Returns the difference between two lists, after applying the provided function to each list element of both.

Use Iterable.toSet() and Iterable.map() to get the unique values in b after applying fn to them.
Use Iterable.map() to apply fn to all the values of a, Iterable.where() in combination with Iterable.contains() to keep only the values in the resulting list not contained in the unique mapped values of b, Iterable.toList() to return the appropriate result.

代码实现

List<Y> differenceBy<T,Y>(Iterable<T> a, Iterable<T> b, Y Function(T) fn) {
  final s = b.map(fn).toSet();
  return a.map((x) => fn(x)).where((x) => !s.contains(x)).toList();
}

使用样例

differenceBy([{ 'x': 2 }, { 'x': 1 }], [{ 'x': 1 }], (v) => v['x']); // [2]