Create infinite scrolling effect using Firebase Realtime Database data in flutter-CodePudding

I am integrating a chat feature in my mobile application, and decided to use Firebase Realtime Database for the backend instad of Firestore as a cost reduction mechanism. I am running into a problem, however. There seems to be very sparse documentation on how to create infinite scrolling using data from Realtime Database instead of Firestore.

Below is the organization of my chat messages. This is the query I want to use:

FirebaseDatabase.instance
            .ref("messages/${widget.placeID}")
            .orderByChild("timeStamp")

And this is the widget I want to return for each result:

MessageWidget(
  message: message.text,
  id: message.uid,
  name: message.name,
  lastSender: message.lastSender,
  date: message.timeStamp,
  profilePicture: message.profilePicture,
);

Here is the database structure

The query works, and I have already programmed the MessageWidget from the JSON response of the query. All I need is for the query to be called whenever it reaches the top of its scroll, and load more MessageWdigets. Also note, this is a chat app where users are scrolling up, to load older messages, to be added above the previous.

Thank you!

EDIT: here is the code I currently have:

Flexible(
  child: StreamBuilder(
    stream: FirebaseDatabase.instance
        .ref("messages/${widget.placeID}")
        .orderByChild("timeStamp")
        .limitToLast(20)
        .onValue,
    builder:
        (context, AsyncSnapshot<DatabaseEvent> snapshot) {
      if (!snapshot.hasData) {
        return const CircularProgressIndicator();
      } else {
        Map<dynamic, dynamic> map =
            snapshot.data!.snapshot.value as dynamic;
        List<dynamic> list = [];
        list.clear();
        list = map.values.toList();
        return Align(
          alignment: Alignment.bottomCenter,
          child: Padding(
            padding: const EdgeInsets.only(bottom: 20),
            child: ListView.builder(
                controller: _scrollController,
                // shrinkWrap: true,
                itemCount: list.length,
                itemBuilder: (context, index) {
                  final json = list[index]
                      as Map<dynamic, dynamic>;
                  final message = Message.fromJson(json);
                  return MessageWidget(
                    message: message.text,
                    id: message.uid,
                    name: message.name,
                    lastSender: message.lastSender,
                    date: message.timeStamp,
                    profilePicture:
                        message.profilePicture,
                  );
                }),
          ),
        );
      }
    },
  ),
),

My initState

void initState() {
   super.initState();

   _scrollController.addListener(() {
     if (_scrollController.position.atEdge) {
       bool isTop = _scrollController.position.pixels == 0;
       if (isTop) {
         //add more messages

       } else {
         print('At the bottom');
       }
     }
   });
 }

CodePudding user response：

Your code already loads all messages.

If you want to load a maximum number of messages, you'll want to put a limit on the number of messages you load. If you want to load only the newest messages, you'd use limitToLast to do so - as the newest messages are last when you order them by their timeStamp value.

So to load for example only the 10 latest messages, you'd use:

FirebaseDatabase.instance
    .ref("messages/${widget.placeID}")
    .orderByChild("timeStamp")
    .limitToLast(10);

This gives you the limited number of messages to initially show in the app.

Now you need to load the 10 previous messages when the scrolling reaches the top of the screen. To do this, you need to know the timeStamp value and the key of the message that is at the top of the screen - so of the oldest message you're showing so far.

With those two values, you can then load the previous 10 with:

FirebaseDatabase.instance
    .ref("messages/${widget.placeID}")
    .orderByChild("timeStamp")
    .endBefore(timeStampValueOfOldestSeenItem, keyOfOldestSeenItem)
    .limitToLast(10);

The database here again orders the nodes on their timeStamp, it then finds the node that is at the top of the screen based on the values you give, and it then returns the 10 nodes right before that.