Parsing Json using arduino-mqtt lib - c++

Question

I am trying to use the arduino-mqtt lib.
I have this working sending the json string. The problem comes with trying to parse the string with ArduinioJson. It just returns no value.
I think it may have todo with the pointer reference in the mqttMessageRecived function ( String &payload).
Function called when there is an MQTT message:
void mqttMessageReceived(String &topic, String &payload){
//Example String for test
String json = "{"id" : "100" , "cmd" : "0xff"}";
jsonout(payload);
Serial.println("Sending Static String");
jsonout(json);
Function to parse json input:
void jsonout(String Json){
StaticJsonDocument<200> doc;
//Deserialize the JSON document
DeserializationError error = deserializeJson(doc, Json);
Serial.println("Got String: ");
Serial.println(Json);
// Test if parsing succeeds.
if (error) {
Serial.print(F("deserializeJson() failed: "));
Serial.println(error.c_str());
return;
}
const char* id = doc["id"];
const char* cmd = doc["cmd"];
// Print values.
Serial.println(id);
Serial.println(cmd);
}
Non parsed output:
Message from MQTT
Got String:
"{\"id\" : 4 , \"cmd\": \"0xee\"}"
Result = No output from json parse
Non parsed output:
Sending Static String
Got String:
{"id" : "100" , "cmd" : "0xff"}
Result = Output from json parse:
100
0xff

Answer 1

The problem is that - in the response from the server
"{\"id\" : 4 , \"cmd\": \"0xee\"}"
the id field is an integer - not a character array.
So you need to change
const char* id = doc["id"];
to
int id = doc["id"];
(and update your test string to use an int for the ID also).

Answer 2

The server returns a id member that's a Number "id":4, while you are generating a id that's a String "id":"200".
You need to adjust your code to either one. If it's a number (and it seems so), you need to send "id":200 and change your code to get a number:
unsigned id = (double)doc["id"];
// And to generate it:
String json = "{\"id\" : 100 , \"cmd\" : \"0xff\"}";
Also, with JSON, beware of hexadecimal encoding, has it's not converted to number (you have to do it yourself by receiving a const char* and calling sscanf or strtol or ...) and it's not convenient. It's better to use base-10 encoding instead:
String json = "{\"id\" : 100 , \"cmd\" : 255}";