Tweeting from the ESP8266

Other blogs have covered how to send data from the ESP8266 to a web server.  Sparkfun suggest a few services (here).  I've started playing around with ThinkSpeak.  The platform is great, and allows you to react to and visualise your data.  I have been looking for a way to post to Twitter from the ESP8266 for a project I've started working on and found that there wasn't a wealth of information.

Firstly, I had to register at ThinkSpeak, then link my Twitter account to my ThinkSpeak account.  Go to ThinkSpeak-->Apps--> ThinkTweet then click on "Link Twitter Account".  This generates an API key that is used in the code to authenticate the post.  For posting from the ESP8266, I used the code here as a start template.

The first time I tried to send a tweet using the code, only the first word of the string was sent.  The problem is that strings containing "special" characters such as space non-alphanumerical characters need to be percent-encoded.  Thankfully this is trivial to do.  I couldn't find a standard function to do this, so I wrote my own:

String encodeURL(char *src)
{
  String encoded="";
  char *hex = "0123456789ABCDEF"; 
  char *ptr = src;
  char c=0;

  while (c!='\n')
  {
    c = *ptr;
      if( ('a' <= c && c <= 'z')
          || ('A' <= c && c <= 'Z')
          || ('0' <= c && c <= '9') )
          {
            encoded+=c;
          }
          else
          {
            encoded+="%";
            encoded+=hex[c >> 4];
            encoded+=hex[c & 15];
          }
    ptr++;
  }
  return encoded;
}

In the above code, the address of the string is passed to the function.  The string is parsed - if the character is 0-9, a-z or A-Z it is appended unchanged, otherwise the percent sign is appended, followed by the hexidecimal value of the character.  The encoded string is returned and is called like so:

char tweet[] = "This is my tweet, with spaces and characters!! :)\n";
.
.
.
Serial.println(encodeURL(&tweet[0]);

Confusingly, using a String object to hold the tweet string caused problems with some characters - in particular the '?' character was reported as '|'.  I never got to the bottom of this, but using a character array as above worked.  I found I needed to explicitly add the end of line character '\n' though, as the compiler didn't add it automatically.  It is then simply a matter of calling the below SendTweet function to send the tweet!

String API = "XXXXXXXXXXXXXXX"; // Your ThingSpeak-Twitter API key goes here
.
.
.
void SendTweet(void)
{
if (client.connect("184.106.153.149", 80))
{
  client.print("GET /apps/thingtweet/1/statuses/update?key=" + API + "&status=" + encodeURL(&tweet[0]) + " HTTP/1.1\r\n");
  client.print("Host: api.thingspeak.com\r\n");
  client.print("Accept: */*\r\n");
  client.print("User-Agent: Mozilla/4.0 (compatible; esp8266 Lua; Windows NT 5.1)\r\n");
  client.print("\r\n");
  Serial.println("Success!");
}
else Serial.println("Could not connect to ThingSpeak!");
}

Note, that it seems you can't send the same tweet twice.  Happy tweeting from your sensors!