Flutter API JSON映射报错：List<dynamic>无法转为String求助

问题概述 #

调用API时，单独读取响应中的单个值功能正常，但将JSON映射到MovieLists数据模型时失败，调试控制台报错：type 'List<dynamic>' is not a subtype of type 'String'。

错误日志 #

E/flutter ( 4602): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: type 'List<dynamic>' is not a subtype of type 'String'
E/flutter ( 4602): #0      new MovieLists.fromMapJson (package:movie_app_1/data/models/movies_list_model.dart:29:20)
E/flutter ( 4602): #1      getDada.&lt;anonymous closure&gt; (package:movie_app_1/screens/splashScreen.dart:44:54)
E/flutter ( 4602): #2      MappedListIterable.elementAt (dart:_internal/iterable.dart:415:31)
E/flutter ( 4602): #3      ListIterator.moveNext (dart:_internal/iterable.dart:344:26)
E/flutter ( 4602): #4      new _GrowableList._ofEfficientLengthIterable (dart:core-patch/growable_array.dart:189:2

相关代码 #

API请求代码 #

Future<void> getDada(context) async {
  var response = await Dio().get('*****');

  List<MovieLists> moviesList = response.data['data']
      .map<MovieLists>((jsonMapObject) => MovieLists.fromMapJson(jsonMapObject))
      .toList();
  print('---------------&gt; ${moviesList.length}');
  Navigator.push(
    context,
    MaterialPageRoute(
      builder: (context) => homeScreen(listMovies: moviesList),
    ),
  );
}

MovieLists数据模型 #

class MovieLists {
  int id;
  String title;
  String poster;
  String year;
  String country;
  String imdbRating;
  String genres;
  String images;

  MovieLists(
    this.id,
    this.title,
    this.poster,
    this.year,
    this.country,
    this.imdbRating,
    this.genres,
    this.images,
  );
  factory MovieLists.fromMapJson(Map<String, dynamic> jsonMapObject) {
    return MovieLists(
      jsonMapObject['id'],
      jsonMapObject['title'],
      jsonMapObject['poster'],
      jsonMapObject['year'],
      jsonMapObject['country'],
      jsonMapObject['imdb_rating'],
      jsonMapObject['genres'],
      jsonMapObject['images'],
    );
  }
}

问题原因 #

API返回的JSON中，genres或images字段是数组类型（比如["动作", "喜剧"]），但MovieLists模型中把这两个字段定义成了String类型，导致类型转换失败。

解决方案 #

根据需求选择以下两种修改方式：

方式1：将字段类型改为数组（推荐，保留原始数据结构） #

修改MovieLists类，把genres和images的类型改为List<String>，并在fromMapJson中处理类型转换和空值：

class MovieLists {
  int id;
  String title;
  String poster;
  String year;
  String country;
  String imdbRating;
  List<String> genres;
  List<String> images;

  MovieLists(
    this.id,
    this.title,
    this.poster,
    this.year,
    this.country,
    this.imdbRating,
    this.genres,
    this.images,
  );
  
  factory MovieLists.fromMapJson(Map<String, dynamic> jsonMapObject) {
    return MovieLists(
      jsonMapObject['id'] as int,
      jsonMapObject['title'] as String,
      jsonMapObject['poster'] as String,
      jsonMapObject['year'] as String,
      jsonMapObject['country'] as String,
      jsonMapObject['imdb_rating'] as String,
      // 处理数组转List<String>，空值时返回空列表
      List<String>.from(jsonMapObject['genres'] ?? []),
      List<String>.from(jsonMapObject['images'] ?? []),
    );
  }
}

方式2：将数组转为字符串（适合需要展示为单一字符串的场景） #

如果需要把数组转为逗号分隔的字符串，修改fromMapJson中的对应字段处理逻辑：

class MovieLists {
  int id;
  String title;
  String poster;
  String year;
  String country;
  String imdbRating;
  String genres;
  String images;

  MovieLists(
    this.id,
    this.title,
    this.poster,
    this.year,
    this.country,
    this.imdbRating,
    this.genres,
    this.images,
  );
  
  factory MovieLists.fromMapJson(Map<String, dynamic> jsonMapObject) {
    return MovieLists(
      jsonMapObject['id'] as int,
      jsonMapObject['title'] as String,
      jsonMapObject['poster'] as String,
      jsonMapObject['year'] as String,
      jsonMapObject['country'] as String,
      jsonMapObject['imdb_rating'] as String,
      // 将数组转为逗号分隔的字符串，空值时返回空字符串
      (jsonMapObject['genres'] as List?)?.join(', ') ?? '',
      (jsonMapObject['images'] as List?)?.join(', ') ?? '',
    );
  }
}

额外优化建议 #

• 为所有字段添加显式类型转换（as int/as String），避免隐式类型转换带来的潜在问题。
• 处理API返回的空值情况（比如??运算符），防止空指针异常。

内容的提问来源于stack exchange，提问作者Parsa