Your First Flutter App: Polishing the App

Apr 12 2022 · Dart 2.14.1, Flutter 2.5, Visual Studio Code

Part 3: Style the App

25. Style the Slider Thumb

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Previous episode: 24. Style the Slider Next episode: 26. Conclusion

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Our slider looks really good, but it would be nice to upgrade the thumb. We want to add a custom image to it that looks like bullseye. Now, there are a couple of challenging aspects when updating the thumb. We first need to get the image from our app bundle.

import 'package:flutter/material.dart';
import 'dart:ui' as ui;
class SliderThumbImage extends SliderComponentShape {

final ui.Image? image;
Size getPreferredSize(bool isEnabled, bool isDiscrete) {
  return const Size(0, 0);
  void paint(
    PaintingContext context,
    Offset center, {
    required Animation<double> activationAnimation,
    required Animation<double> enableAnimation,
    required bool isDiscrete,
    required TextPainter labelPainter,
    required RenderBox parentBox,
    required SliderThemeData sliderTheme,
    required TextDirection textDirection,
    required double value,
    required double textScaleFactor,
    required Size sizeWithOverflow,
  }) {

final thumbImage = image;
if (thumbImage != null) {

final canvas = context.canvas;
final imageOffset = Offset(
    center.dx - (thumbImage.width / 2),
    center.dy - (thumbImage.height / 2),
var paint = Paint();
paint.filterQuality = FilterQuality.high;
canvas.drawImage(thumbImage, imageOffset, paint);
import 'slider_thumb_image.dart';
import 'dart:ui' as ui;
import 'package:flutter/services.dart';
ui.Image? _sliderImage;
Future<ui.Image> _load(String asset) async {

final data = await rootBundle.load(asset);
final codec = await ui.instantiateImageCodec(data.buffer.asUint8List());
final fi = await codec.getNextFrame();
return fi.image;
void initState() {
  _load('images/nub.png').then((image) {
      setState(() {
      _sliderImage = image;
thumbShape: SliderThumbImage(_sliderImage),