Flutter: HttpClient post contentLength -- exception - flutter

Question

Very weird...
In order to post some JSON data to my server, I define the contentLength to the length of the JSON encoded data but I then receive an exception that says "Content size exceeds specified contentLength". Difference is 1 byte.
Here is the source code:
Future<Map> ajaxPost(String serviceName, Map data) async {
var responseBody = json.decode('{"data": "", "status": "NOK"}');
try {
var httpClient = new HttpClient();
var uri = mid.serverHttps ? new Uri.https(mid.serverUrl, _serverApi + serviceName)
: new Uri.http(mid.serverUrl, _serverApi + serviceName);
var request = await httpClient.postUrl(uri);
var body = json.encode(data);
request.headers
..add('X-mobile-uuid', await _getDeviceIdentity())
..add('X-mobile-token', await mid.getMobileToken());
request.headers.contentLength = body.length;
request.headers.set('Content-Type', 'application/json; charset=utf-8');
request.write(body);
var response = await request.close();
if (response.statusCode == 200){
responseBody = json.decode(await response.transform(utf8.decoder).join());
//
// If we receive a new token, let's save it
//
if (responseBody["status"] == "TOKEN"){
await mid.setMobileToken(responseBody["data"]);
// Let's change the status to "OK", to make it easier to handle
responseBody["status"] = "OK";
}
}
} catch(e){
// An error was received
throw new Exception("AJAX ERROR");
}
return responseBody;
}
Some other times, it works fine...
Am I doing anything wrong with this code?
Many thanks for your help.
EDITED WITH SOLUTION:
Many thanks for your help. The simply fact of using utf8.encode(json.encode(data)) did not fully work. So, I turned to the http library and it now works like a charm. The code is even lighter!
Here is the new version of the code:
Future<Map> ajaxPut(String serviceName, Map data) async {
var responseBody = json.decode('{"data": "", "status": "NOK"}');
try {
var response = await http.put(mid.urlBase + '/$_serverApi$serviceName',
body: json.encode(data),
headers: {
'X-mobile-uuid': await _getDeviceIdentity(),
'X-mobile-token': await mid.getMobileToken(),
'Content-Type': 'application/json; charset=utf-8'
});
if (response.statusCode == 200) {
responseBody = json.decode(response.body);
//
// If we receive a new token, let's save it
//
if (responseBody["status"] == "TOKEN") {
await mid.setMobileToken(responseBody["data"]);
// Let's change the status to "OK", to make it easier to handle
responseBody["status"] = "OK";
}
}
} catch (e) {
// An error was received
throw new Exception("AJAX ERROR");
}
return responseBody;
}

Answer 1

I got it working with
req.headers.contentLength = utf8.encode(body).length;
From an indirect tip of the Utf8Codec documentation which states
decode(List codeUnits, { bool allowMalformed }) → String
Decodes the UTF-8 codeUnits (a list of unsigned 8-bit integers) to the corresponding string.
That means thatutf8.encode() returns codeUnits which actually means List<uint8>.
Encoding a String payload would in theory return a list which length is the length of the payload in bytes.
So using httpClient means to always measure the length of the payload in bytes, not the length of a String which may differ.

Answer 2

Günter is right. Content-Length has to be the length of the byte array after encoding from a String to bytes in whatever encoding you server requires.
There's a package called http which provides a slightly higher level api (it uses dart.io httpClient under the hood) which takes care of encoding the post body and length for you. For example, when you need to send application/x-www-form-urlencoded form it will even take a Map and do all the encoding for you (you still need to encode to json yourself). It's equally happy to send just a String or List<int>. Here's an example:
Map<String, String> body = {
'name': 'doodle',
'color': 'blue',
'teamJson': json.encode({
'homeTeam': {'team': 'Team A'},
'awayTeam': {'team': 'Team B'},
}),
};
Response r = await post(
url,
body: body,
);

Answer 3

Seems your string contains multibyte characters.
UTF8-encode the string to get the correct length:
var body = utf8.encode(json.encode(data));