Error 400 and 415 when exchanging code for access_token - objective-c

Question

I am writing an application to authorise itself into Spotify. I used the node app.js example to get things working and an now re-writing natively into Objective C. I have extracted the authorisation code via a callback function
NSMutableURLRequest * request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:#"https://accounts.spotify.com/api/token"]];
request.HTTPMethod = #"POST";
// put in the header fields
NSString *headerString = [NSString stringWithFormat:#"%#:%#",clientID,clientSecret];
NSData *nsdata = [headerString dataUsingEncoding:NSUTF8StringEncoding];
NSString *base64Encoded = [nsdata base64EncodedStringWithOptions:0];
base64Encoded = [ NSString stringWithFormat:#"Basic %#", base64Encoded];
[request setValue:base64Encoded forHTTPHeaderField:#"Authorization"];
NSLog(#"request.header %#", request.allHTTPHeaderFields.description);
// put in the body form
NSString * stringData = [NSString stringWithFormat:#"grant_type:%#, code:%#, redirect_uri:\"%#\"", #"authorization_code",authorisationCode,redirectUri];
NSData * requestBodyData = [stringData dataUsingEncoding:NSUTF8StringEncoding];
request.HTTPBody = requestBodyData;
NSLog(#"request.body %#", [[NSString alloc] initWithData:request.HTTPBody encoding:NSUTF8StringEncoding]);
I then post this request and get
{"error":"server_error","error_description":"Unexpected status: 415"}
returned.
I have found a variety of questions on this topic revolving around the Content-Type needing to be application/x-www-form-urlencoded but this was no direct cure.
Anyone got any suggestions for me?

Answer 1

I found the answer here NSURLRequest : Post data and read the posted page
It appears that the raw data within the HTTPBody needs to be formatted differently - no JSON at all. The above link has a useful function which I updated slightly
-(NSData*)encodeDictionary:(NSDictionary*)dictionary {
NSMutableArray *parts = [[NSMutableArray alloc] init];
for (NSString *key in dictionary) {
NSString *encodedValue = [[dictionary objectForKey:key] stringByAddingPercentEncodingWithAllowedCharacters:[NSCharacterSet URLPasswordAllowedCharacterSet]];
NSString *encodedKey = [key stringByAddingPercentEncodingWithAllowedCharacters:[NSCharacterSet URLPasswordAllowedCharacterSet]];
NSString *part = [NSString stringWithFormat: #"%#=%#", encodedKey, encodedValue];
[parts addObject:part];
}
NSString *encodedDictionary = [parts componentsJoinedByString:#"&"];
return [encodedDictionary dataUsingEncoding:NSUTF8StringEncoding];
}
and ..
[request setValue:#"application/x-www-form-urlencoded" forHTTPHeaderField:#"Content-Type"];
//NSLog(#"request.header %#", request.allHTTPHeaderFields.description);
// put in the body form using the new function
NSDictionary * parameters = [[NSDictionary alloc] initWithObjectsAndKeys:#"authorization_code",#"grant_type",authorisationCode,#"code",redirectUri,#"redirect_uri", nil ];
NSData * requestBodyData = [self encodeDictionary:parameters];
request.HTTPBody = requestBodyData;