groupBy<TKey> method

GroupByIterable<T, TKey> groupBy<TKey>(
  1. TKey keySelector(
    1. T element
    ), {
  2. EqualityComparer<TKey>? keyComparer,
})

Groups the elements in the iterable by a key.

After applying the groupBy 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.

Optionally, an EqualityComparer can be supplied to handle key comparisons. If one is provided, the groupBy method will use the compare and hash properties in order to determine equivalency. If omitted, groupBy will resort to strict equivalency (i.e. checking if (value == element)).

(For the groupBy method, only the compare and hash properties of the EqualityComparer need be supplied.)

The resulting iterable will consist of a series of Grouping constructs that contain the elements. If no two elements in the iterable share a common key, the resulting iterable will consist of Grouping objects (each containing a single element) of the same length as the source iterable.

Example:

class Pet {
  Pet(this.name, this.age);

  String name;
  double age;
}

void main() {
  final pets = [
    Pet('Barley', 8),
    Pet('Boots', 4),
    Pet('Whiskers', 1),
    Pet('Daisy', 4),
  ];

  final results = pets.groupBy((pet) => pet.age);

  // Resulting Iterable:
  // [
  //   [ Pet('Barley', 8) ],
  //   [ Pet('Boots', 4), Pet('Daisy', 4) ],
  //   [ Pet('Whiskers', 1) ],
  // ]
}

Implementation

GroupByIterable<T, TKey> groupBy<TKey>(
  TKey Function(T element) keySelector, {
  EqualityComparer<TKey>? keyComparer,
}) {
  keyComparer ??= EqualityComparer.forType<TKey>();
  return GroupByIterable<T, TKey>(this, keySelector, keyComparer);
}