这个问题似乎有点愚蠢,但是作为一个从来没有使用异步功能的人,这并不是一件容易的事。
我从http请求中获取一些json数据并建立一个列表。例如,假设用户ID和用户名。
[
{"userid":1,"username":"JohnDoe"},
{"userid":2,"username":"SamSmith"}
]
代码:
Future<UsersList> fetchUsers() async {
final response = await http.get(
Uri.encodeFull('https://www.myurl.com/users'),
headers: {'Accept': 'application/json'});
if (response.statusCode == 200) {
return UsersList.fromJson(json.decode(response.body));
} else {
throw Exception('Failed to load users');
}
}
class User {
final String userid;
final String username;
String tag;
User({this.userid, this.username});
factory User.fromJson(Map<String, dynamic> json){
return User(
userid: json['userid'],
username: json['username'],
);
}
}
class UsersList {
final List<User> Users;
UsersList({this.Users});
factory UsersList.fromJson(List<dynamic> parsedJson) {
List<User> Users = new List<User>();
Users = parsedJson.map((i) => User.fromJson(i)).toList();
return new UsersList(Users: Users);
}
}
class UsersTab extends StatefulWidget {
@override
State<StatefulWidget> createState() {
return UsersTabState();
}
}
class UsersTabState extends State<UsersTab> {
Future<UsersList> Users;
@override
void initState() {
super.initState();
Users = fetchUsers();
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text('users'), backgroundColor: Colors.blue),
body: Center(
child: FutureBuilder<usersList>(
future: users,
builder: (context, snapshot) {
if (snapshot.hasData) {
return ListView.builder(
itemCount: snapshot.data.users.length,
itemBuilder: (BuildContext context, int index) {
return Container(
child: Text('User: ' +
snapshot.data.users[index].username +
'\nTag: ' +
snapshot.data.users[index].tag),
);
},
);
} else if (snapshot.hasError) {
return Text('${snapshot.error}');
}
return CircularProgressIndicator();
},
),
),
);
}
}
现在,我还具有来自shared_preferences的本地数据,可以在其中按ID标记用户。所以我有一个功能,如
Future<String> getTag(String id) async {
final prefs = await SharedPreferences.getInstance();
return prefs.getString(id) ?? "none";
}
我的问题是,在哪里可以调用此函数?显然,它必须在FutureBuilder构建列表之前,但必须在http请求完成之后。我有一些想法,例如UsersTabState的initState或User类构造函数,但它总是在将来需要使用String的地方结束。将本地存储的标签放入User类的最佳方法是什么?
因此,我的解决方案是将getTag
方法放入User
类中,并使User.fromJson
和UsersList.fromJson
都变为返回Future<User>
和Future<UsersList>
的静态方法。这样,我们可以将所有等待中的内容放入fetchUsers
中,以便此方法最终返回一个在等待后完成的UsersList对象。
Future<UsersList> fetchUsers() async {
final response = await http.get(
Uri.encodeFull('https://www.myurl.com/users'),
headers: {'Accept': 'application/json'});
if (response.statusCode == 200) {
return await UsersList.fromJson(json.decode(response.body));
} else {
throw Exception('Failed to load users');
}
}
class User {
final String userid;
final String username;
final String tag;
User({this.userid, this.username, this.tag});
static Future<User> fromJson(Map<String, dynamic> json) async {
final userId = json['userid'];
final tag = await _getTag(userId);
return User(
userid: json['userid'],
username: json['username'],
tag: tag
);
}
static Future<String> _getTag(String id) async {
final prefs = await SharedPreferences.getInstance();
return prefs.getString(id) ?? "none";
}
}
class UsersList {
final List<User> Users;
UsersList({this.Users});
static fromJson(List<dynamic> parsedJson) async {
List<User> Users = new List<User>();
Users = await Future.wait(parsedJson.map((i) => User.fromJson(i));
return new UsersList(Users: Users);
}
}
class UsersTab extends StatefulWidget {
@override
State<StatefulWidget> createState() {
return UsersTabState();
}
}
class UsersTabState extends State<UsersTab> {
Future<UsersList> Users;
@override
void initState() {
super.initState();
Users = fetchUsers();
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text('users'), backgroundColor: Colors.blue),
body: Center(
child: FutureBuilder<usersList>(
future: users,
builder: (context, snapshot) {
if (snapshot.hasData) {
return ListView.builder(
itemCount: snapshot.data.users.length,
itemBuilder: (BuildContext context, int index) {
return Container(
child: Text('User: ' +
snapshot.data.users[index].username +
'\nTag: ' +
snapshot.data.users[index].tag),
);
},
);
} else if (snapshot.hasError) {
return Text('${snapshot.error}');
}
return CircularProgressIndicator();
},
),
),
);
}
}
我使用的一个技巧是等待多个Future返回一个列表:
Users = await Future.wait(parsedJson.map((i) => User.fromJson(i));
您可以在这里阅读:https://api.dart.dev/stable/2.7.2/dart-async/Future/wait.html