You can not use progmem with ESP and mysensors now because PSTR redefined as PSTR(x) (x) so all strings are really in RAM :(

scarce ATMega RAM...

I mean ESP RAM ofcourse... :facepunch:

So this solution is for somebody, who need dynamic reconfiguration of system settings. Remember, that char[] sizes just example and may be reduced or changed to String(...).c_str(). But I don't know arduino well (prefer pure C) and can't predict memory usage/fragmentation on that way.

BTW, as an idea - add wifi and/or mqtt settings as MyMessage to be changed via "standard" way

char ssid[32] = "MyAP";
char pwd[32] = "MyPass";
word mqttPort = 1883;
char mqttHost[64] = "my.mqtt.example.com";

#define MY_GATEWAY_ESP8266
#define MY_GATEWAY_MQTT_CLIENT
#define MY_ESP8266_SSID (ssid)
#define MY_ESP8266_PASSWORD (pwd)

#define MY_PORT (mqttPort)
#define MY_CONTROLLER_URL_ADDRESS (mqttHost)
#define MY_MQTT_CLIENT_ID  (String("MyS_") + String(ESP.getChipId(), HEX)).c_str()

// DOES NOT SUPPORT VARS:
#define MY_MQTT_PUBLISH_TOPIC_PREFIX "/MySe/00/out"
#define MY_MQTT_SUBSCRIBE_TOPIC_PREFIX "/MySe/00/in"

#include <MySensor.h>

void before() {
  // read config into local vars, e.g. from FS via json
  strncpy(ssid, "AltSSID", sizeof(ssid)); // example
}

void setup() {
}

void loop() {
}

FYI, The wifi manager from first post is not so useful here for me - on first look there is some issues with dhcp vs static ip, so I'll try to find another solution or write my own :)

This may solve an issue with AP (compiled ok but untested char[] as MY_ESP8266_SSID) but not for MQTT transport - a lot of PSTR(MY_MQTT_XXX) "/+/+/+" code required refactoring to be used with at least const char*

Example:
https://github.com/mysensors/Arduino/blob/469cbb7d0ccd4698079a44e8632f660fbe50cf18/libraries/MySensors/examples/RelayActuator/RelayActuator.ino#L49-L56

The main issue is that WiFi.begin() and awaiting for WiFi.state == WL_CONNECTED logic executed before setup() so I can not invoke web-config or change hardcoded AP name. In my previous "pure" arduino sensor (https://bitbucket.org/esp8266/sensor-v2) I use WiFiMulti to connect first available pre-configured AP's, so I need at least two AP/password settings (for example home and work wifi)

BTW, de-facto MySensors internally "ignores" last saved AP because calls begin(SSID, PWD) in MyGatewayTransportXXX.cpp but it is good idea as for me. As an option - do not call WiFi.begin() if wifi state already "connected" but required some functionality to force reconnect to /other/ AP

So I need 4 steps:

1. Try connect to saved AP (may be more than one)
2. If not success, bring up softAP and Web/Telnet configurator
3. Reconnect to new configured AP or reboot an goto step 1
4. Start MySensors with already connected WiFi and new SSID

One way: Globally redesign internal initialization sequence and add extra EEPROM arguments. Also provide function to write new SSID/password to EEPROM and reboot. Read SSID/IP/password/etc from config in TransportXXX

Another way: Be more Arduin-ish. All standard Arduino classes have function begin or/and class initializer. So I want to call for example mySensors.begin() for basic init, and some kind of mySensorsTransportXXXBegin(ssid, password, ip, topic, etc) for specific transport init. As an option should be flag like "already connected" or "do not reconnect" for WiFi

As for me, the second way is preferred. If I decide to make my next sensor on MySensors instead of "pure" C/Arduino, I'll refactor at least MyMainESP8266.c:loop_wrapper by removing setup call and adding separate public _begin() alternative