WiFiManager/examples/AutoConnectWithFSParameters/AutoConnectWithFSParameters.ino

#include <FS.h>                   //this needs to be first, or it all crashes and burns...

#include <ESP8266WiFi.h>          //https://github.com/esp8266/Arduino

//needed for library
#include <DNSServer.h>
#include <ESP8266WebServer.h>
#include <WiFiManager.h>          //https://github.com/tzapu/WiFiManager

#include <ArduinoJson.h>          //https://github.com/bblanchon/ArduinoJson

//define your default values here, if there are different values in config.json, they are overwritten.
const char *mqtt_server = NULL;
const char *mqtt_port = "8080";
const char *blynk_token = "YOUR_BLYNK_TOKEN";

  //flag for saving data
  bool shouldSaveConfig = false;

//callback notifying us of the need to save config
void saveConfigCallback () {
  Serial.println("Should save config");
  shouldSaveConfig = true;
}


void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Serial.println();

  //clean FS, for testing
  //SPIFFS.format();

  //read configuration from FS json
  Serial.println("mounting FS...");

  if (SPIFFS.begin()) {
    Serial.println("mounted file system");
    if (SPIFFS.exists("/config.json")) {
      //file exists, reading and loading
      Serial.println("reading config file");
      File configFile = SPIFFS.open("/config.json", "r");
      if (configFile) {
        Serial.println("opened config file");
        size_t size = configFile.size();
        // Allocate a buffer to store contents of the file.
        std::unique_ptr<char[]> buf(new char[size]);

        configFile.readBytes(buf.get(), size);
        StaticJsonBuffer<200> jsonBuffer;
        JsonObject& json = jsonBuffer.parseObject(buf.get());
        json.printTo(Serial);
        if (json.success()) {
          Serial.println("\nparsed json");

          mqtt_server = json["mqtt_server"];
          mqtt_port = json["mqtt_port"];
          blynk_token = json["blynk_token"];
        } else {
          Serial.println("failed to load json config");
        }
      }
    }
  } else {
    Serial.println("failed to mount FS");
  }
  //end read



  // The extra parameters to be configured (can be either global or just in the setup)
  // After connecting, parameter.getValue() will get you the configured value
  // id/name placeholder/prompt default length
  WiFiManagerParameter custom_mqtt_server("server", "mqtt server", mqtt_server, 40);
  WiFiManagerParameter custom_mqtt_port("port", "mqtt port", mqtt_port, 8);
  WiFiManagerParameter custom_blynk_token("blynk", "blynk token", blynk_token, 32);

  //WiFiManager
  //Local intialization. Once its business is done, there is no need to keep it around
  WiFiManager wifiManager;
  
  //set config save notify callback
  wifiManager.setSaveConfigCallback(saveConfigCallback);
  
  //add all your parameters here
  wifiManager.addParameter(&custom_mqtt_server);
  wifiManager.addParameter(&custom_mqtt_port);
  wifiManager.addParameter(&custom_blynk_token);

  //reset settings - for testing
  //wifiManager.resetSettings();

  //sets timeout until configuration portal gets turned off
  //useful to make it all retry or go to sleep
  //in seconds
  //wifiManager.setTimeout(120);

  //fetches ssid and pass and tries to connect
  //if it does not connect it starts an access point with the specified name
  //here  "AutoConnectAP"
  //and goes into a blocking loop awaiting configuration
  if (!wifiManager.autoConnect("AutoConnectAP")) {
    Serial.println("failed to connect and hit timeout");
    delay(3000);
    //reset and try again, or maybe put it to deep sleep
    ESP.reset();
    delay(5000);
  }

  //if you get here you have connected to the WiFi
  Serial.println("connected...yeey :)");

  //read updated parameters
  mqtt_server = custom_mqtt_server.getValue();
  mqtt_port = custom_mqtt_port.getValue();
  blynk_token = custom_blynk_token.getValue();

  //save the custom parameters to FS
  if (shouldSaveConfig) {
    Serial.println("saving config");
    StaticJsonBuffer<200> jsonBuffer;
    JsonObject& json = jsonBuffer.createObject();
    json["mqtt_server"] = mqtt_server;
    json["mqtt_port"] = mqtt_port;
    json["blynk_token"] = blynk_token;

    File configFile = SPIFFS.open("/config.json", "w");
    if (!configFile) {
      Serial.println("failed to open config file for writing");
    }

    json.printTo(Serial);
    json.printTo(configFile);
    configFile.close();
    //end save
  }
}

void loop() {
  // put your main code here, to run repeatedly:


}