import 'package:flutter/gestures.dart';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(
          text:
              '😄 Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.',
          style: TextStyle(color: Colors.black45, fontSize: 18)),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.text, this.style}) : super(key: key);

  final String text;
  final TextStyle style;

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

class _MyHomePageState extends State<MyHomePage> {
  final _textKey = GlobalKey();
  final List<Rect> _textRects = [];
  final List<Rect> _selectionRects = [];
  String selectedText = '';
  Rect _caretRect = Rect.zero;
  MouseCursor _cursor = SystemMouseCursors.basic;
  int _selectionBaseOffset;
  TextSelection _textSelection = TextSelection.collapsed(offset: -1);
  @override
  initState() {
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
      _updateAllTextRects();
    });
  }

  void _onMouseMove(event) {
    if (event is PointerHoverEvent) {
      if (_renderParagraph == null) {
        return;
      }
      final allTextRects = _computeRectsForSelection(
        TextSelection(baseOffset: 0, extentOffset: widget.text.length),
      );
      bool isOverText = false;
      for (final rect in allTextRects) {
        if (rect.contains(event.localPosition)) {
          isOverText = true;
        }
      }
      final newCursor =
          isOverText ? SystemMouseCursors.text : SystemMouseCursors.basic;
      if (newCursor != _cursor) {
        setState(() {
          _cursor = newCursor;
        });
      }
    }
  }

  void _updateAllTextRects() {
    setState(() {
      _textRects
        ..clear()
        ..addAll(_computeRectsForSelection(
            TextSelection(baseOffset: 0, extentOffset: widget.text.length)));
    });
  }

  List<Rect> _computeRectsForSelection(TextSelection textSelection) {
    if (_renderParagraph == null) {
      return [];
    }
    final textBoxes = _renderParagraph.getBoxesForSelection(textSelection);
    return textBoxes.map((box) => box.toRect()).toList();
  }

  RenderParagraph get _renderParagraph =>
      _textKey.currentContext.findRenderObject() as RenderParagraph;

  void _onPanStart(DragStartDetails details) {
    if (_renderParagraph == null) {
      return;
    }
    _selectionBaseOffset =
        _renderParagraph.getPositionForOffset(details.localPosition).offset;
    _textSelection = TextSelection.collapsed(offset: _selectionBaseOffset);
    _updateSelectionDisplay();
  }

  void _updateSelectionDisplay() {
    //Compute selection rectangles
    final selectionRects = _computeRectsForSelection(_textSelection);
    //Update caret display
    final caretOffset =
        _renderParagraph.getOffsetForCaret(_textSelection.extent, Rect.zero);
    final caretHeight =
        _renderParagraph.getFullHeightForCaret(_textSelection.extent);
    setState(() {
      _selectionRects
        ..clear()
        ..addAll(selectionRects);
      _caretRect =
          Rect.fromLTWH(caretOffset.dx - 1, caretOffset.dy, 2, caretHeight);
      // widget.onSelectionChange?.call(_textSelection);
      selectedText = _textSelection.textInside(widget.text);
      print('selectedText: ' + selectedText.toString());
    });
  }

  void _onPanUpdate(DragUpdateDetails details) {
    final selectionExtentOffset =
        _renderParagraph.getPositionForOffset(details.localPosition).offset;
    _textSelection = TextSelection(
        baseOffset: _selectionBaseOffset, extentOffset: selectionExtentOffset);
    _updateSelectionDisplay();
  }

  void _onPanEnd(DragEndDetails details) {}
  void _onPanCancel() {}

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        mainAxisAlignment: MainAxisAlignment.spaceEvenly,
        children: [
          Container(
            color: Colors.white,
            child: Center(
              child: Listener(
                onPointerHover: _onMouseMove,
                child: MouseRegion(
                  cursor: _cursor,
                  child: GestureDetector(
                    onPanStart: _onPanStart,
                    onPanUpdate: _onPanUpdate,
                    onPanEnd: _onPanEnd,
                    onPanCancel: _onPanCancel,
                    child: Stack(
                      children: [
                        CustomPaint(
                          painter: _SelectionPainter(
                              color: Colors.yellow,
                              rects: _selectionRects,
                              fill: true),
                        ),
                        CustomPaint(
                          painter: _SelectionPainter(
                              color: Colors.grey,
                              rects: _textRects,
                              fill: false),
                        ),
                        Text(widget.text, key: _textKey, style: widget.style),
                        CustomPaint(
                          painter: _SelectionPainter(
                              color: Colors.blue,
                              rects: [_caretRect],
                              fill: true),
                        ),
                      ],
                    ),
                  ),
                ),
              ),
            ),
          ),
          Text(selectedText, style: widget.style),
        ],
      ),
    );
  }
}

class _SelectionPainter extends CustomPainter {
  _SelectionPainter({
    @required Color color,
    @required List<Rect> rects,
    bool fill = true,
  })  : _color = color,
        _rects = rects,
        _fill = fill,
        _paint = Paint()..color = color;

  final Color _color;
  final List<Rect> _rects;
  final bool _fill;
  Paint _paint;

  @override
  void paint(Canvas canvas, Size size) {
    //a rectangle
    if (_fill == false) {
      //make an outline of rectangle
      _paint = Paint()
        ..color = _color
        ..style = PaintingStyle.stroke;
    }
    for (Rect _rect in _rects) {
      canvas.drawRect(_rect, _paint);
    }
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    // TODO: implement shouldRepaint
    return true;
  }
}