A flutter example demonstrating a custom painter drawing selectable rects

example.dart

// run in DartPad: <https://dartpad.dev/c6a9111d58c3deb83711106cec6152ee>

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(home: RectsExample()));
}

class RectsExample extends StatefulWidget {
  @override
  _RectsExampleState createState() => _RectsExampleState();
}

class _RectsExampleState extends State<RectsExample> {
  int _index = -1;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Rects(
          rects: [
            Rect.fromLTRB(10, 20, 30, 40),
            Rect.fromLTRB(40, 60, 80, 100),
          ],
          selectedIndex: _index,
          onSelected: (index) {
            setState(() {
              _index = index;
            });
          },
        ),
      ),
    );
  }
}

class Rects extends StatelessWidget {
  final List<Rect> rects;
  final void Function(int) onSelected;
  final int selectedIndex;

  const Rects({
    Key key,
    @required this.rects,
    @required this.onSelected,
    this.selectedIndex = -1,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTapDown: (details) {
        RenderBox box = context.findRenderObject();
        final offset = box.globalToLocal(details.globalPosition);
        final index = rects.lastIndexWhere((rect) => rect.contains(offset));
        if (index != -1) {
          onSelected(index);
          return;
        }
        onSelected(-1);
      },
      child: CustomPaint(
        size: Size(320, 240),
        painter: _RectPainter(rects, selectedIndex),
      ),
    );
  }
}

class _RectPainter extends CustomPainter {
  static Paint _red = Paint()..color = Colors.red;
  static Paint _blue = Paint()..color = Colors.blue;

  final List<Rect> rects;
  final int selectedIndex;

  _RectPainter(this.rects, this.selectedIndex);

  @override
  void paint(Canvas canvas, Size size) {
    var i = 0;
    for (Rect rect in rects) {
      canvas.drawRect(rect, i++ == selectedIndex ? _red : _blue);
    }
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    return true;
  }
}

this is my code below:-

import 'package:BLEMesh/draw_circle.dart';
import 'package:flutter/material.dart';

class DrawCircleExample extends StatefulWidget {
@OverRide
_DrawCircleExampleState createState() => _DrawCircleExampleState();
}

class _DrawCircleExampleState extends State {
int _index = -1;
@OverRide
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: Circles(
circls: [
Circle(center: {"x": 100, "y": 400}, radius: 60),
Circle(center: {"x": 180, "y": 600}, radius: 28),
Circle(center: {"x": 150, "y": 180}, radius: 40),
Circle(center: {"x": 200, "y": 400}, radius: 35),
Circle(center: {"x": 80, "y": 190}, radius: 18),
Circle(center: {"x": 200, "y": 120}, radius: 20),
Circle(center: {"x": 300, "y": 240}, radius: 36),
],
selectedIndex: _index,
onSelected: (index) {
setState(() {
_index = index;
});
},
),
),
);
}
}

class Circles extends StatelessWidget {
final List circls;
final void Function(int) onSelected;
final int selectedIndex;
//final Map<String, double> center;
// final double radius;
const Circles(
{Key key,
@required this.circls,
@required this.onSelected,
this.selectedIndex = -1,
}): super(key: key);
@OverRide
Widget build(BuildContext context) {
return GestureDetector(
onTapDown: (details) {
RenderBox box = context.findRenderObject();
final offset = box.globalToLocal(details.globalPosition);
print(offset);
final index = circls.lastIndexWhere((element) => true);
if (index != -1) {
onSelected(index);
return;
}
onSelected(-1);
},
child: CustomPaint(
size: Size(MediaQuery.of(context).size.width,
MediaQuery.of(context).size.height),
painter: DrawCircle(circls: circls, selectedIndex: selectedIndex),
),
);
}
}

class DrawCircle extends CustomPainter {
Map<String, double> center;
double radius;
final int selectedIndex;
static Paint _red = Paint()..color = Colors.red;
static Paint _blue = Paint()..color = Colors.blue;

final List circls;
DrawCircle({this.circls, this.selectedIndex});
@OverRide
void paint(Canvas canvas, Size size) {
var i = 0;
// Paint brush = new Paint()
// ..color = Colors.red
// ..strokeCap = StrokeCap.round
// ..style = PaintingStyle.fill
// ..strokeWidth = 30;
for (Circle circ in circls) {
canvas.drawCircle(
Offset(circ.center["x"], circ.center["y"]), circ.radius, i++ == selectedIndex ? _red : _blue);
}
}

@OverRide
bool shouldRepaint(CustomPainter oldDelegate) {
return true;
}
}

please help me to make it tappable.how can I get the index of circle.

final index = circls.lastIndexWhere((element) => true);

@rudeboyas you don't seem to be doing any actual checks

To use circles instead of rectangles, you need to implement a different hit test (line 56) and of course a different way to display them (line 84). For example:

56: final index = rects.lastIndexWhere((rect) => (rect.center - offset).distance < rect.shortestSide / 2);
84: canvas.drawCircle(rect.center, rect.shortestSide / 2, i++ == selectedIndex ? _red : _blue);

thanks got it.

Can we detect which geometry shape is draw on screen through custom painter ?

No, only with some kind of model that represents the geometric forms displayed by the painter.

@sma I need to do the same with a circle drawn with multiple arcs. I need to see which arc tapped by the user.
Please have a look at this. https://dartpad.dev/4f1dccf37910f61840d430a3947e4135. Please help me!