Add example "Observer" for later use in more complex examples.

[ilopX]

Important

This pull request includes a commit with an initial flutter launcher for online demos that will be used later.

Observer pattern

Observer is a behavioral design pattern that lets you define a subscription mechanism to notify
multiple objects about any events that happen to the object they’re observing.

Tutorial: here.

AppObserver example

This example was created to be used in a more complex example.
A complex example will be implemented later.

Diagram:

Sequence

Client code:

void main() {
  final observer = AppObserver();

  observer.subscribe<FirstEvent>((e) {
    print('First');
  });


  observer.subscribe((SecondEvent e) {
    print('Second');
  });

  final saveThirdEvent = observer.subscribe((ThirdEvent e) {
    print('Third');
  });

  observer.notify(FirstEvent());
  observer.notify(SecondEvent());
  observer.notify(ThirdEvent());

  print('---unsubscribe "ThirdEvent"---');
  observer.unsubscribe(saveThirdEvent);

  observer.notify(FirstEvent());
  observer.notify(SecondEvent());
  observer.notify(ThirdEvent());
}

Output:

First
Second
Third
---unsubscribe "ThirdEvent"---
First
Second

[neochief]

Thanks a lot!