Dart - Trying to understand the value of 'factory' constructor

Question

If I am understanding correctly:

\"A factory constructor affords an abstract class to be 
    instantiated by another class, despite being abstract.\" 
         


        

Answer 1

I don't think that the problem in the factory.

Your code initially was wrong.

Look at this code and make your conclusions.

Locomotive locomotive = new SomethingWithSmokeFromChimney();

Now look at this code.

Plant plant = new SomethingWithSmokeFromChimney();

You mistakenly believe that all animals on the earth (even the dogs) are cats.

Cat cat = new Animal();

If you want this.

Cat cat = new Animal();
Dog dog = new Animal();

Then (if I correctly understand you) you also want this.

// Cat cat = new Animal();
// Dog dog = new Animal(); 
Dog dog = new Cat();

P.S.

The same wrong conclusions but without factory.

void main() {
  Cat cat = new Animal();
  Dog dog = new Animal();
}

class Animal {
}

class Cat implements Animal {
}

class Dog implements Animal {
}

But this code (depending on documenation) may be considered correct.

void main() {
  Cat cat = new Animal("cat");
  Dog dog = new Animal("dog");
}

abstract class Animal {
  factory Animal(String type) {
    switch(type) {
      case "cat":
        return new Cat();
      case "dog":
        return new Dog();
      default:
        throw "The '$type' is not an animal";
    }
  }
}

class Cat implements Animal {
}

class Dog implements Animal {
}

The factory constructor of the abstract class can return (by default) some default implementation of this abstract class.

abstract class Future {
   factory Future(computation()) {
    _Future result = new _Future();
    Timer.run(() {
      try {
        result._complete(computation());
      } catch (e, s) {
        result._completeError(e, s);
      }
    });
    return result;
  }
}