c - 使用 MQTT paho 异步发布触发新消息回调

Question

我正在使用来自 MQTT paho 的 MQTTAsync_publish.c 示例代码。我所做的是将示例代码中的常量 PAYLOAD 替换为 SendMessageArray。如果有新的无线电消息到达，这个数组的内容会在我的其他代码中发生变化。有效负载在第一次连接时成功发送。

但是，我不明白当我的有效负载更改时如何触发新发布？

函数 MQTTAsync_sendMessage 是在 onConnect 回调中触发的，所以我必须以某种方式再次触发此回调，但是如何。如果我通过不调用 onSend 回调将示例代码更改为在第一次发送后保持连接，那么在完成 = 0 时，它是“等待客户端 ID：%s\n 的主题 %s 上的 %s 的发布”，但我无法放置新的有效载荷。

volatile MQTTAsync_token deliveredtoken;
char SendMessageArray[4096] = { 0 };  // global
int finished = 0;

void connlost(void *context, char *cause)
{
    MQTTAsync client = (MQTTAsync)context;
    MQTTAsync_connectOptions conn_opts = MQTTAsync_connectOptions_initializer;
    int rc;

    printf("\nConnection lost\n");
    printf("     cause: %s\n", cause);

    printf("Reconnecting\n");
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;
    if ((rc = MQTTAsync_connect(client, &conn_opts)) != MQTTASYNC_SUCCESS)
    {
        printf("Failed to start connect, return code %d\n", rc);
        finished = 1;
    }
}


void onDisconnect(void* context, MQTTAsync_successData* response)
{
    printf("Successful disconnection\n");
    finished = 1;
}


void onSend(void* context, MQTTAsync_successData* response)
{
    MQTTAsync client = (MQTTAsync)context;
    MQTTAsync_disconnectOptions opts = MQTTAsync_disconnectOptions_initializer;
    int rc;

    printf("Message with token value %d delivery confirmed\n", response->token);

    opts.onSuccess = onDisconnect;
    opts.context = client;

    if ((rc = MQTTAsync_disconnect(client, &opts)) != MQTTASYNC_SUCCESS)
    {
        printf("Failed to start sendMessage, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }
}


void onConnectFailure(void* context, MQTTAsync_failureData* response)
{
    printf("Connect failed, rc %d\n", response ? response->code : 0);
    finished = 1;
}


void onConnect(void* context, MQTTAsync_successData* response)
{
    MQTTAsync client = (MQTTAsync)context;
    MQTTAsync_responseOptions opts = MQTTAsync_responseOptions_initializer;
    MQTTAsync_message pubmsg = MQTTAsync_message_initializer;
    int rc;

    printf("Successful connection\n");
    
    opts.onSuccess = onSend; // -> something like onHoldConnect???
    opts.context = client;

    pubmsg.payload = SendMessageArray; //PAYLOAD;
    pubmsg.payloadlen = (int)strlen(SendMessageArray);
    pubmsg.qos = QOS;
    pubmsg.retained = 0;
    deliveredtoken = 0;

    if ((rc = MQTTAsync_sendMessage(client, TOPIC, &pubmsg, &opts)) != MQTTASYNC_SUCCESS)
    {
        printf("Failed to start sendMessage, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }
}


int main(int argc, char* argv[])
{
    MQTTAsync client;
    MQTTAsync_connectOptions conn_opts = MQTTAsync_connectOptions_initializer;
    int rc;

    MQTTAsync_create(&client, ADDRESS, CLIENTID, MQTTCLIENT_PERSISTENCE_NONE, NULL);

    MQTTAsync_setCallbacks(client, NULL, connlost, NULL, NULL);

    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;
    conn_opts.onSuccess = onConnect;
    conn_opts.onFailure = onConnectFailure;
    conn_opts.context = client;
    if ((rc = MQTTAsync_connect(client, &conn_opts)) != MQTTASYNC_SUCCESS)
    {
        printf("Failed to start connect, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }

    printf("Waiting for publication of %s\n"
         "on topic %s for client with ClientID: %s\n",
         SendMessageArray, TOPIC, CLIENTID);
    while (!finished)
        #if defined(WIN32)
            Sleep(100);
        #else
            usleep(10000L);
        #endif

    MQTTAsync_destroy(&client);
    return rc;
}

这里是原始代码： http: //gitlab.witium.com.cn/eclipse/paho.mqtt.c/blob/master/src/samples/MQTTAsync_publish.c

我不知何故需要与发送所有新消息的代理建立持久连接。谢谢你的帮助。

编辑：好的，我现在所做的是（1）通过在发送后不断开连接来保持连接：

void onHoldConnect(void* context, MQTTAsync_successData* response)
{
    printf("Holding connection\n");
}

void onConnect(void* context, MQTTAsync_successData* response)
{
    MQTTAsync client = (MQTTAsync)context;
    MQTTAsync_responseOptions opts = MQTTAsync_responseOptions_initializer;
    MQTTAsync_message pubmsg = MQTTAsync_message_initializer;
    int rc;

    printf("Successful connection\n");
    opts.onSuccess = onHoldConnect; //onSend;
    opts.onFailure = onSendFailure;
    opts.context = client;
    pubmsg.payload = SendMessageArray; //PAYLOAD;
    pubmsg.payloadlen = (int)strlen(SendMessageArray); 
    pubmsg.qos = QOS;
    pubmsg.retained = 0;

    if ((rc = MQTTAsync_sendMessage(client, TOPIC, &pubmsg, &opts)) != MQTTASYNC_SUCCESS)
    {
      printf("Failed to start sendMessage, return code %d\n", rc);
      exit(EXIT_FAILURE);
    }
}

(2) 如果 SendMessageArray 已被修改，则在 while 循环中使用全局 bool 运行 onConnect 函数：

while (!finished) {
    if (sendFlag == true) {
      sendFlag = false;
      onConnect(client, &conn_opts);
    }
    #if defined(_WIN32)
        Sleep(100);
    #else
        usleep(10000L);
    #endif
}

这是解决问题的正确方法吗？我只是再次使用 (client, &conn_opts) 作为参数。

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