Lazy Iterable

lazy_iterable.dart

import 'dart:collection';

void main() {
  var list = new WatchList([1,2,3,4,5]);

  var mapped = list.where((x) => x.isEven).map((x) => x*2);

  // What is printed here?
  print(list.accessCount);

  var newList = list.where((x) => x.isEven).map((x) => x*2).toList();

  // What is printed here?
  print(list.accessCount);
}

class WatchList<T> extends ListBase<T> {
  final List<T> _innerList;

  int _count = 0;

  WatchList(List list) : _innerList = list;

  int get accessCount => _count;

  T operator[](int index) {
    _count++;
    return _innerList[index];
  }

  int get length => _innerList.length;
}