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android - Flutter - json_serializable fromJson：在 null 上调用了方法“[]”

问题描述

在我的应用程序中，我将数据导出到 JSON。但是当我尝试导入 JSON 文件时，它会引发如下所示的错误：

E/flutter: [ERROR:flutter/shell/common/shell.cc(184)] Dart 错误：未处理的异常：NoSuchMethodError：方法 '[]' 在 null 上被调用。接收者：null 尝试调用：#0 Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5) #1 _$RecipesModelFromJson (file:///home/stube/IdeaProjects/Flutter_Recipe/lib/JSON/recipes .g.dart:11:11) #2 new RecipesModel.fromJson (package:Time2Eat/JSON/recipes.dart:20:63) #3 _$RecipesModelFromJson (file:///home/stube/IdeaProjects/Flutter_Recipe/lib /JSON/recipes.g.dart:32:22) #4 new RecipesModel.fromJson (package:Time2Eat/JSON/recipes.dart:20:63) #5 RecipebookState.createRecipeJson (package:Time2Eat/recipe/recipebook.dart: 111:32) #6 食谱书状态。getPath (package:Time2Eat/recipe/recipebook.dart:123:9) #7 RecipebookState.build。(package:Time2Eat/recipe/recipebook.dart:185:19) #8 AnimatedChild._performAction (package:flutter_speed_dial/src/animated_child.dart:60:24) #9 _InkResponseState._handleTap (package:flutter/src/material/ink_well .dart:507:14) #10 _InkResponseState.build。(package:flutter/src/material/ink_well.dart:562:30) #11 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:102:24) #12 TapGestureRecognizer._checkUp (package:flutter/src /gestures/tap.dart:242:9) #13 TapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:175:7) #14 PrimaryPointerGestureRecognizer.handleEvent (package:

recipes.g.dart，由 json_serializable 生成：

// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************

RecipesModel _$RecipesModelFromJson(Map<String, dynamic> json) {
  return RecipesModel(
      json['name'] as String,
      json['image'] as String,
      json['description'] as String,
      json['favorite'] as int,
      json['timestamp'] as String,
      json['preperation'] as String,
      json['creation'] as String,
      json['resting'] as String,
      json['people'] as String,
      json['backgroundColor'] as String,
      (json['zutaten'] as List)
          ?.map((e) => e == null
              ? null
              : ZutatenModel.fromJson(e as Map<String, dynamic>))
          ?.toList(),
      (json['zubereitung'] as List)
          ?.map((e) => e == null
              ? null
              : ZubereitungModel.fromJson(e as Map<String, dynamic>))
          ?.toList())
    ..recipeModel =
        RecipesModel.fromJson(json['recipeModel'] as Map<String, dynamic>);
}

Map<String, dynamic> _$RecipesModelToJson(RecipesModel instance) =>
    <String, dynamic>{
      'name': instance.name,
      'image': instance.image,
      'description': instance.description,
      'favorite': instance.favorite,
      'timestamp': instance.timestamp,
      'preperation': instance.preperation,
      'creation': instance.creation,
      'resting': instance.resting,
      'people': instance.people,
      'backgroundColor': instance.backgroundColor,
      'zutaten': instance.zutaten,
      'zubereitung': instance.zubereitung,
      'recipeModel': instance.recipeModel
    };

ZutatenModel _$ZutatenModelFromJson(Map<String, dynamic> json) {
  return ZutatenModel(
      json['zutat'] as String,
      json['number'] as String,
      json['measure'] as String
  );
}

Map<String, dynamic> _$ZutatenModelToJson(ZutatenModel instance) =>
    <String, dynamic>{
      'zutat': instance.zutat,
      'number': instance.number,
      'measure': instance.measure
    };

ZubereitungModel _$ZubereitungModelFromJson(Map<String, dynamic> json) {
  return ZubereitungModel(json['number'] as String, json['steps'] as String);
}

Map<String, dynamic> _$ZubereitungModelToJson(ZubereitungModel instance) =>
    <String, dynamic>{'number': instance.number, 'steps': instance.steps};

我在函数内部调用RecipesModel.fromJson() 方法，它应该能够将这些数据保存在数据库中。该函数如下所示：

createJson() async{
      FlutterDocumentPickerParams params = FlutterDocumentPickerParams(
          allowedFileExtensions: ['json'],
          invalidFileNameSymbols: ['/']
      );
      final path = await FlutterDocumentPicker.openDocument(params: params);
      File file = new File(path);
      createRecipeJson(file);
      Map<String,dynamic> jSON = json.decode(path.readAsStringSync());
      var model = RecipesModel.fromJson(jSON);
      showBottomSnack("Name: ${model.name}", ToastGravity.BOTTOM);
    }

我希望有人可以帮助我并解决我的问题。那将是真棒。

标签： androidiosjsondartflutter

解决方案

检查@JsonSerializable(nullable: false)您的班级成员（从 ZutatenModel 和 RecipeModel 开始）。

如果您希望它们null有时会出现，请不要nullable: false在注释它们的模型类时进行标记。

如果是这种情况，请将其更改为@JsonSerializable(nullable: true). 顺便说一句，它默认设置为 true。

此异常通常发生在提供了某些类属性null但(nullable: false)向 JsonSerializer 建议这些属性不会为空时。

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