类型"Future< String>"在flutter中类型转换中不是'String'类型的子类型

问题描述：

我使用以下功能从Web服务获取用户ID

I used following function to get user Id from web service

Future<String> getUserid() async {
  final storage = new FlutterSecureStorage();
  // Read value
  String userID = await storage.read(key: 'userid');
  return userID;
}

使用此功能时发生错误

类型'Future'不是类型转换类型'字符串'的子类型

type 'Future' is not a subtype of type 'String' in type cast

这是我尝试过的

otherFunction() async {

  final String userID = await getUserid();
 return userID;
}

Future<String> getUserid() async {
  final storage = new FlutterSecureStorage();
  // Read value
  String userID = await storage.read(key: 'userid');
  return userID;
}

print(otherFunction());

静止错误消息显示为

I/flutter(18036):未来"的实例

I/flutter (18036): Instance of 'Future'

答

您将需要等待.如果您完全不了解 Dart 中的Future，则应通读

You will need to await your function. If you are totally unaware of Future's in Dart, you should read through this comprehensive article.

在 Flutter 中，现在有两种方法可以处理这种情况.您要么想在其他常规函数中调用函数.在这种情况下，您可以将该功能标记为async或使用getUserid().then((String userID) {...}).如果要使用async，则还需要使用await:

In Flutter, there are now two ways to handle this case. Either you want to call your function in a regular other function. In that case, you would either mark that function as async or use getUserid().then((String userID) {...}). If you were to use async, you would need to use await as well:

otherFunction() async {
  ...
  final String userID = await getUserid();
  ...
}

但是，在Flutter中的中，您很有可能希望在小部件的 build方法中使用您的值.在这种情况下，您应该使用 FutureBuilder :

However, in Flutter it is very likely that you want to use your value in the build method of your widget. In that case, you should use a FutureBuilder:

@override
Widget build(BuildContext context) {
  return FutureBuilder(future: getUserid(),
                       builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
                         if (!snapshot.hasData) return Container(); // still loading
                         // alternatively use snapshot.connectionState != ConnectionState.done
                         final String userID = snapshot.data;
                         ...
                         // return a widget here (you have to return a widget to the builder)
                       });
}

类型"Future< String>"在flutter中类型转换中不是'String'类型的子类型

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