How to autoscroll text in flutter?

Question

What I am trying to achieve is a multiline text that will autoscroll at a fixed speed until the end of the list.

This is a different application that does similar thing. Is there anyway to do this in flutter?

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  ScrollController _scrollController = ScrollController();
  _scrollToBottom() {
    _scrollController.animateTo(_scrollController.position.maxScrollExtent,
        duration: Duration(seconds: 30), curve: Curves.linear);
  }

  @override
  Widget build(BuildContext context) {
    WidgetsBinding.instance.addPostFrameCallback((_) => _scrollToBottom());
    String val = '''
BUNCH OF TEXT HERE.
''';
    return Scaffold(
      appBar: AppBar(
        title: Text('Lyrics'),
      ),
      body: SingleChildScrollView(
        controller: _scrollController,
        child: Text(
          val,
          maxLines: 1000,
        ),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

I added bunch of lorem ipsum text to autoscroll. to the bottom at a certain speed. In the current setup the text auto scrolls to the bottom in 30 seconds.

But what I would like to have is a button which I can press to start this scroll. And instead of time if possible use a speed.

Edit: With the answer below the text does scroll. But the issue I am having is the scrolling stops if I do any manual intervention. Is there any way to resume scrolling without repressing the button again?

Thanks in advance.

Answer 1

Added a variable called speedFactor. Increasing it will increase the scroll speed and vice versa.

Having a constant duration would decrease the speed as we scroll towards the bottom. This is because we are trying to cover different distances in the same duration which would mean your speed has to vary.

So to handle that I am adjusting the duration to maintain a constant speed factor.

double maxExtent = _scrollController.position.maxScrollExtent;
double distanceDifference = maxExtent - _scrollController.offset;
double durationDouble = distanceDifference / speedFactor;

To stop the animation I just animateTo the current offset which would just stop the animation as we are already at the offset and there is nothing more to animate to.

Here is a working example

import 'package:flutter/material.dart';
import 'dart:async';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

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

class _MyHomePageState extends State<MyHomePage> with TickerProviderStateMixin {
  ScrollController _scrollController = ScrollController();
  bool scroll = false;
  int speedFactor = 20;

  _scroll() {
    double maxExtent = _scrollController.position.maxScrollExtent;
    double distanceDifference = maxExtent - _scrollController.offset;
    double durationDouble = distanceDifference / speedFactor;

    _scrollController.animateTo(_scrollController.position.maxScrollExtent,
        duration: Duration(seconds: durationDouble.toInt()),
        curve: Curves.linear);
  }

  _toggleScrolling() {
    setState(() {
      scroll = !scroll;
    });

    if (scroll) {
      _scroll();
    } else {
      _scrollController.animateTo(_scrollController.offset,
          duration: Duration(seconds: 1), curve: Curves.linear);
    }
  }

  @override
  Widget build(BuildContext context) {
    String val = '''
BUNCH OF TEXT HERE. BUNCH OF TEXT HERE. BUNCH OF TEXT HERE. BUNCH OF TEXT HERE. BUNCH OF TEXT HERE. BUNCH OF TEXT HERE. BUNCH OF TEXT HERE. BUNCH OF TEXT HERE. BUNCH OF TEXT HERE.BUNCH OF TEXT HERE.BUNCH OF TEXT HERE.BUNCH OF TEXT HERE.BUNCH OF TEXT HERE.BUNCH OF TEXT HERE.BUNCH OF TEXT HERE. BUNCH OF TEXT HERE.v 
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''';
    return Scaffold(
      appBar: AppBar(
        title: Text('Lyrics'),
      ),
      body: NotificationListener(
        onNotification: (notif) {
          if (notif is ScrollEndNotification && scroll) {
            Timer(Duration(seconds: 1), () {
              _scroll();
            });
          }

          return true;
        },
        child: SingleChildScrollView(
          controller: _scrollController,
          child: Text(
            val,
            maxLines: 1000,
          ),
        ),
      ),
      floatingActionButton: FloatingActionButton(onPressed: () {
        _toggleScrolling();
      }), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

UPDATE: Added code which makes the scrollview continue scrolling after user interruption.