What is the purpose of ._() in naming the constructor of a theme?

Question

Was watching a tutorial and saw instructor defining the constructor of theme file like this

MyTheme._();

I expect to know why he didn't declare a normal constructor and if the the other constructor mean something is it a must ?

Answer 1

Identifiers that start with an underscore (_) are visible only inside the library. See Dart libraries.

The underscore indicates that for example a class, constructor, method, variable, getter, etc is library-private and can only be accessed from within the library where it is defined.

In the example below, A._() is a private constructor, _a is a private instance variable, and _B is a private class.

class A{
  
  /// Default constructor
  A(this._a);
  
  /// Private constructor
  A._(): _a = 89;
  
  /// Named factory constructor
  factory A.named({required int a}){
    if (a > 100){
      return A._();
    }else{
      return A(a);
    }
  }
  
  /// Public getter
  int get a => _a;
  
  /// Private instance variable
  int _a;
  
  /// Public getter
  bool get isEven => a.isEven;
}

/// Private class
class _B extends A{
  _B(): super._();
}

So the purpose of naming the constructor MyTheme._() is to make it private.