Rest API Consumption in flutter - rest

Question

Problem
I'm trying to receive data from a rest API in my flutter app, inside a future function but i keep getting the following error:
type 'CastList<dynamic, List>' is not a subtype of type 'List'.
The function I'm using to fetch the data is as follows:
static Future<List<Questionnaire>> fetchQuestionnaires() async {
try {
final response =
await http.get('http://10.0.2.2:4010/phone/questionnaires');
if (response.statusCode == 200) {
List<Questionnaire> resp =
json.decode(response.body).cast<List<Questionnaire>>();
List<Questionnaire> questionnaires = resp
.map<Questionnaire>((dynamic item) => Questionnaire.fromJson(item))
.toList();
log(
questionnaires.toString(),
);
return questionnaires;
} else {
throw Exception('Unable to fetch questionnaires');
}
} catch (error) {
log(error.toString());
}
}
I don't understand why this is. Why does using cast, cast to CastList<dynamic, Type> and not the original Type? What changes do I do to get my data?
The data model is given below.
Data Model
The Data i expect from my backend is like this. An array called questionnaires, containing multiples of a questionnaire, each containing an id and a displayQuestion. A displayQuestion in turn has the question text and the answers Array_.
For this, I have the following structure in my Json.
[
{
"questionId": 1,
"displayQuestions": [
{
"question": "how old are you",
"answers": [
"1",
"2",
"3",
"4"
]
}
]
}
]
This is my questionnaires.dart model
class Questionnaires {
List<Questionnaire> _questionnaires;
Questionnaires.fromJson(this._questionnaires);
}
This is questionnaire.dart model
class Questionnaire {
int questionnaireId;
List<DisplayQuestion> displayQuestions = [];
Questionnaire(this.questionnaireId, this.displayQuestions);
Questionnaire.fromJson(Map<String, dynamic> json)
: questionnaireId = json['questionnaireId'],
displayQuestions = json['displayQuestions'];
}
The code from display-question.dart model
class DisplayQuestion {
String _question;
List<String> _answers;
String get question => this._question;
List get answers => this._answers;
DisplayQuestion(this._question, [List<String> answers])
: this._answers = answers ?? [];
DisplayQuestion.fromJson(Map<String, dynamic> json)
: _question = json['question'],
_answers = json['answers'];
}

Answer 1

Why does using cast, cast to CastList<dynamic, Type> and not the original Type?
From the docs, myList.cast<MyType> returns a List<MyType>. In your case you're calling resp.cast<List<Questionnaire>>, so the return will be List<List<Questionnaire>>, which is not what you want.
If you're asking about CastList<dynamic, Type>, it's a subclass of List<Type>, see the source code. It's useful because CastList doesn't need to create a new list, it's just a wrapper around the original list where each element is cast with as Type before being returned.
What changes do I do to get my data?
The problem is you're calling resp.cast<Type> where resp is not a list that constains Type.
Here's a working sample based on the code you provided:
import 'dart:convert';
final sampleJson = """[
{
"questionId": 1,
"displayQuestions": [
{
"question": "how old are you",
"answers": [
"1",
"2",
"3",
"4"
]
}
]
}
]
""";
class DisplayQuestion {
String question;
List<String> answers;
DisplayQuestion.fromJson(Map<String, dynamic> json)
: question = json["question"],
answers = json["answers"].cast<String>();
String toString() => 'question: $question | answers: $answers';
}
class Questionnaire {
int questionnaireId;
List<DisplayQuestion> displayQuestions;
Questionnaire.fromJson(Map<String, dynamic> json)
: questionnaireId = json['questionnaireId'],
displayQuestions = (json['displayQuestions'] as List<dynamic>)
.map((questionJson) => DisplayQuestion.fromJson(questionJson))
.toList();
String toString() => '$displayQuestions';
}
List<Questionnaire> parseQuestionnaires() {
List<Questionnaire> questionnaires =
(json.decode(sampleJson) as List<dynamic>)
.map((questionnaireJson) => Questionnaire.fromJson(questionnaireJson))
.toList();
return questionnaires;
}
void main() {
print(parseQuestionnaires());
// => [[question: how old are you | answers: [1, 2, 3, 4]]]
}