Dart Map and Parsing JSON - android-studio

Question

As a new Dart Fan, I would like to understand the concept of Map/List.
I tried to do HTTP requests, getting JSON data. And it's ok till I have to assign to the Map.
Let me show you the example of JSON data:
{
"error":"",
"error_number":"",
"response_code":200,
"result":[
{
"id":1,
"name":"Great Deal",
"day_aired":"2015-07-05 11:06:09",
"trend":"Noone",
"trend_details": [{
"name":"Great Deal",
}
]
},
{
"id":2,
....
}
]
}
The code:
import 'dart:async';
import 'dart:convert';
import 'package:http/http.dart' as http;
Future<ApiResponse> fetchData(String command, Map params) async {
final String url =
'https://example.com/api/v2/....';
final response = await http.get(url);
if (response.statusCode == 200) {
return ApiResponse.fromJson(json.decode(response.body));
} else {
// If that call was not successful, throw an error.
throw Exception('Failed to load post');
}
}
}
final response = await http.get(url);
dynamic data = json.decode(response.body);
List<String> parsed = data['result'] as List<String>;
// List<String> parsedList = new List<String>.from(parsed);
if (response.statusCode == 200) {
//return json.decode(response.body);
List<ApiResponse> list = List<ApiResponse>.from(
parsed.map((i) => ApiResponse.fromJson(i as Map<String, dynamic>)));
}
I do the same as this article. But I read this article too. I'm trying to create Future<ApiResponse> with data from json.decode(response.body) (result entry inside of it).
factory ApiResponse.fromJson(Map<String, dynamic> json) {...}
But as I understand, result is not Map<String, dynamic> but when I try to invoke the code below it says:
Unhandled Exception: type 'List<dynamic>' is not a subtype of type 'List<String>' in type cast and it referred to List<String> parsed = data['result'] as List<String>;.
I'm confused and I know the code is a mess. I read in the second article that I should do an additional cast to trend_details but it did not work as I expected. Obviously data['result'] is an array but how to cast it properly? What are the good practices?

Answer 1

result stores a list of Map<String, dynamic>
final parsed = data['result'] as List<Map<String, dynamic>>;

Answer 2

You can parse json string with the following structure and code
You can see picture display correct result name
code snippet
var payload = payloadFromJson(jsonStr);
print('${payload.result[0].name}');
related class
// To parse this JSON data, do
//
// final payload = payloadFromJson(jsonString);
import 'dart:convert';
Payload payloadFromJson(String str) => Payload.fromJson(json.decode(str));
String payloadToJson(Payload data) => json.encode(data.toJson());
class Payload {
String error;
String errorNumber;
int responseCode;
List<Result> result;
Payload({
this.error,
this.errorNumber,
this.responseCode,
this.result,
});
factory Payload.fromJson(Map<String, dynamic> json) => Payload(
error: json["error"] == null ? null : json["error"],
errorNumber: json["error_number"] == null ? null : json["error_number"],
responseCode: json["response_code"] == null ? null : json["response_code"],
result: json["result"] == null ? null : List<Result>.from(json["result"].map((x) => Result.fromJson(x))),
);
Map<String, dynamic> toJson() => {
"error": error == null ? null : error,
"error_number": errorNumber == null ? null : errorNumber,
"response_code": responseCode == null ? null : responseCode,
"result": result == null ? null : List<dynamic>.from(result.map((x) => x.toJson())),
};
}
class Result {
int id;
String name;
DateTime dayAired;
String trend;
List<TrendDetail> trendDetails;
Result({
this.id,
this.name,
this.dayAired,
this.trend,
this.trendDetails,
});
factory Result.fromJson(Map<String, dynamic> json) => Result(
id: json["id"] == null ? null : json["id"],
name: json["name"] == null ? null : json["name"],
dayAired: json["day_aired"] == null ? null : DateTime.parse(json["day_aired"]),
trend: json["trend"] == null ? null : json["trend"],
trendDetails: json["trend_details"] == null ? null : List<TrendDetail>.from(json["trend_details"].map((x) => TrendDetail.fromJson(x))),
);
Map<String, dynamic> toJson() => {
"id": id == null ? null : id,
"name": name == null ? null : name,
"day_aired": dayAired == null ? null : dayAired.toIso8601String(),
"trend": trend == null ? null : trend,
"trend_details": trendDetails == null ? null : List<dynamic>.from(trendDetails.map((x) => x.toJson())),
};
}
class TrendDetail {
String name;
TrendDetail({
this.name,
});
factory TrendDetail.fromJson(Map<String, dynamic> json) => TrendDetail(
name: json["name"] == null ? null : json["name"],
);
Map<String, dynamic> toJson() => {
"name": name == null ? null : name,
};
}
full code
import 'package:flutter/material.dart';
// To parse this JSON data, do
//
// final payload = payloadFromJson(jsonString);
import 'dart:convert';
Payload payloadFromJson(String str) => Payload.fromJson(json.decode(str));
String payloadToJson(Payload data) => json.encode(data.toJson());
class Payload {
String error;
String errorNumber;
int responseCode;
List<Result> result;
Payload({
this.error,
this.errorNumber,
this.responseCode,
this.result,
});
factory Payload.fromJson(Map<String, dynamic> json) => Payload(
error: json["error"] == null ? null : json["error"],
errorNumber: json["error_number"] == null ? null : json["error_number"],
responseCode: json["response_code"] == null ? null : json["response_code"],
result: json["result"] == null ? null : List<Result>.from(json["result"].map((x) => Result.fromJson(x))),
);
Map<String, dynamic> toJson() => {
"error": error == null ? null : error,
"error_number": errorNumber == null ? null : errorNumber,
"response_code": responseCode == null ? null : responseCode,
"result": result == null ? null : List<dynamic>.from(result.map((x) => x.toJson())),
};
}
class Result {
int id;
String name;
DateTime dayAired;
String trend;
List<TrendDetail> trendDetails;
Result({
this.id,
this.name,
this.dayAired,
this.trend,
this.trendDetails,
});
factory Result.fromJson(Map<String, dynamic> json) => Result(
id: json["id"] == null ? null : json["id"],
name: json["name"] == null ? null : json["name"],
dayAired: json["day_aired"] == null ? null : DateTime.parse(json["day_aired"]),
trend: json["trend"] == null ? null : json["trend"],
trendDetails: json["trend_details"] == null ? null : List<TrendDetail>.from(json["trend_details"].map((x) => TrendDetail.fromJson(x))),
);
Map<String, dynamic> toJson() => {
"id": id == null ? null : id,
"name": name == null ? null : name,
"day_aired": dayAired == null ? null : dayAired.toIso8601String(),
"trend": trend == null ? null : trend,
"trend_details": trendDetails == null ? null : List<dynamic>.from(trendDetails.map((x) => x.toJson())),
};
}
class TrendDetail {
String name;
TrendDetail({
this.name,
});
factory TrendDetail.fromJson(Map<String, dynamic> json) => TrendDetail(
name: json["name"] == null ? null : json["name"],
);
Map<String, dynamic> toJson() => {
"name": name == null ? null : name,
};
}
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
// This is the theme of your application.
//
// Try running your application with "flutter run". You'll see the
// application has a blue toolbar. Then, without quitting the app, try
// changing the primarySwatch below to Colors.green and then invoke
// "hot reload" (press "r" in the console where you ran "flutter run",
// or simply save your changes to "hot reload" in a Flutter IDE).
// Notice that the counter didn't reset back to zero; the application
// is not restarted.
primarySwatch: Colors.blue,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
// This widget is the home page of your application. It is stateful, meaning
// that it has a State object (defined below) that contains fields that affect
// how it looks.
// This class is the configuration for the state. It holds the values (in this
// case the title) provided by the parent (in this case the App widget) and
// used by the build method of the State. Fields in a Widget subclass are
// always marked "final".
final String title;
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
String jsonStr = '''
{
"error":"",
"error_number":"",
"response_code":200,
"result":[
{
"id":1,
"name":"Great Deal",
"day_aired":"2015-07-05 11:06:09",
"trend":"Noone",
"trend_details": [{
"name":"Great Deal"
}
]
}
]
}
''';
void _incrementCounter() {
var payload = payloadFromJson(jsonStr);
print('${payload.result[0].name}');
setState(() {
// This call to setState tells the Flutter framework that something has
// changed in this State, which causes it to rerun the build method below
// so that the display can reflect the updated values. If we changed
// _counter without calling setState(), then the build method would not be
// called again, and so nothing would appear to happen.
_counter++;
});
}
#override
Widget build(BuildContext context) {
// This method is rerun every time setState is called, for instance as done
// by the _incrementCounter method above.
//
// The Flutter framework has been optimized to make rerunning build methods
// fast, so that you can just rebuild anything that needs updating rather
// than having to individually change instances of widgets.
return Scaffold(
appBar: AppBar(
// Here we take the value from the MyHomePage object that was created by
// the App.build method, and use it to set our appbar title.
title: Text(widget.title),
),
body: Center(
// Center is a layout widget. It takes a single child and positions it
// in the middle of the parent.
child: Column(
// Column is also a layout widget. It takes a list of children and
// arranges them vertically. By default, it sizes itself to fit its
// children horizontally, and tries to be as tall as its parent.
//
// Invoke "debug painting" (press "p" in the console, choose the
// "Toggle Debug Paint" action from the Flutter Inspector in Android
// Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
// to see the wireframe for each widget.
//
// Column has various properties to control how it sizes itself and
// how it positions its children. Here we use mainAxisAlignment to
// center the children vertically; the main axis here is the vertical
// axis because Columns are vertical (the cross axis would be
// horizontal).
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.display1,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
);
}
}