[사용자]
    ↓ 로그인 및 동의
[Authorization Server] --(인가 코드)--> [Client]
[Client] --(인가 코드로 토큰 요청)--> [Authorization Server]
[Authorization Server] --(Access Token 발급)--> [Client]
[Client] --(토큰으로 요청)--> [Resource Server]
[Resource Server] --> 보호된 사용자 데이터 반환

-H "Content-Type: application/x-www-form-urlencoded" \
-d "grant_type=authorization_code" \
-d "client_id={REST_API_KEY}" \
--data-urlencode "redirect_uri={REDIRECT_URI}" \
-d "code={AUTHORIZATION_CODE}"

#include <WiFi.h>
#include <HTTPClient.h>

const char *ssid = "Your_SSID";          // 사용하는 WiFi 네트워크 이름 (SSID)
const char *password = "Your_Password";  // 사용하는 WiFi 네트워크 비밀번호

const String rest_api_key = "REST API KEY"; 
String access_token = "Access token";
String refresh_token = "Refresh token";

#define MsgSendInterval 3600 // 60 * 60 초, 즉 한시간 간격으로 전송
long timeout = 3600;  //시간을 초로 나타냄
int sensorValue = 0;
int sensorPin = 34; // 토양 습도 센서 핀

void setup() {
  Serial.begin(115200);

  WiFi.begin(ssid, password);
  Serial.println("Connecting");
  while (WiFi.status() != WL_CONNECTED) {
    Serial.print(".");
    delay(1000);
  }
  Serial.print("\nConnected to WiFi : ");
  Serial.println(WiFi.localIP());
}
void loop() {
  if (timeout++ > MsgSendInterval)  // 1시간(60 * 60)에 1번씩 전송
  {
    if (isAccessTokenExpired() == true) { //access token 만료 여부 확인
      if (update_access_token() == false) { // access token 재발급
        Serial.println("Access token update failed");
      }
    }
    sensorValue = analogRead(sensorPin);//토양 센서값 읽기
    send_message();
    timeout = 0;
  }

  delay(1000);
}


// str문자열에서 start_string와 end_string사이의 문자열을 추출하는 함수
String extract_string(String str, String start_string, String end_string) {
  int index1 = str.indexOf(start_string) + start_string.length();
  int index2 = str.indexOf(end_string, index1);
  String value = str.substring(index1, index2);
  return value;
}

bool isAccessTokenExpired() {
  HTTPClient http;
  bool returnVal = true;
/*
curl -v -X GET "https://kapi.kakao.com/v1/user/access_token_info" \
  -H "Authorization: Bearer ${ACCESS_TOKEN}"
*/
  if (!http.begin("https://kapi.kakao.com/v1/user/access_token_info")) {
    Serial.println("\nfailed to begin http\n");
  }
  http.addHeader("Authorization", "Bearer " + access_token);

  int httpCode = http.GET();

  // httpCode will be negative on error
  if (httpCode > 0) {
    // file found at server
    if (httpCode == HTTP_CODE_OK) {
      String payload = http.getString();
      Serial.println(payload);
      String expireT = extract_string(payload, "\"expires_in\":", ",");
      Serial.println(expireT.toInt());
      if (expireT.toInt() > 0) {
        returnVal = false;
      } else {
        returnVal = true;
      }
    }
  } else {
    Serial.printf("[HTTP] GET... failed, error: %s\n", http.errorToString(httpCode).c_str());
  }
  http.end();
  return returnVal;
}

void send_message() {
  HTTPClient http;
  String url = "https://kapi.kakao.com/v2/api/talk/memo/default/send";
  if (!http.begin(url)) {
    Serial.println("\nfailed to begin http\n");
  }
  http.addHeader("Authorization", "Bearer " + access_token);
  http.addHeader("Content-Type", "application/x-www-form-urlencoded");

  int http_code;
  /*
   template_object={
        "object_type": "text",
        "text": "텍스트 영역입니다. 최대 200자 표시 가능합니다.",
        "link": {
            "web_url": "https://developers.kakao.com",
            "mobile_web_url": "https://developers.kakao.com"
        },
        "button_title": "바로 확인"
    }
    */
  String data = String("template_object={") + 
                String("\"object_type\": \"text\",") + 
                String("\"text\": \"") + String("토양 센서 값 :") + 
                String(sensorValue) +  //토양 센서 값
                String("\",\"link\": {}}"); //link가 없으면 오류메세지 받음
  Serial.println(data);
  http_code = http.POST(data);
  Serial.print("HTTP Response code: ");
  Serial.println(http_code);

  String response;
  if (http_code > 0) {
    response = http.getString();
    Serial.println(response);
  }

  http.end();
}

/*
curl -v -X POST "https://kauth.kakao.com/oauth/token" \
 -H "Content-Type: application/x-www-form-urlencoded" \
 -d "grant_type=refresh_token" \
 -d "client_id=${REST_API_KEY}" \
 -d "refresh_token=${USER_REFRESH_TOKEN}"
*/
bool update_access_token() {
  HTTPClient http;
  bool retVal = false;

  String url = "https://kauth.kakao.com/oauth/token";
  String new_refresh_token = "";
  if (!http.begin(url)) {
    Serial.println("\nfailed to begin http\n");
  }
  http.addHeader("Content-Type", "application/x-www-form-urlencoded");
  int http_code;
  String data = "grant_type=refresh_token&client_id=" + rest_api_key + "&refresh_token=" + refresh_token;
  Serial.println(data);
  http_code = http.POST(data);
  Serial.print("HTTP Response code: ");
  Serial.println(http_code);

  String response;
  if (http_code > 0) {
    response = http.getString();
    Serial.println(response);
    access_token = extract_string(response, "{\"access_token\":\"", "\"");
    new_refresh_token = extract_string(response, "\"refresh_token\":\"", "\"");
    //만료 1개월전부터 갱신되므로 data가 없을 수도 있음
    if (new_refresh_token != "") {
      refresh_token = new_refresh_token;
    }
    retVal = true;
  } else {
    retVal = false;
  }
  http.end();
  return retVal;
}