Bad state: Cannot set the body fields of a Request with content-type "application/json"

You need to wrap the body in jsonEncode.

import 'package:http/http.dart' as http;
import 'dart:convert';


Map<String,String> headers = {'Content-Type':'application/json','authorization':'Basic c3R1ZHlkb3RlOnN0dWR5ZG90ZTEyMw=='};
final msg = jsonEncode({"grant_type":"password","username":"******","password":"*****","scope":"offline_access"});


var response = await post(Urls.getToken,
headers: headers,
body: msg,
);

had similar issue with http library...changed for dio 2.1.0 and the problem with headers is gone.

jsonEncode(body) didn't do the trick, because the docs say:

Sends an HTTP POST request with the given headers and body to the given URL, which can be a [Uri] or a [String].

[body] sets the body of the request. It can be a [String], a [List] or a [Map<String, String>]. If it's a String, it's encoded using [encoding] and used as the body of the request. The content-type of the request will default to "text/plain".

If [body] is a List, it's used as a list of bytes for the body of the request.

If [body] is a Map, it's encoded as form fields using [encoding]. The content-type of the request will be set to "application/x-www-form-urlencoded"; this cannot be overridden. [encoding] defaults to [utf8].

For more fine-grained control over the request, use [send] instead.

Future<Response> post(url, {Map<String, String> headers, body, Encoding encoding});

Map<String,String> header = {'Content-Type':'application/json-patch+json','accept':'application/json'};
final msg = jsonEncode({"username":"$emailorPhoneN","password":"$passwrod"});


try {
var response = await http.post(UrlConstants.loginUrl, headers: header, body: msg,
).timeout(Duration(seconds: httpDuration));
var convert = json.decode(response.body);
print('**********Data from server $convert');


if (convert == null) {
return null;
} else {
String token = convert['token'];


if (token != null) {
SignUpModel signUpModel = SignUpModel.fromJson(convert);
return signUpModel;
} else {
//*** GET Error message from the API provider.....
SignUpModel signUpModel = SignUpModel.fromJson(convert);
return signUpModel;
}
}

Use jsonEncode to wrap your body object.

import 'package:http/http.dart' as http;
import 'dart:convert';


var headers = {
'Content-Type':'application/json',
'authorization':'Basic c3R1ZHlkb3RlOnN0dWR5ZG90ZTEyMw=='
};


final body = {
'username':'foo',
'password':'pass123'
}


var response = await post(
Urls.getToken,
headers: headers,
body: jsonEncode(body), // use jsonEncode()
);

Why jsonEncode?

body: It can be a HTML, JSON or XML, etc. This mean is we need to send/recive data in that perticular format. You can set this in content-type header its default value is text/plain.

As you set the content-type header to JSON you must have to pass a "valid" JSON as the body. But you are passing Map<String, String> as the body, which obviously throws an error.

So to solve this issue you need to change (or encode) your Map<String, String> data to JSON data.

Best way to do this is to use jsonEncode function.

For me, jsonencode(body) did not work. This is worked for me instead:

body = json.encode(body);
http.post(Uri.parse(uri), headers: header, body: body);