groupSelectValue<TKey, TValue, TResult> method

GroupSelectValueIterable<T, TKey, TValue, TResult> groupSelectValue<TKey, TValue, TResult>(
  1. TResult resultSelector(
    1. TKey key,
    2. Iterable<TValue> group
    ), {
  2. TKey keySelector(
    1. T element
    )?,
  3. TValue valueSelector(
    1. T element
    )?,
  4. 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 compare and hash 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 compare and hash 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,
}) {
  keySelector ??= (T v) => v as TKey;
  valueSelector ??= (T v) => v as TValue;
  keyComparer ??= EqualityComparer.forType<TKey>();

  return GroupSelectValueIterable(
      this, keySelector, valueSelector, resultSelector, keyComparer);
}