flutter - dart advantage of a factory constructor identifier

Question

I've been investigating JSON parsing for my Flutter app and have a question about factory constructors that I can't resolve. I'm trying to understand the advantage of using a factory constructor versus a plain constructor. For example, I see quite a few JSON parsing examples that create a model class with a JSON constructor like this:

class Student{
  String studentId;
  String studentName;
  int studentScores;

  Student({
    this.studentId,
    this.studentName,
    this.studentScores
  });

  factory Student.fromJson(Map<String, dynamic> parsedJson){
    return Student(
      studentId: parsedJson['id'],
      studentName : parsedJson['name'],
      studentScores : parsedJson ['score']
    );
  }
}

I've also seen an equal number of examples that DON'T declare the constructor as a factory. Both types of classname.fromJSON constructors create an object from the JSON data so is there an advantage to declaring the constructor as a factory or is using a factory here superfluous?

Answer 1

A normal constructor always returns a new instance of the current class (except when the constructor throws an exception).

A factory constructor is quite similar to a static method with the differences that it

• can only return an instance of the current class or one of its subclasses
• can be invoked with new but that is now less relevant since new became optional.
• has no initializer list (no : super())

So a factory constructor can be used

• to create instances of subclasses (for example depending on the passed parameter
• to return a cached instance instead of a new one
• to prepare calculated values to forward them as parameters to a normal constructor so that final fields can be initialized with them. This is often used to work around limitations of what can be done in an initializer list of a normal constructor (like error handling).

In your example this code

  studentId: parsedJson['id'],
  studentName : parsedJson['name'],
  studentScores : parsedJson ['score']

could be moved to the body of a normal constructor because no final fields need to be initialized.