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typedef VoidCallback = void Function();

class ChangeNotifier {
  List<VoidCallback> _listeners = List<VoidCallback>();
  List<int> _toRemove = List<int>();
  bool _notifying = false;

  bool get hasListeners {
    return _listeners.isNotEmpty;
  }

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typedef VoidCallback = void Function();

class _Listener {
  _Listener(this.func);
  final VoidCallback func;
  VoidCallback afterNotify;
  void call() {
    if (afterNotify == null) {
      func();
    }

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class RenderSliverGap extends RenderSliver {
  RenderSliverGap({
    double mainAxisExtent,
  }) : _mainAxisExtent = mainAxisExtent;

  double get mainAxisExtent => _mainAxisExtent;
  double _mainAxisExtent;
  set mainAxisExtent(double value) {
    if (_mainAxisExtent != value) {
      _mainAxisExtent = value;

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const SliverGeometry({
  this.scrollExtent = 0.0,
  this.paintExtent = 0.0,
  this.paintOrigin = 0.0,
  double layoutExtent,
  this.maxPaintExtent = 0.0,
  this.maxScrollObstructionExtent = 0.0,
  double hitTestExtent,
  bool visible,
  this.hasVisualOverflow = false,

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const SliverConstraints({
  @required this.axisDirection,
  @required this.growthDirection,
  @required this.userScrollDirection,
  @required this.scrollOffset,
  @required this.precedingScrollExtent,
  @required this.overlap,
  @required this.remainingPaintExtent,
  @required this.crossAxisExtent,
  @required this.crossAxisDirection,

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class Gap extends LeafRenderObjectWidget {
  const Gap(
    this.mainAxisExtent, {
    Key key,
  })  : assert(mainAxisExtent != null &&
            mainAxisExtent >= 0 &&
            mainAxisExtent < double.infinity),
        super(key: key);

  final double mainAxisExtent;

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class _RenderGap extends RenderBox {
  _RenderGap({
    double mainAxisExtent,
  }) : _mainAxisExtent = mainAxisExtent;

  double get mainAxisExtent => _mainAxisExtent;
  double _mainAxisExtent;
  set mainAxisExtent(double value) {
    if (_mainAxisExtent != value) {
      _mainAxisExtent = value;

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class Gap extends LeafRenderObjectWidget {
  const Gap(
    this.mainAxisExtent, {
    Key key,
  })  : assert(mainAxisExtent != null &&
            mainAxisExtent >= 0 &&
            mainAxisExtent < double.infinity),
        super(key: key);

  final double mainAxisExtent;

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
      builder: (_) => ValueNotifier<String>(null),

letsar

/*

ModelBinding<T>: a simple class for binding a Flutter app to a immutable
model of type T.

This is a complete application. The app shows the state of an instance of
MyModel in a button's label, and replaces its MyModel instance when the
button is pressed.

ModelBinding is an inherited widget that must be created in a context above