How to implement Iterable<E> in dart?

Question

I still havn't understood how to deal with Iterable/Iterator in Dart.

I think I have to give up and simply return Lists but that's not what I want since it will lead bad performance in my case.

What I want is to understand how to implement my own Iterable/Iterator.

Why do both of these attempts fail?

library foo;

import 'dart:collection';

// Both attemps below raises the following error:
// ==============================================
//
// Closure call with mismatched arguments: function 'moveNext'
//
// NoSuchMethodError: incorrect number of arguments passed to method named 'moveNext'
// Receiver: Closure: (dynamic) => Iterator<int> from Function 'iterator':.
// Tried calling: moveNext()


main() {
  Iterable<int> iterable1 = new OddsIterableDartStyle([1,2,4,6,7,8,9]);
  for (int i in iterable1)
    print("ODD: $i");

  Iterable<int> iterable2 = new OddsIterableJavaStyle([1,2,4,6,7,8,9]);
  for (int i in iterable2)
    print("ODD: $i");
}

// ------------------------------------------

class OddsIterableDartStyle extends Object with IterableMixin<int> {
  List<int> _ints;

  OddsIterableDartStyle(this._ints);

  Iterator<int> iterator() {
    return new OddsIterator(this);
  }
}

// ------------------------------------------

class OddsIterableJavaStyle implements Iterable<int> {
  List<int> _ints;

  OddsIterableJavaStyle(this._ints);

  Iterator<int> iterator() {
    return new OddsIterator(this);
  }

}

// ------------------------------------------

class OddsIterator implements Iterator<int> {  // Iterate over odd numbers
  List<int> _ints;
  int _index;

  OddsIterator(this._ints) {
    _index = -1;
  }

  bool moveNext() {
    while (++_index < _ints.length) {
      if (_ints[_index].isOdd)
        return true;
    }
    return false;
  }

  int get current => (_index < 0) ? null : _ints[_index];
}

Answer 1

I see two immediate problems:

1. iterator is a getter. The code shouldn't read Iterator<int> iterator() { ... }, it should be Iterator<int> get iterator { ... } instead.

2. Your iterators are expecting the underlying integer lists, but you are passing in the wrapper. You probably want to construct your iterator like new OddsIterator(_ints), not like new OddsIterator(this).

Btw, Iterator is supposed to return null if you call current and you have already moved beyond the end.

Answer 2

class Count extends Iterable with Iterator {
  Count([this.limit = 10]);
  int limit;
  int i = 0;

  @override
  int get current => i;

  @override
  bool moveNext() {
    i++;    
    return i <= limit;
  }

  @override
  Iterator get iterator => this;
}