CustomPaint({Key key, CustomPainter painter, CustomPainter foregroundPainter, Size size: Size.zero, Widget child })

Creates a widget that delegates its painting.

Source 

CustomPaint({ Key key, this.painter, this.foregroundPainter, this.size: Size.zero, Widget child })
  : super(key: key, child: child) {
  assert(size != null);
}