参数类型“X”不能分配给参数类型“X”dartargument_type_not_assignable

Question

由于用户出错，我无法运行我的系统。并且在 ListTile Text(plant.name) 中也是错误的。参数类型“字符串？”不能分配给参数类型“字符串”。有人可以帮我吗？

The argument type 'List<Plants>?' can't be assigned to the parameter type 'List<Plants>'.dartargument_type_not_assignable List<Plants>? users

  body: FutureBuilder<List<Plants>>(
      future: PlantsApi.getPlantsLocally(context),
      builder:(context, snapshot) {
        final users = snapshot.data;
        switch (snapshot.connectionState) {
          case ConnectionState.waiting:
            return Center(child: CircularProgressIndicator());
          default:
          if (snapshot.hasError) {
            return Center(child: Text('Some error'),);
          }
          else {
            return buildPlants(users);
          }
        }
      }, 


Widget buildPlants(List<Plants> plants) => 
ListView.builder(
  
  itemCount: plants.length,
  itemBuilder: (context, index) {
    final plant = plants[index];
    return 
    
    ListTile (
      title: Text(plant.name),
    );
  }
  );

我的 fetch api

class PlantsApi {
  static Future<List<Plants>> getPlantsLocally(BuildContext context) async {
    final assetBundle = DefaultAssetBundle.of(context);
    final data = await assetBundle.loadString('assets/plants.json');
    final body = json.decode(data);

    return body.map<Plants>((e) => Plants.fromJson(e)).toList();
  }
}

这是使用数据类的json示例

 [
        {
            "id": 1,
            "name": "ALOCASIA-ELEPHANT EARS",
            "image": "assets/images/ALOCASIA.jpeg",
            "descript": "Alocasia plant (Alocasia mortfontanensis) is a hybrid species between Alocasia longiloba and Alocasia sanderiana. The Alocasia is known for its large leaves and wide variety of cultivars within the species. Alocasia plant is native to tropical Asia and Australia.",
            "charac": [{
              "planttype": "Herb",
              "lifespan": "Perennial",
              "bloomtime": "Spring, summer",
              "plantheight": "1-2 feet",
              "spread": "7 feet",
              "leafColor": "PurpleGreenGreySilver"
            }
        ],
            "scienclass": [
                {
                "genus": "Alocasia - Elephant's-ears, Taro, Kris plant",
                "family": " Araceae - Arum, Aroids ",
                "order": "Alismatales - Water plantains and allies",
                "classes": "Liliopsida - Monocotyledons, Monocots ",
                "phylum":"Tracheophyta - Vascular plants, Seed plants, Ferns, Tracheophytes"
            }
        ],
            "pestdesease": " Stem rot, crown rot, root rot, leaf spot, mealy bugs, aphids",
            "requirements":[{
                "difficultyrating": "Alocasia plant is super easy to take care of, with resistance to almost all pests and diseases. It is a perfect option for gardeners with brown thumbs.",
                "sunlight": "Full shade to partial sun",
                "hardenesszone": " 9-11 ",
                "soil": "Loose, fertile and well-drained humus soil"
            }
        ],
            "careguide":[
                {
                "water": "Moisture-loving, keep the soil moist but do not let water accumulate.",
                "fertilizaton": "Fertilization once in spring. ", 
                "pruning": "Fertilization once in spring. ", 
                "plantingtime": "Spring, summer, autumn ",
                "propagation": "Division ",
                "pottingsuggestion": " Needs excellent drainage in pots."
            }
            ],
            "toxictohuman": "Is the alocasia plant poisonous for humans? The sap of alocasia plant is toxic to humans topically and when ingested. When the leaves are chewed or swallowed, symptoms may include swelling, stinging, and irritation of the mouth and gastrointestinal tract. In rare cases, it can cause swelling of the upper airway and difficulty breathing. Contact with the sap can also lead to skin irritation where the contact occurred. Poisoning is most likely to occur from accidental ingestion of the leaves or rough handling of the plant, particularly by children. Alocasia plant is often encountered as an ornamental plant in gardens or as a houseplant."
        },
        {
            "id": 2,
            "name": "Sample Sample",
            "image": "assets/images/ALOCASIA.jpeg",
            "descript": "Alocasia plant (Alocasia mortfontanensis) is a hybrid species between Alocasia longiloba and Alocasia sanderiana. The Alocasia is known for its large leaves and wide variety of cultivars within the species. Alocasia plant is native to tropical Asia and Australia.",
            "charac": [{
              "planttype": "Herb",
              "lifespan": "Perennial",
              "bloomtime": "Spring, summer",
              "plantheight": "1-2 feet",
              "spread": "7 feet",
              "leafColor": "PurpleGreenGreySilver"
            }
            ],
            "scienclass": [
                {
                "genus": "Alocasia - Elephant's-ears, Taro, Kris plant",
                "family": " Araceae - Arum, Aroids ",
                "order": "Alismatales - Water plantains and allies",
                "classes": "Liliopsida - Monocotyledons, Monocots ",
                "phylum":"Tracheophyta - Vascular plants, Seed plants, Ferns, Tracheophytes"
            }
            ],
            "pestdesease": " Stem rot, crown rot, root rot, leaf spot, mealy bugs, aphids",
            "requirements":[{
                "difficultyrating": "Alocasia plant is super easy to take care of, with resistance to almost all pests and diseases. It is a perfect option for gardeners with brown thumbs.",
                "sunlight": "Full shade to partial sun",
                "hardenesszone": " 9-11 ",
                "soil": "Loose, fertile and well-drained humus soil"
            }
            ],
            "careguide":[
                {
                "water": "Moisture-loving, keep the soil moist but do not let water accumulate.",
                "fertilizaton": "Fertilization once in spring. ", 
                "pruning": "Fertilization once in spring. ", 
                "plantingtime": "Spring, summer, autumn ",
                "propagation": "Division ",
                "pottingsuggestion": " Needs excellent drainage in pots."
            }
            ],
            "toxictohuman": "Is the alocasia plant poisonous for humans? The sap of alocasia plant is toxic to humans topically and when ingested. When the leaves are chewed or swallowed, symptoms may include swelling, stinging, and irritation of the mouth and gastrointestinal tract. In rare cases, it can cause swelling of the upper airway and difficulty breathing. Contact with the sap can also lead to skin irritation where the contact occurred. Poisoning is most likely to occur from accidental ingestion of the leaves or rough handling of the plant, particularly by children. Alocasia plant is often encountered as an ornamental plant in gardens or as a houseplant."
        }
    ]
  

我使用 json 的数据类将其转换为 dart

Answer 1

Future builder 将始终具有可为空的数据类型，即 snapshot.data 的类型为 [your_type]? 因此，代码必须编写如下：

FutureBuilder<List<Plants>>(
      future: PlantsApi.getPlantsLocally(context),
      builder:(context, snapshot) {
        switch (snapshot.connectionState) {
          case ConnectionState.waiting:
            return Center(child: CircularProgressIndicator());
          default:
            if (snapshot.hasData) {
              return buildPlants(snapshot.data!)
            } else {
              return Center(child: Text('Some error'),);
            }
           
        }
      }

关于Text(plant.name) 的问题，这是因为Text 小部件需要一个不可为空的字符串，但您的Plant 类的name 是一个可以为空的字符串。

因此，要解决此问题，您可以给它一个默认值（如果它为 null，即 plant.name ?? 'default'），或者在您的类中将类型更改为不可为空。