如何连接 const char* 以通过 https 获取另一个 JSON

如何解决如何连接 const char* 以通过 https 获取另一个 JSON

在使用带有 ArduinoJSON 库的 ESP32 板响应 JSON 后，我尝试多次获取 JSON（YouTube 数据 API）（以获取 activeLiveChatId 和评论）。但我无法连接 const char 值来创建新 URL。也许我的代码处理 const char* 应该是错误的。 你能给我建议一些解决方案吗？下面是我的草图。

#include <string>
#include <WiFi.h>
#include <WiFiClientSecure.h>
#include <HTTPClient.h>
#include <ArduinoJson.h>
#include <stdlib.h>

// Fingerprint for demo URL,expires on June 2,2021,needs to be updated well before this date

char ssid[] = "myssid";       // your network SSID (name)
char password[] = "mypass";  // your network key

#define API_KEY "myapikey"  // your google apps API Token
#define CHANNEL_ID "mychannelid" // makes up the url of channel
#define servername "www.googleapis.com"

WiFiClientSecure client;

void setup() {
  Serial.begin(115200);
  // Set WiFi to station mode and disconnect from an AP if it was previously connected
  WiFi.mode(WIFI_STA);
  WiFi.disconnect();
  delay(100);
  // Attempt to connect to Wifi network:
  Serial.print("Connecting Wifi: ");
  Serial.println(ssid);
  WiFi.begin(ssid,password);
  while (WiFi.status() != WL_CONNECTED) {
    Serial.print(".");
    delay(500);
  }
  Serial.println("");
  Serial.println("WiFi connected");
  Serial.println("IP address: ");
  IPAddress ip = WiFi.localIP();
  Serial.println(ip);
  delay(3000);

  if ((WiFi.status() == WL_CONNECTED)) {
    String url1 = ("https://"servername"/youtube/v3/search?eventType=live&part=id&channelId="CHANNEL_ID"&type=video&key="API_KEY);
    String url2prefix = "https://"servername"/youtube/v3/videos?part=liveStreamingDetails&field=activeLiveChatId&id=";
    String url2postfix = "&key="API_KEY;
    const char* items_0_id_videoId;
    HTTPClient https;
    
    Serial.print("[HTTPS] begin...\n");
    if (https.begin(client,url1)) {  // HTTPS
      Serial.print("[HTTPS] GET...\n");
      // start connection and send HTTP header
      int httpCode = https.GET();
      // httpCode will be negative on error
      if (httpCode > 0) {
        // HTTP header has been send and Server response header has been handled
        Serial.printf("[HTTPS] GET... code: %d\n",httpCode);
        // file found at server
        if (httpCode == HTTP_CODE_OK || httpCode == HTTP_CODE_MOVED_PERMANENTLY) {
          String payload = https.getString();
          DynamicJsonDocument doc(96);
          StaticJsonDocument<64> filter;
          filter["items"][0]["id"]["videoId"] = true;
          DeserializationError error = deserializeJson(doc,payload,DeserializationOption::Filter(filter));
          if (error) {
            Serial.print(F("deserializeJson() failed: "));
            Serial.println(error.c_str());
            return;
          }
 
          const char* kind = doc["kind"]; // "youtube#searchListResponse"
          const char* etag = doc["etag"]; // 
          const char* regionCode = doc["regionCode"]; // "JP"
          int pageInfo_totalResults = doc["pageInfo"]["totalResults"]; // 1
          int pageInfo_resultsPerPage = doc["pageInfo"]["resultsPerPage"]; // 1
          JsonObject items_0 = doc["items"][0];
          const char* items_0_kind = items_0["kind"]; // "youtube#searchResult"
          const char* items_0_etag = items_0["etag"]; // 
          const char* items_0_id_kind = items_0["id"]["kind"]; // "youtube#video"
          items_0_id_videoId = items_0["id"]["videoId"]; // 

          Serial.printf(
          "ID: %s\n",items_0_id_videoId);
        }
      }
    }
    String url2 = url2prefix + items_0_id_videoId + url2postfix;
    //retrieve activeLiveChatId
    if (https.begin(client,url2)) {  // HTTPS
      Serial.print("[HTTPS] GET...\n");
      // start connection and send HTTP header
      int httpCode = https.GET();
      // httpCode will be negative on error
      if (httpCode > 0) {
        // HTTP header has been send and Server response header has been handled
        Serial.printf("[HTTPS] GET... code: %d\n",httpCode);
        // file found at server
        if (httpCode == HTTP_CODE_OK || httpCode == HTTP_CODE_MOVED_PERMANENTLY) {
          String payload = https.getString();
          Serial.println(payload);
          // Stream& input;
          DynamicJsonDocument doc(96);
          StaticJsonDocument<64> filter;
          filter["items"][0]["liveStreamingDetails"]["activeLiveChatId"] = true;
          DeserializationError error = deserializeJson(doc,DeserializationOption::Filter(filter));

          if (error) {
            Serial.print(F("deserializeJson() failed: "));
            Serial.println(error.c_str());
            return;
          }
  
          const char* items_0_liveStreamingDetails_activeLiveChatId = doc["items"][0]["liveStreamingDetails"]["activeLiveChatId"];
        
          Serial.printf(
          "livechatID: %s\n",items_0_liveStreamingDetails_activeLiveChatId
          );
        }
      } else {
        Serial.printf("[HTTP] GET... failed,error: %s\n",https.errorToString(httpCode).c_str());
      }
      https.end();
    } else {
      Serial.printf("[HTTP] Unable to connect\n");
    }
  }
}  

void loop() {
  // wait for WiFi connection
  Serial.println("Wait 10s before next round...");
  delay(10000);
}

解决方法

您从 JSON 解析器获得了 const char* 类型的只读字符串 - 没关系。您可以无限制地连接它们，但您必须提供内存来保存连接的结果。

有几种方法可以解决这个问题

1. strncat()（很好的老式 C）。看看这个thread explaining strcat()。本质上，分配足够的内存来保存新字符串，然后将连接结果存储在那里（为清楚起见省略了错误处理）。假设您要连接的字符串是 str1 和 str2：

    const size_t cat_buf_len = strlen(str1) + strlen(str2) + 1;
    char* cat_buf = (char*) malloc(cat_buf_len);
    strncpy(cat_buf,str1,cat_buf_len);
    strncat(cat_buf,str2,cat_buf_len - strlen(cat_buf));
    // ... use the concatenated string in cat_buf
    free(cat_buf); 

2. snprintf() (C)。与 strcat() 类似 - 看起来更容易一些，但效率不高。

    const size_t cat_buf_len = strlen(str1) + strlen(str2) + 1;
    char* cat_buf = (char*) malloc(cat_buf_len);
    snprintf(cat_buf,cat_buf_len,"%s%s",str2);
    // ... use the concatenated string in cat_buf
    free(cat_buf); 

3. Arduino String (C++)。 String 类型在堆上操作，管理自己的内存，使用起来更容易一些。但是，如果不小心使用，它确实存在堆碎片化的风险。

    String cat_str = String(str1) + str2;