你能在Flutter中多次调用FireStore.Instance吗？

Question

final FirebaseApp app = await FirebaseApp.configure(
    name: 'test',
    options: const FirebaseOptions(
      googleAppID: '1:79601577497:ios:5f2bcc6ba8cecddd',
      gcmSenderID: '79601577497',
      apiKey: 'AIzaSyArgmRGfB5kiQT6CunAOmKRVKEsxKmy6YI-G72PVU',
      projectID: 'flutter-firestore',
    ),
  );
  final Firestore firestore = Firestore(app: app);

  runApp(MaterialApp(
      title: 'Firestore Example', home: MyHomePage(firestore: firestore)));
}

@override
  Widget build(BuildContext context) {
    DMProfile dmProfile = Provider.of<DMProfile>(context);
    PlayerProfile playerProfile = Provider.of<PlayerProfile>(context);
    return StreamBuilder<List<Message>>(
      stream: Firestore.instance
          .collection('chats')
          .document(widget.chatId)
          .collection('messages')
          .orderBy('timestamp', descending: true)
          .limit(20)
          .snapshots()
          .map((list) =>
              list.documents.map((doc) => Message.fromFirestore(doc)).toList()),
      builder: (context, snapshot) {
        if (!snapshot.hasData) {
          return LoadingPage();
        }
        List<Message> messageStream = snapshot.data;
        if (messages.length == 0) {
          messages = messageStream;
          hasMore = !(messageStream.length < 20);
        } else if (messages.first.timestamp != null &&
            messageStream.first.timestamp != null) {
          if (messageStream.first.timestamp
                  .compareTo(messages.first.timestamp) !=
              0) {
            print('Not equal');
            messages.insert(0, messageStream.first);
          }
        }
        return Expanded(
          child: ListView.builder(
            reverse: true,
            padding: EdgeInsets.symmetric(horizontal: 10.0, vertical: 20.0),
            controller: listScrollController,
            itemCount: messages.length,
            itemBuilder: (context, index) {
              final messageText = messages[index].message;
              final messageSender = messages[index].senderId;

              final currentUser =
                  widget.isPlayer ? playerProfile.userId : dmProfile.userId;

              return MessageBubble(
                imageUrl: messageSender == playerProfile.userId
                    ? playerProfile.imageUrl
                    : dmProfile.imageUrl,
                text: messageText,
                isMe: currentUser == messageSender,
              );
            },
          ),
        );
      },
    );
  }

sendMessage({
    String sender,
    String receiver,
    String message,
    bool isPlayer,
    String dmName,
    String playerName,
    String dmImage,
    String playerImage,
  }) async {
    String sender1 = sender.compareTo(receiver) < 0 ? sender : receiver;
    String sender2 = sender.compareTo(receiver) > 0 ? sender : receiver;
    String chatName = '${sender1}_$sender2';
    await Firestore.instance
        .collection(CHAT_COLLECTION)
        .document(chatName)
        .setData({
      'dm': isPlayer ? receiver : sender,
      'dm_image': dmImage,
      'dm_name': dmName,
      'player': isPlayer ? sender : receiver,
      'player_image': playerImage,
      'player_name': playerName,
      'dm_hidden': false,
      'player_hidden': false,
      'timestamp': FieldValue.serverTimestamp(),
      'dm_seen': isPlayer ? false : true,
      'player_seen': isPlayer ? true : false,
      'last_message': {
        'message': message,
        'sender_name': isPlayer ? playerName : dmName,
        'sender_id': sender,
        'receiver_id': receiver,
      },
    }, merge: true);
    await Firestore.instance
        .collection(CHAT_COLLECTION)
        .document(chatName)
        .collection(MESSAGES_COLLECTION)
        .add({
      'sender_id': sender,
      'receiver_id': receiver,
      'sender_name': isPlayer ? playerName : dmName,
      'message': message,
      'timestamp': FieldValue.serverTimestamp(),
    });
  }

Ive尝试了所有的方法，尽可能多地查看文档，但似乎找不到解决方案。