Hi guys, I was wondering how is it possible to introduce a condition in a standard spin function. More precisely: at the moment the spin function for my code looks like:

    ros::Rate loop(frequency);
    while(ros::ok()){
    //do some computations and publish messages
    loop.sleep();
    ros::spinOnce();
    }

By doing so, the computations and publish messages are executed at every loop. I would now like to introduce the following condition:

do the computation and publish messages only if a new message on the topic A is received.

So the new spin function should look like:

 ros::Rate loop(frequency);
    while(ros::ok()){
    if (new message on topic A is received){
//do some computations and publish messages
}
    loop.sleep();
    ros::spinOnce();
    }

I would like to ask you what is the best way to introduce this condition.

bool is_receive;

void callback_topic_A(msgtype &msg)
{
    is_receive = true;
}

// ...
    ros::Rate loop(frequency);
    is_receive = false;
    while(ros::ok()){
        if (is_receive){
            //do some computations and publish messages
            is_receive = false;
        }
        loop.sleep();
        ros::spinOnce();
    }

void callback_topic_A(msgtype &msg)
{
    //do some computations and publish messages
}

// ...
    ros::Rate loop(frequency);
    while(ros::ok()){
        loop.sleep();
        ros::spinOnce();
    }