Unexpected iterator function and Iterable.take behavior - dart

Question

I was writing a simple function called batches that should get an iterable split it to iterable of iterables of size size.
Then I ran into weird behavior of generator functions and Iterable.take method (when expecting/knowing python generators behavior).
this code:
Iterable<T> iterate<T>(Iterable<T> iterable) sync* {
print('generator started');
for (var item in iterable)
yield item;
}
void main() {
List l = [1, 2, 3, 4, 5];
final it = iterate(l);
print(it.take(2));
print(it.take(2));
}
outputs:
generator started
(1, 2)
generator started
(1, 2)
while the expected output is:
generator started
(1, 2)
(3, 4)
Why does iterate is being called twice instead of continuing to the next iteration after the yield?
Any buildin or more elegant way of writing the following workaround?
my workaround:
Iterable<List<T>> batches<T>(Iterable<T> iterable, int size) sync* {
final iter = iterable.iterator;
List group = takeN(iter, size).toList();
while (group.length > 0){
yield group;
group = takeN(iter, size).toList();
}
}
Iterable<T> takeN<T>(Iterator<T> iterator, int n) sync* {
for (var i = 0; i < n && iterator.moveNext(); i++)
yield iterator.current;
}
void main() {
List l = [1, 2, 3, 4, 5];
print(batches(l, 2));
}

Answer 1

That's how iterables and iterators work.
An Iterable is a simple object which does nothing until your start iterating.
An Iterator is the one maintaining the iteration state.
When you call a sync* function, it immediately returns an Iterable.
When you start iterating that iterable, by reading its iterator getter and using the returned Iterator, the sync* function body starts running. Every call to moveNext runs the body until the next yield.
Every time you get a new iterator, the function body starts from scratch.
That is why your two calls to it.take(2) both do the same thing, each work by getting a new iterator and calling moveNext twice.
As for an easier approach to what you want to do, maybe:
Iterable<List<T>> batch<T>(Iterable<T> source, int size) {
List<T> accumulator;
for (var value in source) {
(accumulator ??= []).add(value);
if (accumulator.length == size) {
yield accumulator;
accumulator = null;
}
}
if (accumulator != null) yield accumulator;
}