Programmatically scrolling to the end of a ListView

Question

I have a scrollable ListView where the number of items can change dynamically. Whenever a new item is added to the end of the list, I would like to programmatically scroll the ListView to the end. (e.g., something like a chat message list where new messages can be added at the end)

My guess is that I would need to create a ScrollController in my State object and pass it manually to the ListView constructor, so I can later call animateTo() / jumpTo() method on the controller. However, since I cannot easily determine the maximum scroll offset, it seems impossible to simply perform a scrollToEnd() type of operation (whereas I can easily pass 0.0 to make it scroll to the initial position).

Is there an easy way to achieve this?

Using reverse: true is not a perfect solution for me, because I would like the items to be aligned at the top when there are only a small number of items that fit within the ListView viewport.

Answer 1

If you use a shrink-wrapped ListView with reverse: true, scrolling it to 0.0 will do what you want.

import 'dart:collection';

import 'package:flutter/material.dart';

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Example',
      home: new MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  List<Widget> _messages = <Widget>[new Text('hello'), new Text('world')];
  ScrollController _scrollController = new ScrollController();

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new Center(
        child: new Container(
          decoration: new BoxDecoration(backgroundColor: Colors.blueGrey.shade100),
          width: 100.0,
          height: 100.0,
          child: new Column(
            children: [
              new Flexible(
                child: new ListView(
                  controller: _scrollController,
                  reverse: true,
                  shrinkWrap: true,
                  children: new UnmodifiableListView(_messages),
                ),
              ),
            ],
          ),
        ),
      ),
      floatingActionButton: new FloatingActionButton(
        child: new Icon(Icons.add),
        onPressed: () {
          setState(() {
            _messages.insert(0, new Text("message ${_messages.length}"));
          });
          _scrollController.animateTo(
            0.0,
            curve: Curves.easeOut,
            duration: const Duration(milliseconds: 300),
          );
        }
      ),
    );
  }
}