Why Finite State Machine (FSM) is important to Arduino ESP32

1. Introduction
- Arduino ESP32 core is built over FreeRTOS. If you look into the cores/esp32 folder you will see a file called "main.cpp" with code:

#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "Arduino.h"

#if CONFIG_AUTOSTART_ARDUINO

#if CONFIG_FREERTOS_UNICORE
#define ARDUINO_RUNNING_CORE 0
#else
#define ARDUINO_RUNNING_CORE 1
#endif

void loopTask(void *pvParameters)
{
    setup();
    for(;;) {
        micros(); //update overflow
        loop();
    }
}

extern "C" void app_main()
{
    initArduino();
    xTaskCreatePinnedToCore(loopTask, "loopTask", 4096, NULL, 1, NULL, ARDUINO_RUNNING_CORE);
}

#endif

- Here you can see our Arduino program is just a task of FreeRTOS. When the task is invoked it will invoke “setup()” function and then invoke the “loop()” in an infinite loop.
- We only have one task so we can only execute one job. Suppose that we implement an application that have 2 jobs:
+ job A to scan the input from keyboard.
+ job B to send a message and wait for response.
Our implementation:

void loop(){
 jobA_scan_the_keyboard();
 jobB_send_message();
 while(1){
  if(jobB_get_response()){
   break;
  }
 }
}

If job B takes long time to finish, it will block job A. So job A may miss some keyboard pressing event. It is not a good design.
2. How to apply Finite State Machine (FSM)
- In order to overcome this we will use a method called Finite State Machine. The purpose of this method is to divide a machine into a collection of states. At a specific time, only one state is active and in each state the machine just executes a specific action. After finishing that action, it may transit to another state to execute another action. For example: we have a lighting system. It can have 3 states: ON, OFF, IDLE. Its normal state is IDLE and waiting for trigger to change state ON or OFF and then back to IDLE.
Note: people often implement Finite State Machine using switch/case structure.
- Back to the example above we can re-implement the application like below:

void loop(){
 jobA_scan_the_keyboard();
 switch(jobB_state){
  case SEND_MESSAGE:
   jobB_send_message();
   jobB_state = WAIT_FOR_RESPONSE;
   break;
  case WAIT_FOR_RESPONSE:
   if(jobB_get_response()){
    jobB_state = IDLE;
    break;
   }
  case IDLE:
   if(need_send_message){
                jobB_state = SEND_MESSAGE;
                break;
            }
        case default:
            break;
    }
}

- Here jobB is divided into 3 states: SEND_MESSAGE, WAIT_FOR_RESPONSE and IDLE. jobB executes a specific action according to its state in every cycle and do not block jobA anymore. Each job has a chance to execute itself in every cycle, look like multitasking.
- I applied this method in Demo 15: How to build a system to update Price Tag automatically using Arduino ESP32

void loop() {
  /* if client was disconnected then try to reconnect again */
  if (!client.connected()) {
    mqttconnect();
  }
  /* this function will listen for incomming 
  subscribed topic-process-invoke receivedCallback */
  client.loop();
  /* because OLED waste time 
  so we use state machine to display 
  step by step every loop */
  switch(dispState)
  {
    case 1:
    display.clearDisplay();
    dispState = 2;
    break;
    case 2:
    display.setCursor(2,2);
    dispState = 3;
    break;
    case 3:
    display.println((char *)rec);
    dispState = 4;
    break;
    case 4:
    display.setCursor(2,22);
    dispState = 5;
    break;
    case 5:
    display.println((char *)&amp;rec[separate]);
    dispState = 6;
    break;
    case 6:
    display.display();
    dispState = 0;
    memset(rec, 0, BUF_SIZE);
    break;
    default:
    break;
  }
}

You can see in the code to display the received message on OLED, I used the switch/case structure, it is a style of FSM and it has states 1,2,3,4,5,6 but I did not mention the detail name of states since I just apply it to avoid blocking. Certainly we can apply it to make the program well-organized and easy to maintain. I will take one more example:
In Demo 14: How to use MQTT and Arduino ESP32 to build a simple Smart home system .
The loop() function have 2 jobs: MQTT handler and DHT temperature measure. In the old code we use counter:

void loop() {
  /* if client was disconnected then try to reconnect again */
  if (!client.connected()) {
    mqttconnect();
  }
  /* this function will listen for incomming 
  subscribed topic-process-invoke receivedCallback */
  client.loop();
  /* we measure temperature every 3 secs
  we count until 3 secs reached to avoid blocking program if using delay()*/
  long now = millis();
  if (now - lastMsg > 3000) {
    lastMsg = now;
    /* read DHT11/DHT22 sensor and convert to string */
    temperature = dht.readTemperature();
    if (!isnan(temperature)) {
      snprintf (msg, 20, "%lf", temperature);
      /* publish the message */
      client.publish(TEMP_TOPIC, msg);
    }
  }
}

Applying Finite State Machine, we can implement like below:

/* these are available states of DHT */
typedef enum {
    /* wait until 3 seconds */
    WAIT_FOR_TIMEOUT = 1,
    /* read DHT sensor for temperature */
    START_MEASURE,
    /* publish MQTT topic temperature */
    PUBLISH_MEASURE
} DHT_State;

/* create a variable to hold current state of DHT */
DHT_State state = WAIT_FOR_TIMEOUT;

void loop() {
  /* if client was disconnected then try to reconnect again */
  if (!client.connected()) {
    mqttconnect();
  }
  /* this function will listen for incomming 
  subscribed topic-process-invoke receivedCallback */
  client.loop();
  
    /* we measure temperature every 3 secs
  this code is to avoid blocking program 
  and easy to read maintain when using FinitSate Machine */
  switch(state){
    case WAIT_FOR_TIMEOUT:
        long now = millis();
        if (now - lastMsg > 3000) {
            state = START_MEASURE;
        }
        break;
    case START_MEASURE:
        temperature = dht.readTemperature();
        if (!isnan(temperature)) {
            snprintf (msg, 20, "%lf", temperature);
            state = PUBLISH_MEASURE;
        }
        break;
    case PUBLISH_MEASURE:
        /* publish the message */
        client.publish(TEMP_TOPIC, msg);
        lastMsg = now;
        state = WAIT_FOR_TIMEOUT;
        break;
  }
}