In this section, we will activate our web server on the ESP32 board. When we receive a SYSTEM_EVENT_STA_GOT_IP event from Wi-Fi service, we start a web server by calling the start_webserver() function.

1. We will call the stop_webserver() function when the ESP32 board is disconnected on a SYSTEM_EVENT_STA_DISCONNECTED event:

static esp_err_t event_handler(void *ctx, system_event_t *event)
{
    httpd_handle_t *server = (httpd_handle_t *) ctx;

    switch(event->event_id) {
    case SYSTEM_EVENT_STA_START:
        ESP_LOGI(TAG, "SYSTEM_EVENT_STA_START");
        ESP_ERROR_CHECK(esp_wifi_connect());
        break;
    case SYSTEM_EVENT_STA_GOT_IP:
        ESP_LOGI(TAG, "SYSTEM_EVENT_STA_GOT_IP");
        ESP_LOGI(TAG, "Got IP: '%s'",
                ip4addr_ntoa(&event->event_info.got_ip.ip_info.ip));

        /* Start the web server */
        if (*server == NULL) {
            *server = start_webserver();
        }
        break;
    case SYSTEM_EVENT_STA_DISCONNECTED:
        ESP_LOGI(TAG, "SYSTEM_EVENT_STA_DISCONNECTED");
        ESP_ERROR_CHECK(esp_wifi_connect());

        /* Stop the web server */
        if (*server) {
            stop_webserver(*server);
            *server = NULL;
        }
        break;
    default:
        break;
    }
    return ESP_OK;
}

2. Now, we will implement the start_webserver() function. We will first use httpd_start() to start our web server. Then, we will register all HTTP requests using the httpd_register_uri_handler() function:

httpd_handle_t start_webserver(void)
{
    httpd_handle_t server = NULL;
    httpd_config_t config = HTTPD_DEFAULT_CONFIG();

    // Start the httpd server
    ESP_LOGI(TAG, "Starting server on port: '%d'", config.server_port);
    if (httpd_start(&server, &config) == ESP_OK) {
        // Set URI handlers
        ESP_LOGI(TAG, "Registering URI handlers");
        httpd_register_uri_handler(server, &hello);
        httpd_register_uri_handler(server, &echo);
        httpd_register_uri_handler(server, &ctrl);
        return server;
    }

    ESP_LOGI(TAG, "Error starting server!");
    return NULL;
}

3. To stop the web server service from working, we will implement the stop_webserver() function. We will also call the httpd_stop() function to stop our web server service in ESP32:

void stop_webserver(httpd_handle_t server)
{
    // Stop the httpd server
    httpd_stop(server);
}

4. Save all the programs.