countItems<K, V> static method

@useResult int countItems<K, V>(Map<K, Iterable<V>> inputMap)

Total number of elements across every iterable value in inputMap.

Folds the .length of each value into a single running int, so the result is the combined element count of all the iterables, ignoring the keys entirely. Generic over key type K and element type V, so it works on any Map<K, List<V>>, Map<K, Set<V>>, or Map<K, Iterable<V>>. Typical use is sizing grouped collections such as Map<Section, List<Item>> (e.g. counting selected items across grouped sections).

Edge cases:

• Empty map or all-empty values: returns 0.
• Set values: counts the post-deduplication .length, not the number of values that were inserted ({1, 1} contributes 1, not 2).
• Lazy iterables (from Iterable.generate, map(...), where(...)): .length materializes the iterable, so the count is correct.
• Element CONTENT is irrelevant — only .length is read. A multi-code- unit emoji, a combining-mark string, and null each count as exactly one element.
• Order-independent: rearranging keys or empty/non-empty values does not change the result, because addition is commutative.
• Non-mutating: neither inputMap nor any of its iterable values is altered; fold only reads .length.

Example:

MapExtensions.countItems(<String, List<int>>{
  'a': <int>[1, 2],
  'b': <int>[3],
  'c': <int>[],
}); // 3

Audited: 2026-06-12 11:26 EDT

Implementation

@useResult
static int countItems<K, V>(Map<K, Iterable<V>> inputMap) =>
    inputMap.values.fold<int>(0, (int sum, Iterable<V> iter) => sum + iter.length);