How to publish a message from a rosserial_arduino subscriber

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How do you make a subscriber in a rosserial_arduino node that can publish a message?

I'm trying to make a node that listens for a "force_sensors" message, and in response, publishes sensor readings.

This is my Arduino Uno sketch:

#include <ros.h>
#include <std_msgs/Empty.h>
#include <std_msgs/Bool.h>
#include <std_msgs/Byte.h>
#include <std_msgs/Int64.h>
#include <std_msgs/Float32.h>
#include <std_msgs/Float64MultiArray.h>

#include "BooleanSensor.h"

sensor = BooleanSensor();

// If true, all sensors will push their current readings to the host, even if they haven't changed
// since last polling.
bool force_sensors = false;

ros::NodeHandle nh;

std_msgs::Bool bool_msg;

ros::Publisher sensor_publisher = ros::Publisher("sensor", &bool_msg);

void on_force_sensors(const std_msgs::Empty& msg) {
    force_sensors = true;
}
ros::Subscriber<std_msgs::Empty> force_sensors_sub("force_sensors", &on_force_sensors);

void on_toggle_led(const std_msgs::Empty& msg) {
    force_sensors = true;
    digitalWrite(STATUS_LED_PIN, HIGH-digitalRead(STATUS_LED_PIN));   // blink the led
}
ros::Subscriber<std_msgs::Empty> toggle_led_sub("toggle_led", &on_toggle_led);

void setup() {

    pinMode(STATUS_LED_PIN, OUTPUT);
    digitalWrite(STATUS_LED_PIN, true);

    nh.getHardware()->setBaud(57600);
    nh.initNode();
    nh.subscribe(toggle_led_sub);
    nh.subscribe(force_sensors_sub);
    nh.advertise(sensor_publisher);

}

void loop() {

    if (sensor.changed() || force_sensors) {
        bool_msg.data = sensor.get();
        sensor_publisher.publish(&bool_msg);
    }

    nh.spinOnce();
    delay(1);

    force_sensors = false;
}

It has two subscribers, one that listens for a "toggle_led" message, and toggles the state of the onboard LED.

The other listens for a "force_sensors" message, and sets a boolean flag to force a sensor message to be published.

Attached to the Uno is a simple button whose state is checked when sensor.changed() is called. I've confirmed that when I push this button, I see a message with rostopic echo /uno/sensor.

I would also expect to see this same message after I run rostopic pub /torso_arduino/force_sensors std_msgs/Empty --once. However, no message is received.

I modified the toggle_led subscriber to also set the force_sensor flag. Yet even though this still responds to a message and toggles the LED, it still doesn't force the sending of any sensor messages.

What am I doing wrong?