Custom Painter

.dart

import 'package:flutter/material.dart';

class HeaderSlash extends StatelessWidget {
  @override
  Widget build(BuildContext context) => CustomPaint(
        painter: _HeaderSlashCustom(),//The container shape
        child: Container(
          height: double.infinity,
          width: double.infinity,
        ),
      );
}

class _HeaderSlashCustom extends CustomPainter {
  
  //Size means the child's size
  @override
  void paint(Canvas canvas, Size size) {
    
    final path = new Path();//To draw 
    final pen = new Paint();// To configure the pen to draw

    //Setup the pen
    pen.strokeWidth = 5;
    pen.color = Colors.deepPurple;
    //If there is a design hard. The propierty stroke should be used
    pen.style = PaintingStyle.fill; //Or PaintingStyle.stroke Just a line

    //Drawing

    //path.moveTo() MEANS CHANGE POSITION OF THE PEN TO DRAWING
    //path.lineTo() MEAN Draw frm x position to y position to do that we neeed to methods like this...

    //X means  = width and Y means = height
    path.lineTo(0.0,size.height / 2);
    //Remember, lineTo take the last position and draw to the new positon
    path.lineTo( size.width, size.height * 0.35);
    path.lineTo( size.width, size.height * 0.0);
    path.close();//To 0,0


    canvas.drawPath(path, pen);//Draw the lines
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    //If this return true means that shape shouuld be repaint else is not repaint
    return true;
  }
}