countBy<TKey> method
Applies keySelector
to every element in this iterable and returns an iterable
containing each resulting key and the number of times that key appears in this
iterable.
The order of the resulting iterable is not guaranteed to be any particular order.
Implementation
Iterable<MapEntry<TKey, int>> countBy<TKey>(
TKey Function(T element) keySelector,
) {
final countMap = HashMap<TKey, int>();
for (var o in this) {
final key = keySelector(o);
countMap[key] = (countMap[key] ?? 0) + 1;
}
return countMap.entries;
}