group<GK> method
Implementation
Map<GK, List<E>> group<GK>(GK Function(E item) groupKeyProvider)
{
Map<GK, List<E>> map = {};
if (this == null) {
return map;
}
for (var item in this!) {
GK key = groupKeyProvider(item);
map.putIfAbsent(key, () => []).add(item);
}
return map;
}