颤振未处理异常：类型 'Future<Group>' 不是类型 'Group' 的子类型

Question

在使用Firebase Firestore时，我遇到了以下错误:

E/flutter ( 4477): [ERROR:flutter/shell/common/shell.cc(93)] Dart Unhandled Exception: type 'Future<Group>' is not a subtype of type 'Group', stack trace: #0      new List.from (dart:core-patch/array_patch.dart:41:5)
E/flutter ( 4477): #1      GroupService.getGroups (package:money_manager/core/services/group_service.dart:56:21)
E/flutter ( 4477): <asynchronous suspension>
E/flutter ( 4477): #2      GroupViewModel._getAllGroups (package:money_manager/core/view_models/group_viewmodel.dart:44:26)
E/flutter ( 4477): <asynchronous suspension>
E/flutter ( 4477):

虽然我认为我已经在必要时正确使用async-await，但我无法找出这个错误。相关代码:

ingroup-service.dart:

Future<List<Group>> getGroups() async {
final userData = await getUserData();
List<Group> groups = [];
if (userData['groups'] != null) {
List<String> groupIds = List.from(userData['groups'].map((e) => e['id']));
groups = List.from(groupIds.map((e) async {
return await getGroupFromId(e);
}));
}
return groups;
}
Future<Group> getGroupFromId(String groupId) async {
final groupData = await getGroupData(groupId);
return Group.fromJson(groupData);
}
Future<Map<String, dynamic>> getGroupData(String groupId) async {
DocumentReference groupDoc =
FirebaseFirestore.instance.collection('groups').doc(groupId);
final snapshot = await groupDoc.get();
Map<String, dynamic> groupData = snapshot.data() as Map<String, dynamic>;
return groupData;
}

ingroup_viewmodel.dart:

List<Group> _userGroups = [];
void _getAllGroups() async {
List<Group> groups = await _groupService.getGroups();
_userGroups = groups;
}

Answer 1

问题出在下一行:

groups = List.from(groupIds.map((e) async {
return await getGroupFromId(e);
}));

即使您在map()函数的return之前使用await，它仍然会返回Future。map()是一个同步函数，它不会异步运行内部函数。因此，它将返回一个Iterable<Future<Group>>，在List.from()函数中无法转换为List<Group>。

有一个方便的函数，它接受一个可迭代的期货并等待它们中的每一个，Future.wait()。下面是您的代码使用它的样子:

groups = List.from(await Future.wait(groupIds.map((e) async {
return await getGroupFromId(e);
})));

和更好的撕掉:

groups = List.from(await Future.wait(groupIds.map(getGroupFromId)));