ESPaSyncWebServer relay example

relayWebserver.ino

  static int relayPin=0;

server.on("/relay/toggle", HTTP_POST, [](AsyncWebServerRequest *request)
{
  AsyncResponseStream *response;
  response = request->beginResponseStream( "text/plain" );
  bool newState = !digitalRead(relayPin);
  digitalWrite(relayPin, newState);
  response->printf( "{\"relay\" : \"%s\"}", newState ? "on" : "off" );
  request->send( response );
});