paintWithProgress method
Implementation
@override
void paintWithProgress(Canvas canvas, Size size, Paint paint, double p) {
double width = size.width;
double height = size.height;
// the best ratio is width: height => 1 : 1
Path pathLeft = Path();
pathLeft.moveTo(0, 0);
pathLeft.lineTo(width * p, height * p);
Path pathRight = Path();
pathRight.moveTo(width, 0);
pathRight.lineTo(width - (width * p), height * p);
canvas.drawPath(pathLeft, paint);
canvas.drawPath(pathRight, paint);
}