使用TMDB API获取热门电影第三页时出现类型不匹配错误

我使用TMDB接口获取电影数据，前两页数据正常，请求第三页热门电影时抛出如下错误：

Exception occured: type 'Null' is not a subtype of type 'String' stackTrace: #0

返回错误的获取数据函数 #

class DioApiClient implements ApiClient {
  final Dio dio = Dio();
  final Api api = Api();

  @override
  Future<List<MovieModel>> getDataFrom(String endPoint,
      {Map<String, dynamic>? params}) async {
    final String url = "${api.baseURL}/$endPoint";
    log("from api client :$url/$params");

    Map<String, dynamic> queryParameters = {
      'api_key': api.apiKey,
      // 'language': 'en-US',
    };
    if (params != null) {
      queryParameters.addAll(params);
    }

    try {
      final response = await dio.get(url, queryParameters: queryParameters);

      log(" status code : ${response.statusCode}");
      if (response.statusCode == 200) {
        final Map<String, dynamic> data = response.data;
        final List<dynamic> results = data['results'];
        final List<MovieModel> movies =
            results.map((dynamic movie) => MovieModel.fromJson(movie)).toList();

        return movies;
      }

      
    } catch (error, stacktrace) {
      log("Exception occured: $error stackTrace: $stacktrace");
      throw Exception('Failed to load data ');
    }
    throw Exception("data not found");
  }
}

对应的MovieModel代码 #

import '../../domain/entities/movie_entity.dart';

class MovieModel {
  final int id;
  final String title;
  final String overview;
  final String posterPath;
  final num voteAverage;
  final String backdropPath;
  final String releaseDate;

  MovieModel({
    required this.id,
    required this.title,
    required this.overview,
    required this.posterPath,
    required this.voteAverage,
    required this.backdropPath,
    required this.releaseDate,
  });

  factory MovieModel.fromJson(Map<String, dynamic> json) {
    return MovieModel(
      id: json['id'],
      title: json['title'],
      overview: json['overview'],
      posterPath: json['poster_path'],
      voteAverage: json['vote_average'],
      backdropPath: json['backdrop_path'],
      releaseDate: json['release_date'],
    );
  }

  MovieEntity toEntity() {
    return MovieEntity(
      id: id,
      title: title,
      overview: overview,
      posterPath: posterPath,
      voteAverage: voteAverage,
      backdropPath: backdropPath,
      releaseDate: releaseDate,
    );
  }
}

我曾尝试将backdropPath和posterPath设为可空类型，但问题依旧。从API响应中可以看到，部分电影的backdrop_path、poster_path等字段为null值。

问题原因 #

错误本质是TMDB返回的JSON中部分字段为null，但MovieModel里对应字段被声明为非空String类型，导致解析时类型不匹配。仅修改backdropPath和posterPath可能不够，其他字段（如overview、release_date）也可能出现null情况，或者你修改可空类型的方式未覆盖所有场景。

解决方案 #

方案1：将可能为null的字段设为可空，并在解析时兜底默认值

修改MovieModel代码，确保所有可能返回null的字段处理到位：

import '../../domain/entities/movie_entity.dart';

class MovieModel {
  final int id;
  final String? title;
  final String? overview;
  final String? posterPath;
  final num voteAverage;
  final String? backdropPath;
  final String? releaseDate;

  MovieModel({
    required this.id,
    this.title,
    this.overview,
    this.posterPath,
    required this.voteAverage,
    this.backdropPath,
    this.releaseDate,
  });

  factory MovieModel.fromJson(Map<String, dynamic> json) {
    return MovieModel(
      id: json['id'] ?? 0, // 兜底默认ID
      title: json['title'] ?? '',
      overview: json['overview'] ?? '',
      posterPath: json['poster_path'] ?? '',
      voteAverage: json['vote_average'] ?? 0.0,
      backdropPath: json['backdrop_path'] ?? '',
      releaseDate: json['release_date'] ?? '',
    );
  }

  MovieEntity toEntity() {
    return MovieEntity(
      id: id,
      title: title ?? '',
      overview: overview ?? '',
      posterPath: posterPath ?? '',
      voteAverage: voteAverage,
      backdropPath: backdropPath ?? '',
      releaseDate: releaseDate ?? '',
    );
  }
}

方案2：保持字段非空，强制给null字段赋默认值

如果不想修改字段可空性，可在fromJson中直接为null字段赋值默认空字符串或默认值：

factory MovieModel.fromJson(Map<String, dynamic> json) {
  return MovieModel(
    id: json['id'] ?? 0,
    title: json['title'] ?? '',
    overview: json['overview'] ?? '',
    posterPath: json['poster_path'] ?? '',
    voteAverage: json['vote_average'] ?? 0.0,
    backdropPath: json['backdrop_path'] ?? '',
    releaseDate: json['release_date'] ?? '',
  );
}

额外检查 #

确认请求第三页的参数是否正确（如page=3是否正确传递），但从错误类型判断，核心问题还是JSON解析时的null值处理。

内容的提问来源于stack exchange，提问作者Ydev