Jatra

class _ColourSliderState extends State<ColourSlider> {
  var fraction = 0.0;

  @override
  Widget build(BuildContext context) {
    debugPaintPointersEnabled = true;
    return GestureDetector(
      onPanUpdate: (DragUpdateDetails details) {
        RenderBox box = context.findRenderObject();
        Offset localOffset = box.globalToLocal(details.globalPosition);

Jatra

  void performLayout(Size size) {
    var trackSize = layoutChild(ColourSlider.track, BoxConstraints.loose(size));
    var thumbSize = layoutChild(ColourSlider.thumb, BoxConstraints.loose(size));
    positionChild(ColourSlider.track, Offset(0, thumbSize.height/2 - trackSize.height/2));
    positionChild(ColourSlider.thumb, Offset(-thumbSize.width/2, 0));
  }

Jatra

class _ColourSliderDelegate extends MultiChildLayoutDelegate {
  @override
  void performLayout(Size size) {
    layoutChild(ColourSlider.track, BoxConstraints.loose(size));
    positionChild(ColourSlider.track, Offset(0, 0));
    layoutChild(ColourSlider.thumb, BoxConstraints.loose(size));
    positionChild(ColourSlider.thumb, Offset(0, 0));
  }

  @override

Jatra

class ColourSlider extends StatefulWidget {
  static final track = "track";
  static final thumb = "thumb";

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

class _ColourSliderState extends State<ColourSlider> {
  @override

Jatra

      body: Center(
        child: ColourSlider(),
      ),

Jatra

package uk.co.jatra.recorder

import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import android.support.v7.widget.LinearLayoutManager
import android.support.v7.widget.RecyclerView
import android.support.v7.widget.helper.ItemTouchHelper
import android.support.v7.widget.helper.ItemTouchHelper.DOWN
import android.support.v7.widget.helper.ItemTouchHelper.UP
import android.view.LayoutInflater