The argument type 'Set<String>' can't be assigned to the parameter type 'Map<String, String>'

Question

I'm trying to create an user authentication system using Auth0 in my Flutter app. The Auth0 REST documentation gave an example of cURL but I didn't find any flutter package which does the job of cURL. So, I used http. Here's the code:

  Future<String> getToken(String userId) async {
    final response = await http.post(
      Uri.parse('https://my-auth0-subdomain.auth0.com/oauth/token'),  // I used my real subdomain
      body: jsonEncode({
        'grant_type=client_credentials',
        'client_id=my_project_client_id',  // I used my real client id
        'client_secret=my_project_client_secret',  // I used my real client secret
        'audience=https://my-auth0-subdomain.auth0.com/api/v2/'  // I used my real subdomain
      }),
      headers: {
        'content-type: application/x-www-form-urlencoded'
      },
    );
    final token = jsonDecode(response.body)["access_token"];

    return token;
  }

This gives me an error that The argument type 'Set<String>' can't be assigned to the parameter type 'Map<String, String>'. on line 10 (headers: {...}).
I can resolve this error by using headers: {'content-type': 'application/x-www-form-urlencoded'},.
But this then gives the error from Auth0 {"error":"access_denied","error_description":"Unauthorized"}. The API is set up properly, because on running

curl --request POST \
  --url 'https://my-auth0-subdomain.auth0.com/oauth/token' \
  --header "content-type: application/x-www-form-urlencoded" \
  --data grant_type=client_credentials \
  --data 'client_id=my_project_client_id' \
  --data client_secret=my_project_client_secret \
  --data 'audience=https://my-auth0-subdomain.auth0.com/api/v2/'

it returns a "access_token", "scope", "expires_in" and "token_type".

Please help. It's very important.
Thanks in advance :)

Answer 1

Try to send data as url encoded using :

Map<String, String> myBody= {

'grant_type' : 'client_credentials',
        'client_id' : 'my_project_client_id',  // I used my real client id
        'client_secret' : 'my_project_client_secret',  // I used my real client secret
        'audience ' : 'https://my-auth0-subdomain.auth0.com/api/v2/'  // I used my real subdomain     
};

Future<String> getToken(String userId) async {
    final response = await http.post(
      Uri.parse('https://my-auth0-subdomain.auth0.com/oauth/token'),  // I used my real subdomain
      body: myBody,
      headers: {
        'content-type: application/x-www-form-urlencoded'
      },
    );
    final token = jsonDecode(response.body)["access_token"];

    return token;
  }