Reactivity explained in dart

reactivity.dart

import 'dart:html';

class Storage {
  Map db = {'count': 0};
  Map deps = {'count': []};

  void add(String key, dynamic value) {
    db[key] = value;
  }

  dynamic getter(String key) {
    print('$key was used.');
    return db[key];
  }

  dynamic setter(String key, dynamic value) {
    db[key] = value;
    print('$key was changed.');

    for (var fn in deps[key]) {
      fn(db[key]);
    }
  }

  void depend(String key, Function dep) {
    deps[key].add(dep);

    dep(db[key]);
  }

  int get count => getter('count');
  set count(value) => setter('count', value);
}

void render(int count) {
  querySelector('#counter').text = '$count';
}

void main() {
  var storage = Storage();
  storage.depend('count', render);

  querySelector('#increment').onClick.listen((e) {
    ++storage.count;
  });
}