttClient 类的基本结构：

DEFINE CLASS MqttClient AS Session OLEPUBLIC
    PROCEDURE Connect(tcServer AS STRING, tcClientId AS STRING, tcUsername AS STRING, tcPassword AS STRING, tnTimeout AS INTEGER) AS LOGIC
    PROCEDURE Disconnect()
    PROCEDURE Publish(tcTopic AS STRING, tcMessage AS STRING, tnQosLevel AS INTEGER) AS LOGIC
    PROCEDURE Subscribe(tcTopic AS STRING, tnQosLevel AS INTEGER) AS LOGIC
    PROCEDURE Unsubscribe(tcTopic AS STRING) AS LOGIC
ENDDEFINE

MqttClient 类继承自 Visual FoxPro 中的 Session 类，具有连接、断开连接等基本方法，同时还增加了消息发布和订阅的方法。其中，Connect 方法用于连接 MQTT 服务器，Disconnect 方法用于断开连接，Publish 方法用于发布消息，Subscribe 和 Unsubscribe 方法用于订阅和取消订阅指定的主题。