Flutter Http getting a response but passing null to then when the method is call-CodePudding

I actually get the response from my API, using this method.

static Future<String> saveData(String url, Vehicle vehicle) async {
dio.Dio d = dio.Dio();


Map<String, dynamic> headers = {};
headers['Cookie'] = "JSESSIONID="   "fff";// SessionUtils().getSession().toString();
dio.Options options = dio.Options(
    headers: headers,
    contentType: 'application/json',
    responseType: dio.ResponseType.json,
);

final dio.Response response = await d.post(url, options: options, data: jsonEncode(vehicle));

print(response.data);
dynamic item;
String result = "";
if (response.statusCode == HttpStatus.ok) {
  item = response.data; //json.decode(response.data);

  print("****************");
  print(item);
  print("****************");
  print(item["id"]);

  if(item["success"]){
    result = item["id"]; // **result is correct here** 
    print("OK");
  }
  print(item["success"]); // Data is printed out correctly
}
print(result);

return result;

}

But when I call this method using the code below I cannot get the returned data in the then.

VehicleHttpService.saveData(Constant.POST_VEHICLE_URL, widget.vehicle).then((value){
          }).then((String? value){
            print("#########");
            print(value); //**I get null here. why**
            print("#########");
          }).whenComplete((){
            Navigator.of(context).pop();
          });

I was expecting the result returned by saveData. Please help me check what is wrong with my code.

CodePudding user response：

saveData is async method and you should await for the result and also why do you call two then after each other? it is wrong, try this:

String? value = await VehicleHttpService.saveData(Constant.POST_VEHICLE_URL, widget.vehicle);

print("#########");
print(value); //**I get null here. why**
print("#########");
Navigator.of(context).pop();

CodePudding user response：

Let's check your executing code by parts:

VehicleHttpService.saveData(Constant.POST_VEHICLE_URL, widget.vehicle).then((value){
      }).then((String? value){
        print("#########");
        print(value); //**I get null here. why**
        print("#########");
      }).whenComplete((){
        Navigator.of(context).pop();
      });

The first step is executing the API request and return a response

VehicleHttpService.saveData(Constant.POST_VEHICLE_URL, widget.vehicle)

The second step is receives the response, converts it in null and return to the Future chain

then((value){})

The third step always will receive the null

then((String? value){
        print("#########");
        print(value); //**I get null here. why**
        print("#########");
      }).whenComplete((){
        Navigator.of(context).pop();
      })

Remove then in the middle and you will receive the response and can able to handle it