Flutter Freezed包fromJson生成对象的runtimeType为何不符合预期？

我在研究Flutter的freezed包，使用如下模型类：

import 'package:freezed_annotation/freezed_annotation.dart';
part 'ledger_summary_model.freezed.dart';
part 'ledger_summary_model.g.dart';

@freezed
class LedgerSummary with _$LedgerSummary {
  const factory LedgerSummary({
    required int ledgerId,
  }) = _LedgerSummary;
  factory LedgerSummary.fromJson(Map<String, dynamic> json) =>
      _$LedgerSummaryFromJson(json);
}

测试代码：

LedgerSummary ledgerSummary = const LedgerSummary(ledgerId: 1);
LedgerSummary ledgerSummaryAgain =
    LedgerSummary.fromJson(jsonDecode(jsonEncode(ledgerSummary)));
print(ledgerSummaryAgain.runtimeType.toString());

打印结果是_$_LedgerSummary，但我预期是LedgerSummary。

我的pubspec.yaml配置：

name: test_freeze
description: A new Flutter project.
publish_to: 'none' 
version: 1.0.0+1
environment:
  sdk: '>=3.1.0 <4.0.0'
dependencies:
  flutter:
    sdk: flutter
  freezed: ^2.4.0
  freezed_annotation: ^2.1.0
  flutter_localizations:
    sdk: flutter
  dart_mappable:
  dart_mappable_builder:
dev_dependencies:
  flutter_test:
    sdk: flutter
  build_runner: ^2.1.0
  flutter_launcher_icons: ^0.13.1
  flutter_lints: ^2.0.1
flutter:
  uses-material-design: true

flutter doctor -v输出：

[√] Flutter (Channel stable, 3.13.1, on Microsoft Windows [Version 10.0.19044.3086], locale et-EE)
    • Flutter version 3.13.1 on channel stable at c:\flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision e1e47221e8 (6 weeks ago), 2023-08-22 21:43:18 -0700
    • Engine revision b20183e040
    • Dart version 3.1.0
    • DevTools version 2.25.0

[√] Windows Version (Installed version of Windows is version 10 or higher)

[√] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
    • Android SDK at C:\Users\pavel\AppData\Local\Android\Sdk
    • Platform android-34, build-tools 34.0.0
    • ANDROID_HOME = C:\Users\pavel\AppData\Local\Android\Sdk
    • ANDROID_SDK_ROOT = C:\Users\pavel\AppData\Local\Android\Sdk
    • Java binary at: c:\Program Files\Android\Android Studio\jbr\bin\java
    • Java version OpenJDK Runtime Environment (build 17.0.6+0-b2043.56-10027231)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe

[√] Visual Studio - develop Windows apps (Visual Studio Community 2022 17.1.4)
    • Visual Studio at C:\Program Files\Microsoft Visual Studio\2022\Community
    • Visual Studio Community 2022 version 17.1.32407.343
    • Windows 10 SDK version 10.0.19041.0

[√] Android Studio (version 2022.3)
    • Android Studio at c:\Program Files\Android\Android Studio
    • Flutter plugin can be installed from:
       https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
       https://plugins.jetbrains.com/plugin/6351-dart
    • android-studio-dir = c:\Program Files\Android\Android Studio
    • Java version OpenJDK Runtime Environment (build 17.0.6+0-b2043.56-10027231)

[√] VS Code (version 1.82.2)
    • VS Code at C:\Users\pavel\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.72.0

[√] Connected device (4 available)
    • sdk gphone64 x86 64 (mobile) • emulator-5554 • android-x64    • Android 14 (API 34) (emulator)
    • Windows (desktop)            • windows       • windows-x64    • Microsoft Windows [Version 10.0.19044.3086]
    • Chrome (web)                 • chrome        • web-javascript • Google Chrome 116.0.5845.141
    • Edge (web)                   • edge          • web-javascript • Microsoft Edge 117.0.2045.43

[√] Network resources
    • All expected network resources are available.

• No issues found!

我想实现一个包含多个freezed类的仓库，把这些类作为泛型对象交给单个滚动视图控制器。

问题原因与解决方案 #

这是Freezed的正常行为：LedgerSummary是抽象类，_$_LedgerSummary是Freezed自动生成的具体实现类。不管是通过构造函数还是fromJson创建实例，实际运行时类型都是生成的具体类，但实例在类型检查时会被视为LedgerSummary类型。

泛型场景处理方案 #

如果你的滚动视图控制器需要基于泛型区分不同Freezed类，可以通过以下方式处理：

1. 统一基类标识
   定义一个抽象基类，让所有Freezed类实现它：

   abstract class BaseModel {}
   
   @freezed
   class LedgerSummary with _$LedgerSummary implements BaseModel {
     const factory LedgerSummary({required int ledgerId}) = _LedgerSummary;
     factory LedgerSummary.fromJson(Map<String, dynamic> json) => _$LedgerSummaryFromJson(json);
   }
   
   // 其他Freezed类同样实现BaseModel
   

   滚动视图控制器可以接受List<BaseModel>作为数据源，通过is关键字判断具体类型：

   if (item is LedgerSummary) {
     // 渲染LedgerSummary对应的UI组件
   } else if (item is OtherModel) {
     // 渲染OtherModel对应的UI组件
   }
   

2. 密封类+when方法匹配
   如果所有模型属于同一业务体系，可以用Freezed的密封类特性，通过when方法做类型匹配：

   @freezed
   sealed class AppModel with _$AppModel {
     const factory AppModel.ledgerSummary({required int ledgerId}) = LedgerSummary;
     const factory AppModel.userInfo({required String name}) = UserInfo;
   
     factory AppModel.fromJson(Map<String, dynamic> json) => _$AppModelFromJson(json);
   }
   
   // 使用时直接通过when分发UI
   item.when(
     ledgerSummary: (ledgerId) => LedgerSummaryWidget(ledgerId: ledgerId),
     userInfo: (name) => UserInfoWidget(name: name),
   );
   

3. 依赖类型检查而非runtimeType
   无需纠结runtimeType的字符串值，代码中用is关键字做类型判断足够可靠：

   print(ledgerSummaryAgain is LedgerSummary); // 输出true
   

内容的提问来源于stack exchange，提问作者Pavel Murnikov