How to super() call with complex arguments in Dart?

Question

Up to what I researched, in Dart you gotta call super outside the function body of the constructor.

Suppose this situation:

/// Unmodifiable given class
class Figure{
  final int sides;
  const Figure(this.sides);
}

/// Own class
class Shape extends Figure{
  Shape(Form form){
    if(form is Square) super(4);
    else if(form is Triangle) super(3);
  }
}

That throws analysis errors (the superclass doesn't have 0 parameters constructor & the expression super(3) doesn't evaluate to a function, so it can't be invoked). How could I achieve the desidered functionality of the example?

Answer 1

In Dart for calling the super constructor the initializer list is used.

class Shape extends Figure{
  Shape(Form form) : super(form is Square ? 4 : form is Triangle ? 3 : null);
}

if you need to execute statements you can add a factory constructor that forwards to a (private) regular constructor like

class Shape extends Figure{

  factory Shape(Form form) {
    if (form is Square) return new Shape._(4);
    else if(form is Triangle) return new Shape._(3);
  }
  Shape._(int sides) : super(sides)
}