[英]Parsing JSON that has a nested array of objects in Dart?
我正在制作一個 Flutter 應用程序,我正在使用 MovieDB api 來獲取數據。 當我調用 api 並要求特定電影時,這是我返回的一般格式:
{
"adult": false,
"backdrop_path": "/wrqUiMXttHE4UBFMhLHlN601MZh.jpg",
"belongs_to_collection": null,
"budget": 120000000,
"genres": [
{
"id": 28,
"name": "Action"
},
{
"id": 12,
"name": "Adventure"
},
{
"id": 878,
"name": "Science Fiction"
}
],
"homepage": "http://www.rampagethemovie.com",
"id": 427641,
"imdb_id": "tt2231461",
"original_language": "en",
"original_title": "Rampage",
...
}
我已經設置了一個模型類來解析它,並且該類定義如下:
import 'dart:async';
class MovieDetail {
final String title;
final double rating;
final String posterArtUrl;
final backgroundArtUrl;
final List<Genre> genres;
final String overview;
final String tagline;
final int id;
const MovieDetail(
{this.title, this.rating, this.posterArtUrl, this.backgroundArtUrl, this.genres, this.overview, this.tagline, this.id});
MovieDetail.fromJson(Map jsonMap)
: title = jsonMap['title'],
rating = jsonMap['vote_average'].toDouble(),
posterArtUrl = "http://image.tmdb.org/t/p/w342" + jsonMap['backdrop_path'],
backgroundArtUrl = "http://image.tmdb.org/t/p/w500" + jsonMap['poster_path'],
genres = (jsonMap['genres']).map((i) => Genre.fromJson(i)).toList(),
overview = jsonMap['overview'],
tagline = jsonMap['tagline'],
id = jsonMap['id'];
}
class Genre {
final int id;
final String genre;
const Genre(this.id, this.genre);
Genre.fromJson(Map jsonMap)
: id = jsonMap['id'],
genre = jsonMap['name'];
}
我的問題是我無法從 JSON 中正確解析流派。 當我獲取 JSON 並通過我的模型類傳遞它時,我收到以下錯誤:
I/flutter (10874): type 'List<dynamic>' is not a subtype of type 'List<Genre>' where
I/flutter (10874): List is from dart:core
I/flutter (10874): List is from dart:core
I/flutter (10874): Genre is from package:flutter_app_first/models/movieDetail.dart
我認為這會起作用,因為我為Genre
對象創建了一個不同的類,並將 JSON 數組作為列表傳入。 我不明白List<dynamic>
怎么不是List<Genre>
的孩子,因為關鍵字dynamic
暗示任何對象嗎? 有誰知道如何將嵌套的 JSON 數組解析為自定義對象?
嘗試genres = (jsonMap['genres'] as List).map((i) => Genre.fromJson(i)).toList()
問題:在沒有Genre.fromJson
情況下調用map
使其成為動態調用,這意味着來自Genre.fromJson
的返回類型也是動態的(不是 Genre)。
查看https://flutter.io/json/以獲得一些提示。
有一些解決方案,比如https://pub.dartlang.org/packages/json_serializable ,可以讓這變得更容易
我認為JSONtoDart 轉換器非常有用,必須嘗試...
這是一個簡單易懂的例子。
將 JSON 字符串作為這樣的嵌套對象。
{
"title": "Dart Tutorial",
"description": "Way to parse Json",
"author": {
"name": "bezkoder",
"age": 30
我們可以考慮 2 個類:
author
User
{title, description, author}
教程所以我們可以像這樣定義一個新的教程。
class User {
String name;
int age;
...
}
class Tutorial {
String title;
String description;
User author;
Tutorial(this.title, this.description, this.author);
factory Tutorial.fromJson(dynamic json) {
return Tutorial(json['title'] as String, json['description'] as String, User.fromJson(json['author']));
}
@override
String toString() {
return '{ ${this.title}, ${this.description}, ${this.author} }';
}
}
main() 函數現在看起來像下面的代碼。 導入“飛鏢:轉換”;
main() {
String nestedObjText =
'{"title": "Dart Tutorial", "description": "Way to parse Json", "author": {"name": "bezkoder", "age": 30}}';
Tutorial tutorial = Tutorial.fromJson(jsonDecode(nestedObjText));
print(tutorial);
查看結果,可以看到我們嵌套的對象:
{ Dart Tutorial, Way to parse Json, { bezkoder, 30 } }
收到響應后,首先需要分別提取數組。 然后就可以輕松映射了。 這就是我這樣做的方式。
List<Attempts> attempts;
attempts=(jsonDecode(res.body)['message1'] as List).map((i) => Attempts.fromJson(i)).toList();
List<Posts> posts;
attempts=(jsonDecode(res.body)['message2'] as List).map((i) => Post.fromJson(i)).toList();
參考下面的例子。
Future<List<Attempts>> getStatisticData() async {
String uri = global.serverDNS + "PaperAttemptsManager.php";
var res = await http.post(
uri,
headers: <String, String>{
'Content-Type': 'application/json; charset=UTF-8',
},
body: jsonEncode(<String, String>{
'userName': widget.userId,
'subject': widget.subjectName,
'method': "GETPTEN",
}),
);
if (res.statusCode == 200) {
List<Attempts> attempts;
attempts=(jsonDecode(res.body)['message'] as List).map((i) => Attempts.fromJson(i)).toList();
return attempts;
} else {
throw "Can't get subjects.";
}
}
模型類
class Attempts {
String message, userName, date, year, time;
int status, id, marks, correctAnswers, wrongAnswers, emptyAnswers;
Attempts({
this.status,
this.message,
this.id,
this.userName,
this.date,
this.year,
this.marks,
this.time,
this.correctAnswers,
this.wrongAnswers,
this.emptyAnswers,
});
factory Attempts.fromJson(Map<String, dynamic> json) {
return Attempts(
status: json['status'],
message: json['message'],
id: json['ID'],
userName: json['USERNAME'],
date: json['DATE'],
year: json['YEAR'],
marks: json['MARKS'],
time: json['TIME'],
correctAnswers: json['CORRECT_ANSWERS'],
wrongAnswers: json['WRONG_ANSWERS'],
emptyAnswers: json['EMPTY_ANSWERS'],
);
}
}
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