Parsing JSON response from Stream

Question

I want to parse JSON response from a GET Request. I do this usually by using a String variable to store the response and later parse this string using ArduinoJSON library like shown below:

#include <Arduino.h>
#include "OTA.h"
#include "Credentials.h"
#include <HTTPClient.h>
#include <ArduinoJson.h>

const char url[] = "http://date.jsontest.com/";

HTTPClient getClient;

void getRequest(String &payload)
{
  getClient.begin(url);

  int httpCode = getClient.GET();

  if (httpCode > 0)
  {
    payload = getClient.getString();
    // Serial.print("HTTP Status: ");
    // Serial.println(httpCode);
  }
  else
  {
    Serial.println("Error on HTTP request");
  }
  getClient.end();
}

void parseResponse(String &payload)
{

  StaticJsonDocument<128> doc;

  DeserializationError error = deserializeJson(doc, payload);

  if (error)
  {
    Serial.print("deserializeJson() failed: ");
    Serial.println(error.c_str());
    return;
  }

  const char *time = doc["time"]; // "03:53:25 AM"
  // unsigned long *epoch = doc["milliseconds_since_epoch"]; // 1362196405309
  const char *date = doc["date"]; // "03-02-2013"

  Serial.println(date);
}

void setup()
{
  setupOTA("ESP-N2", mySSID, myPASSWORD);

  Serial.begin(115200);
}

void loop()
{
  ArduinoOTA.handle();

  unsigned long newTime = millis();
  static unsigned long oldTime = 0;
  String payload;

  if (newTime - oldTime >= 5000)
  {
    oldTime = newTime;
    getRequest(payload);
    parseResponse(payload);
    Serial.println();
  }
}

I want to avoid using String and ArduinoJSON offician reference page mentions that it's possible to deserialize directly from input Stream e.g. wifiClient, ethernetclient or serial but I couldn't find any example showing it's usage. So I am curious to learn how to use it.

From my basic understanding of a Stream, I suppose I need define a buffer or memory location that will hold the incoming bytes and using the address of this buffer need to read data bit by bit. So do I need to do something like this:

char buffer[]= httpClient.getStream();

and then pass this buffer as input to deserialize function? or have I gotten it completely wrong?

Answer 1

httpClient.getStream() returns an instance of Stream that you can pass to deserializeJson(), like so:

deserializeJson(doc, httpClient.getStream());

However, by doing so, you bypass the code in HTTPClient that handles chunked transfer encoding, so you must call httpClient.useHTTP10(true) before calling GET to make sure the server won't return a chunked response.

See How to use ArduinoJson with HTTPClient?