Trivial Flutter app to track HP for RPGs

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'HP Tracker',
      theme: ThemeData(
        brightness: Brightness.dark,
        primaryColor: Colors.deepOrange,
      ),
      home: TrackerPage(),
    );
  }
}

class TrackerPage extends StatefulWidget {
  @override
  _TrackerPageState createState() => _TrackerPageState();
}

class _TrackerPageState extends State<TrackerPage> {
  int _hp = 0;
  bool _show = false;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("HP Tracker")),
      body: Stack(
        fit: StackFit.expand,
        alignment: Alignment.center,
        children: <Widget>[
          SwipeDetector(
            onTap: () => setState(() => _show = !_show),
            onSwipeUp: () => update(1),
            onSwipeDown: () => update(-1),
            child: Padding(
              padding: const EdgeInsets.all(32),
              child: FittedBox(
                fit: BoxFit.scaleDown,
                child: Text(
                  '$_hp',
                  style: TextStyle(fontSize: 320, fontWeight: FontWeight.w200),
                ),
              ),
            ),
          ),
          Positioned(
            bottom: 20,
            child: AnimatedOpacity(
              duration: Duration(milliseconds: 1000),
              opacity: _hp == 0 ? 1 : 0,
              child: Text("Swipe up or down", style: Theme.of(context).textTheme.caption),
            ),
          ),
          AnimatedOpacity(
            duration: Duration(milliseconds: 200),
            opacity: _show ? 1 : 0,
            child: Column(
              children: <Widget>[
                Row(
                  children: <Widget>[
                    Inc(value: 1, update: update),
                    Inc(value: 5, update: update),
                    Inc(value: 10, update: update),
                    Inc(value: 100, update: update),
                  ],
                ),
                Row(
                  children: <Widget>[
                    Inc(value: -1, update: update),
                    Inc(value: -5, update: update),
                    Inc(value: -10, update: update),
                    Inc(value: -100, update: update),
                  ],
                ),
              ],
            ),
          ),
        ],
      ),
    );
  }

  void update(int delta) {
    setState(() => _hp += delta);
  }
}

class SwipeDetector extends StatefulWidget {
  const SwipeDetector({
    Key key,
    this.onTap,
    this.onSwipeUp,
    this.onSwipeDown,
    this.child,
  }) : super(key: key);

  final VoidCallback onTap;
  final VoidCallback onSwipeUp;
  final VoidCallback onSwipeDown;
  final Widget child;

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

class _SwipeDetectorState extends State<SwipeDetector> {
  Offset _start, _end;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onVerticalDragStart: (details) {
        _start = _end = details.globalPosition;
      },
      onVerticalDragUpdate: (details) {
        _end = details.globalPosition;
      },
      onVerticalDragEnd: (details) {
        final delta = _start - _end;
        if (delta.dy > 2) {
          widget.onSwipeUp?.call();
        } else if (delta.dy < -2) {
          widget.onSwipeDown?.call();
        } else {
          widget.onTap?.call();
        }
      },
      child: widget.child,
    );
  }
}

class Inc extends StatelessWidget {
  const Inc({Key key, this.value, this.update}) : super(key: key);

  final int value;
  final void Function(int) update;

  @override
  Widget build(BuildContext context) {
    return Expanded(
      child: Padding(
        padding: const EdgeInsets.symmetric(horizontal: 4),
        child: FlatButton(
          onPressed: () => update(value),
          color: Colors.white10,
          child: Text('$value'),
          textColor: Colors.grey,
          padding: EdgeInsets.all(0),
          shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(0)),
        ),
      ),
    );
  }
}