Always Visible Scrollbar for Flutter - 4th March 2019

always_scrollbar.dart

import 'package:flutter/gestures.dart' show DragStartBehavior;
import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';

void main() {
  runApp(
    MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primaryColor: Colors.indigo,
        accentColor: Colors.pinkAccent,
      ),
      home: ExampleScreen(),
    ),
  );
}

class ExampleScreen extends StatefulWidget {
  @override
  _ExampleScreenState createState() => _ExampleScreenState();
}

class _ExampleScreenState extends State<ExampleScreen> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('SingleChildScrollView With Scrollbar'),
      ),
      body: SingleChildScrollViewWithScrollbar(
        scrollbarColor: Theme.of(context).accentColor.withOpacity(0.75),
        scrollbarThickness: 8.0,
        child: Container(
          height: 1500,
          child: Placeholder(),
        ),
      ),
    );
  }
}

class SingleChildScrollViewWithScrollbar extends StatefulWidget {
  const SingleChildScrollViewWithScrollbar({
    Key key,
    this.scrollDirection = Axis.vertical,
    this.reverse = false,
    this.padding,
    this.primary,
    this.physics,
    this.controller,
    this.child,
    this.dragStartBehavior = DragStartBehavior.down,
    this.scrollbarColor,
    this.scrollbarThickness = 6.0,
  }) : super(key: key);

  final Axis scrollDirection;
  final bool reverse;
  final EdgeInsets padding;
  final bool primary;
  final ScrollPhysics physics;
  final ScrollController controller;
  final Widget child;
  final DragStartBehavior dragStartBehavior;
  final Color scrollbarColor;
  final double scrollbarThickness;

  @override
  _SingleChildScrollViewWithScrollbarState createState() => _SingleChildScrollViewWithScrollbarState();
}

class _SingleChildScrollViewWithScrollbarState extends State<SingleChildScrollViewWithScrollbar> {
  AlwaysVisibleScrollbarPainter _scrollbarPainter;

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    rebuildPainter();
  }

  @override
  void didUpdateWidget(SingleChildScrollViewWithScrollbar oldWidget) {
    super.didUpdateWidget(oldWidget);
    rebuildPainter();
  }

  void rebuildPainter() {
    final theme = Theme.of(context);
    _scrollbarPainter = AlwaysVisibleScrollbarPainter(
      color: widget.scrollbarColor ?? theme.highlightColor.withOpacity(1.0),
      textDirection: Directionality.of(context),
      thickness: widget.scrollbarThickness,
    );
  }

  @override
  void dispose() {
    _scrollbarPainter?.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return RepaintBoundary(
      child: CustomPaint(
        foregroundPainter: _scrollbarPainter,
        child: RepaintBoundary(
          child: SingleChildScrollView(
            scrollDirection: widget.scrollDirection,
            reverse: widget.reverse,
            padding: widget.padding,
            primary: widget.primary,
            physics: widget.physics,
            controller: widget.controller,
            dragStartBehavior: widget.dragStartBehavior,
            child: Builder(
              builder: (BuildContext context) {
                _scrollbarPainter.scrollable = Scrollable.of(context);
                return widget.child;
              },
            ),
          ),
        ),
      ),
    );
  }
}

class AlwaysVisibleScrollbarPainter extends ScrollbarPainter {
  AlwaysVisibleScrollbarPainter({
    @required Color color,
    @required TextDirection textDirection,
    @required double thickness,
  }) : super(
          color: color,
          textDirection: textDirection,
          thickness: thickness,
          fadeoutOpacityAnimation: const AlwaysStoppedAnimation(1.0),
        );

  ScrollableState _scrollable;

  ScrollableState get scrollable => _scrollable;

  set scrollable(ScrollableState value) {
    _scrollable?.position?.removeListener(_onScrollChanged);
    _scrollable = value;
    _scrollable?.position?.addListener(_onScrollChanged);
    _onScrollChanged();
  }

  void _onScrollChanged() {
    update(_scrollable.position, _scrollable.axisDirection);
  }

  @override
  void dispose() {
    _scrollable?.position?.removeListener(notifyListeners);
    super.dispose();
  }
}

This is great and I integrated it in a demo project I made. I did find a bug, when you press on the back button on the application it crashes if you have not touched the pane with the scrollbar. This is because your disposed method on your _SingleChildScrollViewWithScrollbarState is removing the AlwaysVisibleScrollbarPainter when it should not. Deleting the dispose override on the_SingleChildScrollViewWithScrollbarState fixes it. It would be awesome if you could update and revise my finding (this happened on the Android devices I tested with).

@override void dispose() { _scrollbarPainter?.dispose(); super.dispose(); }

Thank you so much!
Alex

I know this thing is named "always visible scrollbar" but is there a possibility to hide it if there are not enough elements that it's scrollable?

@marcelser I don't know how to make the AlwaysVisibleScrollbarPainter paint nothing ("hide") but i think the way to determine whether or not we need a scrollbar at all would be to check _scrollable?.position?.pixels == 0 (preferentially in _onScrollChanged() ?).

Fantastic example guys, helping me a lot with my project ♥️

Actually, I have found using the answer at https://stackoverflow.com/questions/51069712/how-to-know-if-a-widget-is-visible-within-a-viewport/57252652#57252652, I wrapped the last widget inside my ScrollView a VisibilityDetector() and if it was in fact visible, simply change the color of the scrollbar to transparent.

Why is this not a Flutter Package yet, friend of mine?

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following assertion was thrown while finalizing the widget tree:
A ScrollPositionWithSingleContext was used after being disposed.

Once you have called dispose() on a ScrollPositionWithSingleContext, it can no longer be used.
When the exception was thrown, this was the stack: 
#0      ChangeNotifier._debugAssertNotDisposed.<anonymous closure> (package:flutter/src/foundation/change_notifier.dart:106:9)
#1      ChangeNotifier._debugAssertNotDisposed (package:flutter/src/foundation/change_notifier.dart:112:6)
#2      ChangeNotifier.removeListener (package:flutter/src/foundation/change_notifier.dart:167:12)
#3      AlwaysVisibleScrollbarPainter.dispose (package:MY_APP/parts/always_scrollbar.dart:123:28)
#4      _SingleChildScrollViewWithScrollbarState.dispose (package:MY_APP/parts/always_scrollbar.dart:63:24)
...
════════════════════════════════════════════════════════════════════════════════════════════════════