ArduinoJSON :: How to determine Array's size in a DOC conatinaing not only Arrays

Question

Parameter file containing MQTT topics as shown below, is read into StaticJsonDocument<1250> DOC variable.

Size of some groups may differ between MCU's, for example sub_topics_win may contain more topics, if MCU is 8 relay unit. For that I need the flexibility to read a specific topic that can differ in size.

What is the rifht way to do it?

{
    "pub_gen_topics": [
        "myHome/Messages",
        "myHome/log",
        "myHome/debug"
    ],
    "pub_topics": [
        "myHome/Cont_A/Avail",
        "myHome/Cont_A/State"
    ],
    "sub_topics": [
        "myHome/Cont_A",
        "myHome/All",
        "myHome/lockdown"
    ],
    "sub_topics_win": [
        "myHome/Windows/gFloor/TwinWindow",
        "myHome/Windows/gFloor/Pergola"
    ],
    "sub_topics_SW": [
        "myHome/Light/int/gFloor/SalAmbient1",
        "myHome/Light/int/gFloor/SalAmbient2",
        "myHome/Light/int/gFloor/Lobby",
        "myHome/Light/int/gFloor/Corridor"
    ],
    "sub_topics_win_g": [
        "myHome/Windows",
        "myHome/Windows/gFloor"
    ],
    "sub_topics_SW_g": [
        "myHome/Light",
        "myHome/Light/int",
        "myHome/Light/int/gFloor/"
    ],
    "ver": 0.1
}

Answer 1

If your question is How can I read each element of an array when I don't know its size?, then simply use a for-loop, like so:

JsonArray sub_topics_win = doc["sub_topics_win"];

for (const char* topic : pub_gen_topics) {
  Serial.println(topic);
}

See also: JsonArray::begin() / end(), JsonArray::size().

Answer 2

As I understand, your question is How can I determine the best capacity for a JsonDocument when my input varies?

If that's the case, I recommend that you read How to determine the capacity of the JsonDocument?. Here is an extract from the section "What if my input varies?":

If your program must deserialize a JSON document that changes, you must allocate a JsonDocument that is large enough for the largest valid input. I insist on the term valid because your program doesn't need to support any JSON document, but only the ones relevant to your use case.

Applying this to your problem, you need to put eight topics in each list and pass this document to the ArduinoJson Assistant, which results in a recommended capacity of 3027 bytes on ESP8266. Now, this is a bit large for a StaticJsonDocument, so consider switching to a DynamicJsonDocument.