Triangle

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: Scaffold(
          body: Center(
            child: Material(
              elevation: 10,
              child: Container(
                width: 230,
                height: 230,
                decoration: const ShapeDecoration(
                  shape: MessageBorder(),
                  // color: Colors.teal,
                  gradient: RadialGradient(colors: [
                    Colors.red,
                    Colors.green,
                    Colors.blue,
                  ]),
                  shadows: [
                    BoxShadow(
                        color: Colors.black,
                        offset: Offset(0, 0),
                        blurRadius: 0,
                        spreadRadius: 30),
                    BoxShadow(
                      color: Colors.blue,
                      offset: Offset(0, 0),
                      blurRadius: 0,
                    ),
                  ],
                ),
              ),
            ),
          ),
        ));
  }
}

class MessageBorder extends ShapeBorder {
  final bool usePadding;

  const MessageBorder({this.usePadding = true});

  @override
  EdgeInsetsGeometry get dimensions => const EdgeInsets.only(bottom: 0);

  @override
  Path getInnerPath(Rect rect, {TextDirection? textDirection}) =>
      getOuterPath(rect, textDirection: textDirection);

  @override
  Path getOuterPath(Rect rect, {TextDirection? textDirection}) {
    return Path()
      ..moveTo(rect.left + 66.5, rect.top)
      ..lineTo(rect.left + 133, rect.top + 110)
      ..lineTo(rect.left, rect.top + 110)
      ..lineTo(rect.left + 66.5, rect.top)
      ..close();
  }

  @override
  void paint(Canvas canvas, Rect rect, {TextDirection? textDirection}) {}

  @override
  ShapeBorder scale(double t) => this;
}