vfp 定义 MqttClient

以下是定义 MqttClient 的代码：

DEFINE CLASS MqttClient AS Session OLEPUBLIC
    * Private variables
    lcServer = ""
    lcClientId = ""
    lcUsername = ""
    lcPassword = ""
    loClient = NULL

    * Public methods
    PROCEDURE Connect(tcServer AS STRING, tcClientId AS STRING, tcUsername AS STRING, tcPassword AS STRING, tnTimeout AS INTEGER) AS LOGIC
        TRY
            * Initialize MQTT client
            THIS.loClient = CREATEOBJECT("MSMQTT.MQTTClient")

            * Set client options
            THIS.loClient.MQTTServerName = tcServer
            THIS.loClient.MQTTClientID = tcClientId
            THIS.loClient.MQTTUserName = tcUsername
            THIS.loClient.MQTTPassword = tcPassword
            THIS.loClient.MQTTKeepAliveInterval = 60

            * Connect to server
            THIS.loClient.Connect(tnTimeout)

            * Return success
            RETURN .T.
        CATCH
            * Return failure
            RETURN .F.
        ENDTRY
    ENDPROC

    PROCEDURE Disconnect()
        TRY
            * Disconnect from server
            THIS.loClient.Disconnect()

            * Cleanup MQTT client
            THIS.loClient = NULL
        CATCH
        ENDTRY
    ENDPROC

    PROCEDURE Publish(tcTopic AS STRING, tcMessage AS STRING, tnQosLevel AS INTEGER) AS LOGIC
        TRY
            * Publish message
            THIS.loClient.Publish(tcTopic, tcMessage, tnQosLevel)

            * Return success
            RETURN .T.
        CATCH
            * Return failure
            RETURN .F.
        ENDTRY
    ENDPROC

    PROCEDURE Subscribe(tcTopic AS STRING, tnQosLevel AS INTEGER) AS LOGIC
        TRY
            * Subscribe to topic
            THIS.loClient.Subscribe(tcTopic, tnQosLevel)

            * Return success
            RETURN .T.
        CATCH
            * Return failure
            RETURN .F.
        ENDTRY
    ENDPROC

    PROCEDURE Unsubscribe(tcTopic AS STRING) AS LOGIC
        TRY
            * Unsubscribe from topic
            THIS.loClient.Unsubscribe(tcTopic)

            * Return success
            RETURN .T.
        CATCH
            * Return failure
            RETURN .F.
        ENDTRY
    ENDPROC
ENDDEFINE

上述代码中，MqttClient 类继承自 Visual FoxPro 中的 Session 类，定义了私有变量 lcServer、lcClientId、lcUsername、lcPassword 和 loClient，以及公共方法 Connect、Disconnect、Publish、Subscribe 和 Unsubscribe。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。