About factory constructor in Dart

Question

Below is about a usage of factory constructor from Seth Ladd's blog ' Dart - Trying to understand the value of 'factory' constructor'.

class Symbol {
  final String name;
  static Map<String, Symbol> _cache = new Map<String, Symbol>();

  factory Symbol(String name) {
    if (_cache.containsKey(name)) {
      return _cache[name];
    } else {
      final symbol = new Symbol._internal(name);
      _cache[name] = symbol;
      return symbol;
    }
  }

  Symbol._internal(this.name);
}


main() {
  var x = new Symbol('X');
  var alsoX = new Symbol('X');

  print(identical(x, alsoX));  // true
}

IMHO, with general constructor, the same effect can be achieved with a subtle difference, but quite simpler.

class Symbol {
  static final Map<String, Symbol> cache = {};
  final String name;

  Symbol(name) {
    cache[name] = new Symbol._internal();
  }

  Symbol._internal();
}

main(){
var a = new Symbol('something');
var b = new Symbol('something');

print(identical(a, b)); // false!
print(Symbol.cache); //{something: Instance of 'Symbol'}
}

As shown above, though the two instances, a & b, are different objects, the effect is all the same as shown in 'print(Symbol.cache); //{something: Instance of 'Symbol'}' as a map object permit only one of same strings as its key.

So, my question was what are peculiar merits of factory constructor(or factory pattern) over general/const constructors? Because the above sample code alone shows no merit of factory constructor.

Could anyone explain what is so called 'Factory Pattern' in Dart language rather than Java/C#?

Answer 1

The factory PATTERN is the same. It's a general pattern and is not language specific.

Dart provides factory constructors to support the factory pattern. The factory constructor is able to return values (objects). In your first example you check if there is an instance for the key you return it.

In the second example you don't check the key of the map and you don't return the instance. That's why the two instances are not identical.

Regards, Robert