FromJson 返回 null ，颤动

Question

我有这个模型

class Meal {
  Meal({
    this.strMeal,
    this.strMealThumb,
    this.idMeal,
  });

  String strMeal;
  String strMealThumb;
  String idMeal;

  factory Meal.fromJson(Map<String, dynamic> json) => Meal(
    strMeal: json["strMeal"],
    strMealThumb: json["strMealThumb"],
    idMeal: json["idMeal"],
  );

}

在 Flutter 上的 HTTP 包的帮助下，我提出了这个请求：

class RemoteServices {
  static var client = http.Client();

  static Future<Meal> fetchMealById(String id) async {
    var response = await client
        .get('https://www.themealdb.com/api/json/v1/1/lookup.php?i=52772');
    if (response.statusCode == 200) {
        var jasonStr = response.body;
        print("f" + jasonStr);
        Meal  meal = Meal.fromJson(json.decode(response.body));

        print(meal.idMeal);
    
    } else {
      throw Exception("Unable to  Load");
    }
  }
} 

解码响应后我得到一个 Null 值，尽管解码前的响应正文不是 null 并且包含一个保存我的数据的对象

Answer 1

您可以在下面复制粘贴运行完整代码
从您的网址返回的数据是 List<Meal> 而不是 Meal
您可以参考完整代码中的模型详细信息
代码sn-p

Payload payloadFromJson(String str) => Payload.fromJson(json.decode(str));

class Payload {
  Payload({
    this.meals,
  });

  List<Meal> meals;
...
static Future<List<Meal>> fetchMealById(String id) async {
    print(id);
    var response = await client
        .get('https://www.themealdb.com/api/json/v1/1/lookup.php?i=52772');
    if (response.statusCode == 200) {
      var jasonStr = response.body;
      print("f" + jasonStr);
      Payload payload = payloadFromJson(response.body);      
      return payload.meals;
...   
 FutureBuilder(
            future: _future("yourId"),
            builder: (context, AsyncSnapshot<List<Meal>> snapshot) {
              switch (snapshot.connectionState) {
                case ConnectionState.none:
                  return Text('none');
                case ConnectionState.waiting:
                  return Center(child: CircularProgressIndicator());
                case ConnectionState.active:
                  return Text('');
                case ConnectionState.done:
                  if (snapshot.hasError) {
                    return Text(
                      '${snapshot.error}',
                      style: TextStyle(color: Colors.red),
                    );
                  } else {
                    print(snapshot.data.runtimeType);
                    return ListView.builder(
                        itemCount: snapshot.data.length,
                        itemBuilder: (context, index) {
                          return Card(
                              elevation: 6.0,
                              child: Padding(
                                padding: const EdgeInsets.only(
                                    top: 6.0,
                                    bottom: 6.0,
                                    left: 8.0,
                                    right: 8.0),
                                child: Row(
                                  crossAxisAlignment: CrossAxisAlignment.start,
                                  children: <Widget>[
                                    Text(
                                        snapshot.data[index].idMeal.toString()),      

工作演示

完整代码

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'dart:convert';

Payload payloadFromJson(String str) => Payload.fromJson(json.decode(str));

String payloadToJson(Payload data) => json.encode(data.toJson());

class Payload {
  Payload({
    this.meals,
  });

  List<Meal> meals;

  factory Payload.fromJson(Map<String, dynamic> json) => Payload(
        meals: json["meals"] == null
            ? null
            : List<Meal>.from(json["meals"].map((x) => Meal.fromJson(x))),
      );

  Map<String, dynamic> toJson() => {
        "meals": meals == null
            ? null
            : List<dynamic>.from(meals.map((x) => x.toJson())),
      };
}

class Meal {
  Meal({
    this.idMeal,
    this.strMeal,
    this.strDrinkAlternate,
    this.strCategory,
    this.strArea,
    this.strInstructions,
    this.strMealThumb,
    this.strTags,
    this.strYoutube,
    this.strIngredient1,
    this.strIngredient2,
    this.strIngredient3,
    this.strIngredient4,
    this.strIngredient5,
    this.strIngredient6,
    this.strIngredient7,
    this.strIngredient8,
    this.strIngredient9,
    this.strIngredient10,
    this.strIngredient11,
    this.strIngredient12,
    this.strIngredient13,
    this.strIngredient14,
    this.strIngredient15,
    this.strIngredient16,
    this.strIngredient17,
    this.strIngredient18,
    this.strIngredient19,
    this.strIngredient20,
    this.strMeasure1,
    this.strMeasure2,
    this.strMeasure3,
    this.strMeasure4,
    this.strMeasure5,
    this.strMeasure6,
    this.strMeasure7,
    this.strMeasure8,
    this.strMeasure9,
    this.strMeasure10,
    this.strMeasure11,
    this.strMeasure12,
    this.strMeasure13,
    this.strMeasure14,
    this.strMeasure15,
    this.strMeasure16,
    this.strMeasure17,
    this.strMeasure18,
    this.strMeasure19,
    this.strMeasure20,
    this.strSource,
    this.dateModified,
  });

  String idMeal;
  String strMeal;
  dynamic strDrinkAlternate;
  String strCategory;
  String strArea;
  String strInstructions;
  String strMealThumb;
  String strTags;
  String strYoutube;
  String strIngredient1;
  String strIngredient2;
  String strIngredient3;
  String strIngredient4;
  String strIngredient5;
  String strIngredient6;
  String strIngredient7;
  String strIngredient8;
  String strIngredient9;
  String strIngredient10;
  String strIngredient11;
  String strIngredient12;
  String strIngredient13;
  String strIngredient14;
  String strIngredient15;
  dynamic strIngredient16;
  dynamic strIngredient17;
  dynamic strIngredient18;
  dynamic strIngredient19;
  dynamic strIngredient20;
  String strMeasure1;
  String strMeasure2;
  String strMeasure3;
  String strMeasure4;
  String strMeasure5;
  String strMeasure6;
  String strMeasure7;
  String strMeasure8;
  String strMeasure9;
  String strMeasure10;
  String strMeasure11;
  String strMeasure12;
  String strMeasure13;
  String strMeasure14;
  String strMeasure15;
  dynamic strMeasure16;
  dynamic strMeasure17;
  dynamic strMeasure18;
  dynamic strMeasure19;
  dynamic strMeasure20;
  dynamic strSource;
  dynamic dateModified;

  factory Meal.fromJson(Map<String, dynamic> json) => Meal(
        idMeal: json["idMeal"] == null ? null : json["idMeal"],
        strMeal: json["strMeal"] == null ? null : json["strMeal"],
        strDrinkAlternate: json["strDrinkAlternate"],
        strCategory: json["strCategory"] == null ? null : json["strCategory"],
        strArea: json["strArea"] == null ? null : json["strArea"],
        strInstructions:
            json["strInstructions"] == null ? null : json["strInstructions"],
        strMealThumb:
            json["strMealThumb"] == null ? null : json["strMealThumb"],
        strTags: json["strTags"] == null ? null : json["strTags"],
        strYoutube: json["strYoutube"] == null ? null : json["strYoutube"],
        strIngredient1:
            json["strIngredient1"] == null ? null : json["strIngredient1"],
        strIngredient2:
            json["strIngredient2"] == null ? null : json["strIngredient2"],
        strIngredient3:
            json["strIngredient3"] == null ? null : json["strIngredient3"],
        strIngredient4:
            json["strIngredient4"] == null ? null : json["strIngredient4"],
        strIngredient5:
            json["strIngredient5"] == null ? null : json["strIngredient5"],
        strIngredient6:
            json["strIngredient6"] == null ? null : json["strIngredient6"],
        strIngredient7:
            json["strIngredient7"] == null ? null : json["strIngredient7"],
        strIngredient8:
            json["strIngredient8"] == null ? null : json["strIngredient8"],
        strIngredient9:
            json["strIngredient9"] == null ? null : json["strIngredient9"],
        strIngredient10:
            json["strIngredient10"] == null ? null : json["strIngredient10"],
        strIngredient11:
            json["strIngredient11"] == null ? null : json["strIngredient11"],
        strIngredient12:
            json["strIngredient12"] == null ? null : json["strIngredient12"],
        strIngredient13:
            json["strIngredient13"] == null ? null : json["strIngredient13"],
        strIngredient14:
            json["strIngredient14"] == null ? null : json["strIngredient14"],
        strIngredient15:
            json["strIngredient15"] == null ? null : json["strIngredient15"],
        strIngredient16: json["strIngredient16"],
        strIngredient17: json["strIngredient17"],
        strIngredient18: json["strIngredient18"],
        strIngredient19: json["strIngredient19"],
        strIngredient20: json["strIngredient20"],
        strMeasure1: json["strMeasure1"] == null ? null : json["strMeasure1"],
        strMeasure2: json["strMeasure2"] == null ? null : json["strMeasure2"],
        strMeasure3: json["strMeasure3"] == null ? null : json["strMeasure3"],
        strMeasure4: json["strMeasure4"] == null ? null : json["strMeasure4"],
        strMeasure5: json["strMeasure5"] == null ? null : json["strMeasure5"],
        strMeasure6: json["strMeasure6"] == null ? null : json["strMeasure6"],
        strMeasure7: json["strMeasure7"] == null ? null : json["strMeasure7"],
        strMeasure8: json["strMeasure8"] == null ? null : json["strMeasure8"],
        strMeasure9: json["strMeasure9"] == null ? null : json["strMeasure9"],
        strMeasure10:
            json["strMeasure10"] == null ? null : json["strMeasure10"],
        strMeasure11:
            json["strMeasure11"] == null ? null : json["strMeasure11"],
        strMeasure12:
            json["strMeasure12"] == null ? null : json["strMeasure12"],
        strMeasure13:
            json["strMeasure13"] == null ? null : json["strMeasure13"],
        strMeasure14:
            json["strMeasure14"] == null ? null : json["strMeasure14"],
        strMeasure15:
            json["strMeasure15"] == null ? null : json["strMeasure15"],
        strMeasure16: json["strMeasure16"],
        strMeasure17: json["strMeasure17"],
        strMeasure18: json["strMeasure18"],
        strMeasure19: json["strMeasure19"],
        strMeasure20: json["strMeasure20"],
        strSource: json["strSource"],
        dateModified: json["dateModified"],
      );

  Map<String, dynamic> toJson() => {
        "idMeal": idMeal == null ? null : idMeal,
        "strMeal": strMeal == null ? null : strMeal,
        "strDrinkAlternate": strDrinkAlternate,
        "strCategory": strCategory == null ? null : strCategory,
        "strArea": strArea == null ? null : strArea,
        "strInstructions": strInstructions == null ? null : strInstructions,
        "strMealThumb": strMealThumb == null ? null : strMealThumb,
        "strTags": strTags == null ? null : strTags,
        "strYoutube": strYoutube == null ? null : strYoutube,
        "strIngredient1": strIngredient1 == null ? null : strIngredient1,
        "strIngredient2": strIngredient2 == null ? null : strIngredient2,
        "strIngredient3": strIngredient3 == null ? null : strIngredient3,
        "strIngredient4": strIngredient4 == null ? null : strIngredient4,
        "strIngredient5": strIngredient5 == null ? null : strIngredient5,
        "strIngredient6": strIngredient6 == null ? null : strIngredient6,
        "strIngredient7": strIngredient7 == null ? null : strIngredient7,
        "strIngredient8": strIngredient8 == null ? null : strIngredient8,
        "strIngredient9": strIngredient9 == null ? null : strIngredient9,
        "strIngredient10": strIngredient10 == null ? null : strIngredient10,
        "strIngredient11": strIngredient11 == null ? null : strIngredient11,
        "strIngredient12": strIngredient12 == null ? null : strIngredient12,
        "strIngredient13": strIngredient13 == null ? null : strIngredient13,
        "strIngredient14": strIngredient14 == null ? null : strIngredient14,
        "strIngredient15": strIngredient15 == null ? null : strIngredient15,
        "strIngredient16": strIngredient16,
        "strIngredient17": strIngredient17,
        "strIngredient18": strIngredient18,
        "strIngredient19": strIngredient19,
        "strIngredient20": strIngredient20,
        "strMeasure1": strMeasure1 == null ? null : strMeasure1,
        "strMeasure2": strMeasure2 == null ? null : strMeasure2,
        "strMeasure3": strMeasure3 == null ? null : strMeasure3,
        "strMeasure4": strMeasure4 == null ? null : strMeasure4,
        "strMeasure5": strMeasure5 == null ? null : strMeasure5,
        "strMeasure6": strMeasure6 == null ? null : strMeasure6,
        "strMeasure7": strMeasure7 == null ? null : strMeasure7,
        "strMeasure8": strMeasure8 == null ? null : strMeasure8,
        "strMeasure9": strMeasure9 == null ? null : strMeasure9,
        "strMeasure10": strMeasure10 == null ? null : strMeasure10,
        "strMeasure11": strMeasure11 == null ? null : strMeasure11,
        "strMeasure12": strMeasure12 == null ? null : strMeasure12,
        "strMeasure13": strMeasure13 == null ? null : strMeasure13,
        "strMeasure14": strMeasure14 == null ? null : strMeasure14,
        "strMeasure15": strMeasure15 == null ? null : strMeasure15,
        "strMeasure16": strMeasure16,
        "strMeasure17": strMeasure17,
        "strMeasure18": strMeasure18,
        "strMeasure19": strMeasure19,
        "strMeasure20": strMeasure20,
        "strSource": strSource,
        "dateModified": dateModified,
      };
}

class RemoteServices {
  static var client = http.Client();

  static Future<List<Meal>> fetchMealById(String id) async {
    print(id);
    var response = await client
        .get('https://www.themealdb.com/api/json/v1/1/lookup.php?i=52772');
    if (response.statusCode == 200) {
      var jasonStr = response.body;
      print("f" + jasonStr);
      Payload payload = payloadFromJson(response.body);
      //Meal  meal = Meal.fromJson(json.decode(response.body));
      print(payload.meals.length);
      print(payload.meals);
      return payload.meals;
    } else {
      throw Exception("Unable to  Load");
    }
  }
}

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Function _future;

  @override
  void initState() {
    _future = RemoteServices.fetchMealById;
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: FutureBuilder(
            future: _future("yourId"),
            builder: (context, AsyncSnapshot<List<Meal>> snapshot) {
              switch (snapshot.connectionState) {
                case ConnectionState.none:
                  return Text('none');
                case ConnectionState.waiting:
                  return Center(child: CircularProgressIndicator());
                case ConnectionState.active:
                  return Text('');
                case ConnectionState.done:
                  if (snapshot.hasError) {
                    return Text(
                      '${snapshot.error}',
                      style: TextStyle(color: Colors.red),
                    );
                  } else {
                    print(snapshot.data.runtimeType);
                    return ListView.builder(
                        itemCount: snapshot.data.length,
                        itemBuilder: (context, index) {
                          return Card(
                              elevation: 6.0,
                              child: Padding(
                                padding: const EdgeInsets.only(
                                    top: 6.0,
                                    bottom: 6.0,
                                    left: 8.0,
                                    right: 8.0),
                                child: Row(
                                  crossAxisAlignment: CrossAxisAlignment.start,
                                  children: <Widget>[
                                    Text(
                                        snapshot.data[index].idMeal.toString()),
                                    Spacer(),
                                    Text(
                                      snapshot.data[index].strCategory,
                                    ),
                                  ],
                                ),
                              ));
                        });
                  }
              }
            }));
  }
}