[英]NoSuchMethodError: The method 'get' was called on null in Flutter using Dart with Firestore
[英]I'm getting a "1 positional argument(s) expected, but 0 found." error in Flutter using Dart with Cloud Firestore?
return ListView.builder(
itemCount: users.length,
itemBuilder: (context, index){
return UserTile(sport: users[index]);
},
);
我收到“預期有 1 個位置參數,但找到 0 個”。 錯誤,我不知道如何修復它。 這是我的 UserTile 文件
class UserTile extends StatelessWidget {
final Sport sport;
UserTile(this.sport, {users});
@override
Widget build(BuildContext context) {
return Padding(
padding: const EdgeInsets.only(top: 8.0),
child: Card(
margin: EdgeInsets.fromLTRB(20.0, 6.0, 20.0, 0.0),
child: ListTile(
leading: CircleAvatar(
radius: 25.0,
backgroundColor: Colors.brown[100],
),
title: Text(sport.name),
subtitle: Text('User plays ${sport.sport}'),
),
),
);
}
}
final Sport sport;
final Users users;
UserTile(this.sport, {users});
意味着你可以這樣做:
Sport sport = Sport();
UserTile(sport);
UserTile(sport,users:someUsers);
{}
或[]
,表示它是必需的,與其他語言相同。
class UserTile{
final Sport sport;
UserTile(this.sport);
}
UserTile userTile = UserTile(sport);
{}
參數,意味着它需要被命名。
class UserTile{
final Sport? sport; //? means can be now.
final Sport requiredSport;// or required:
UserTile({this.sport,required this.requiredSport});
}
UserTile userTile = UserTile(sport:sport,requiredsport:sport);
[]
參數,意味着它是可選的。
class UserTile{
final Sport? sport; //? means can be now.
final Sport requiredSport;// or required:
UserTile(requiredSport,[this.sport]);
}
UserTile userTile = UserTile(requiredSport,sport:sport);
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