Issue
I’ve been trying to debug this error type ‘Null’ is not a subtype of type ‘String’ in type cast but could not find the the exact place where the error is being produced besides that it is genereated when trigger a POST API call.
Shop Class
import 'package:freezed_annotation/freezed_annotation.dart';
part 'shop.freezed.dart';
part 'shop.g.dart';
@freezed
class Shop with _$Shop {
factory Shop({
String? id,
@JsonKey(name: 'shopNumber')String? number,
@JsonKey(name: 'created') String? createdAt,
@JsonKey(name: 'updated') String? updatedAt,
int? calendar}) = _Shop;
factory Shop.fromJson(Map<String, dynamic> json) => _$ShopFromJson(json);
static Shop fromJsonModel(Map<String, dynamic> json) => Shop.fromJson(json);
}
Post Function – Create shop
Future<void> postShop() async {
Shop shop;
shop = await shopSvc.create(Shop(calendar: widget.calendar?.id, number: _shopNumber));
var newCalendar = widget.calendar?.copyWith(shop: shop);
Navigator.pushReplacement(context, MaterialPageRoute(builder: (context) => ShopInfoScreen(calendar: newCalendar!)));
}
} catch (e) {
print(e);
}
}
}
Shop Service File
class ShopService extends BaseHttpService {
final AppContext appContext;
late String _shopApi;
shopService(http.Client client, this.appContext) : super(httpClient: client) {
_shopApi = '${Globals.ApiEndpoint}/user/${appContext.currentCustomer.id}/shops/';
}
Future<Shop> create(Shop shop) async {
var token = await this.appContext.currentUser!.getIdToken();
final response = await callApi('$_cardApi/', token, method: 'post', payload: shop.toJson());
// return Shop.fromJsonModel(json.decode(response));
return shop;
}
}
Base Http Service file
class BaseHttpService {
final Client httpClient;
BaseHttpService({required this.httpClient});
@protected
Future<String> callApi(String url, String token, {String method: 'get', Map<String, dynamic>? payload}) async {
late Response response;
Map<String, String> headers = {'Authorization': 'Bearer $token'};
if (method == 'get') response = await httpClient.get(Uri.parse(url), headers: headers);
else if (method == 'post') response = await httpClient.post(Uri.parse(url), headers: headers, body: payload);
else if (method == 'put') response = await httpClient.put(Uri.parse(url), headers: headers, body: payload);
else if (method == 'delete') response = await httpClient.delete(Uri.parse(url), headers: headers);
if (response.statusCode >= 300) {
print('statusCode : ' + response.statusCode.toString());
print(response.body.toString());
throw ClientException('Failed to load story with id $url');
}
return response.body;
}
}
Basically all I’m trying to do is create shop which only requires 2 fields in the body, number and calendar and the other fields will be defaulted in the DB.
The code is failing at the end of this line else if (method == 'post') response = await httpClient.post(Uri.parse(url), headers: headers, body: payload); but I do not know where the problem is as I’ve put ? in the variables already.
The http package is already on the newest version http: ^0.13.4
I’ved tried the below body for the POST call in POSTMAN and it works without a problem:
//Test 1
{
"id": null,
"shopNumber": "87678675",
"created": null,
"updated": null,
"calendar": 1
}
//Test 2
{
"shopNumber": "87678675",
"calendar": 1
}
Stacktrace:
#0 CastMap.forEach.<anonymous closure> (dart:_internal/cast.dart:288:25)
#1 _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:400:8)
#2 CastMap.forEach (dart:_internal/cast.dart:287:13)
#3 mapToQuery (package:http/src/utils.dart:17:7)
#4 Request.bodyFields= (package:http/src/request.dart:137:12)
#5 BaseClient._sendUnstreamed (package:http/src/base_client.dart:87:17)
#6 BaseClient.post (package:http/src/base_client.dart:32:7)
#7 BaseHttpService.callApi (package:shop/services/base-http.dart:18:60)
#8 ShopService.create (package:shop/services/shop_service.dart:20:28)
<asynchronous suspension>
#9 _ShopAddScreenState.postShop (package:shop/shop_add_screen.dart:137:16)
<asynchronous suspension>
Solution
Changing the code snippet below in http-0.13.4/lib/src/utils.dart sorted the error: "type ‘Null’ is not a subtype of type ‘String’ in type cast". Reference to github issue where I got this code https://github.com/dart-lang/http/issues/75
This removes all the null from the payload, or you can put some other code in there to manage nulls.
FROM:
String mapToQuery(Map<String, String> map, {Encoding encoding}) {
var pairs = <List<String>>[];
map.forEach((key, value) =>
pairs.add([Uri.encodeQueryComponent(key, encoding: encoding),
Uri.encodeQueryComponent(value, encoding: encoding)]));
return pairs.map((pair) => "${pair[0]}=${pair[1]}").join("&");
}
TO:
String mapToQuery(Map<String, String> map, {Encoding encoding}) {
+ map.keys
+ .where((k) => (map[k] == null)).toList() // -- keys for null elements
+ .forEach(map.remove);
+
var pairs = <List<String>>[];
map.forEach((key, value) =>
pairs.add([Uri.encodeQueryComponent(key, encoding: encoding),
Uri.encodeQueryComponent(value, encoding: encoding)]));
return pairs.map((pair) => "${pair[0]}=${pair[1]}").join("&");
}
enter code here
I also faced another similar error after resolving this "type ‘int’ is not a subtype of type ‘String’ in type cast". I believe it is due to this code in http-0.13.4/lib/src/utils.dart where the http client tries to map everything to string as referenced above.
String mapToQuery(Map<String, String> map, {Encoding encoding})
To avoid all these issues I just changed my function:
FROM:
Future<String> callApi(String url, String token, {String method: 'get', Map<String, dynamic>? payload})
TO:
Future<String> callApi(String url, String token, {String method: 'get', String? payload})
Then instead of using shop.toJson() I replaced it with json.encode(shop)
Future<Shop> create(Shop shop) async {
var token = await this.appContext.currentUser!.getIdToken();
final response = await callApi('$_cardApi/', token, method: 'post', payload: json.encode(shop));
// return Shop.fromJsonModel(json.decode(response));
return shop;
}
By changing the payload to a String rather than Map it triggers a different output as you can see in this peiece of code in http-0.13.4/lib/src/base_client.dart
if (headers != null) request.headers.addAll(headers);
if (encoding != null) request.encoding = encoding;
if (body != null) {
if (body is String) {
request.body = body;
} else if (body is List) {
request.bodyBytes = body.cast<int>();
} else if (body is Map) {
request.bodyFields = body.cast<String, String>();
} else {
throw ArgumentError('Invalid request body "$body".');
}
Previously I was triggering body is Map which cast the values to Strings:
else if (body is Map) {
request.bodyFields = body.cast<String, String>();
}
This was casting my null and int values to String which is causing the two errors.
Now I am triggering:
if (body is String) {
request.body = body;
}
Hope this helps someone who is facing this problem in the future.
Answered By – TEZZ
Answer Checked By – Willingham (FlutterFixes Volunteer)