groupSelectValue<TKey, TValue, TResult> method
- TResult resultSelector(
- TKey key,
- Iterable<
TValue> group
- {TKey keySelector(
- T element
- TValue valueSelector(
- T element
- EqualityComparer<
TKey> keyComparer}
Groups the elements in the iterable by a key, maps the elements to a new value, and maps the groups to a new element.
After applying the groupSelectValue
method to an iterable, the resulting
iterable will be a series of groups of elements. Each group will consist
of all elements in the source iterable that share a common key as defined
by passing the element to the keySelector
function stored as a value
obtained by passing the elements to the valueSelector
function. Finally,
each group will then be passed to the resultSelector
function along with
its associated key and the returned value of that function will be returned
as an element of the resulting iterable.
Optionally, an EqualityComparer
can be supplied to handle key comparisons.
If one is provided, the groupSelectValue
method will use the comparer
and
hasher
properties in order to determine equivalency. If omitted,
groupSelectValue
will resort to strict equivalency (i.e. checking if (value == element)
).
(For the groupSelectValue
method, only the comparer
and hasher
properties of the EqualityComparer
need be supplied.)
Example:
class Pet {
Pet(this.name, this.age);
String name;
double age;
}
void main() {
final pets = [
Pet('Barley', 8.3),
Pet('Boots', 4.9),
Pet('Whiskers', 1.5),
Pet('Daisy', 4.3),
];
final ageComparer = (Pet p1, Pet p2) => p1.age.compareTo(p2.age);
final result = pets.groupSelectValue(
(age, pets) => {
'key': age,
'count': pets.length,
'names': pets,
},
keySelector: (pet) => pet.age.floor(),
valueSelector: (pet) => pet.name,
);
// Resulting Iterable:
// [
// { 'key': 8, 'count': 1, 'names': ['Barley'] },
// { 'key': 4, 'count': 2, 'names': ['Boots', 'Daisy'] },
// { 'key': 1, 'count': 1, 'names': ['Whiskers'] },
// ]
}
Implementation
GroupSelectValueIterable<T, TKey, TValue, TResult>
groupSelectValue<TKey, TValue, TResult>(
TResult Function(TKey key, Iterable<TValue> group) resultSelector, {
TKey Function(T element) keySelector,
TValue Function(T element) valueSelector,
EqualityComparer<TKey> keyComparer,
}) {
checkNullError(this);
ArgumentError.checkNotNull(resultSelector, 'resultSelector');
keySelector ??= (T v) => v as TKey;
valueSelector ??= (T v) => v as TValue;
keyComparer ??= EqualityComparer.forType<TKey>();
return GroupSelectValueIterable(
this, keySelector, valueSelector, resultSelector, keyComparer);
}