Factory constructors in Dart
Dart supports factory constructors, which can return subtypes or even null. To create a factory constructor, use the factory keyword:
class Square extends Shape {}
class Circle extends Shape {}
class Shape {
Shape();
factory Shape.fromTypeName(String typeName) {
if (typeName == 'square') return Square();
if (typeName == 'circle') return Circle();
print('I don\'t recognize $typeName');
return null;
}
}