尝试对作为列表流快照的列表进行排序时，出现Flutter/Dart "Cannot modify an unmodifiable“(无法修改不可修改的列表)

Question

这是导致问题的Streambuilder的builder方法中的代码：

List<User> users = snapshot.data;
users.sort((user1, user2) => (user1.distanceInKm ?? 1000).compareTo(user2.distanceInKm ?? 1000));

如果我对Streambuilder使用下面的流，上面的排序是有效的：

static Stream<List<User>> getUsersStreamWithDistance(
      {@required User loggedInUser}) {
    try {
      var userSnapshots = _fireStore.collection('users').snapshots().map(
          (snap) => snap.documents
                  .map((doc) => User.fromMap(map: doc.data))
                  .where((user) => user.email != loggedInUser.email)
                  .map((user) {
                user.updateDistanceToOtherUser(otherUser: loggedInUser);
                return user;
              }).toList());
      return userSnapshots;
    } catch (e) {
      print(e);
      return null;
    }
  }

但当我使用下面的流时就不会了，这是我需要的流(ZipStream来自rxdart包)：

static Stream<List<User>> getSpecifiedUsersStreamWithDistance(
      {@required User loggedInUser, @required List<String> uids}) {
    try {
      List<Stream<User>> listOfStreams = [];
      for (var uid in uids) {
        Stream<User> streamToAdd = _fireStore
            .collection('users')
            .where('email', isEqualTo: uid)
            .snapshots()
            .map((snap) => snap.documents
                    .map((doc) => User.fromMap(map: doc.data))
                    .map((user) {
                  user.updateDistanceToOtherUser(otherUser: loggedInUser);
                  return user;
                }).toList()[0]);
        listOfStreams.add(streamToAdd);
      }

      Stream<List<User>> usersStream = ZipStream.list(listOfStreams);

      return usersStream;
    } catch (e) {
      print(e);
      return null;
    }
  }

Answer 1

因为，ZipStream.list()创建了一个新的Stream of List.unmodifiable() list。

List<User> users = List.from(snapshot.data); // to convert it editable list
users.sort((user1, user2) => (user1.distanceInKm ?? 1000).compareTo(user2.distanceInKm ?? 1000));