[flutter-chat-demo]3.dart

StreamBuilder(
  stream: Firestore.instance
      .collection('messages')
      .document(groupChatId)
      .collection(groupChatId)
      .orderBy('timestamp', descending: true)
      .limit(_limit)
      .snapshots(),
  builder: (context, snapshot) {
    if (!snapshot.hasData) {
      return Center(
          child: CircularProgressIndicator(valueColor: AlwaysStoppedAnimation<Color>(themeColor)));
    } else {
      listMessage = snapshot.data.documents;
      return ListView.builder(
        padding: EdgeInsets.all(10.0),
        itemBuilder: (context, index) => buildItem(index, snapshot.data.documents[index]),
        itemCount: snapshot.data.documents.length,
        reverse: true,
        controller: listScrollController,
      );
    }
  },
),