In your programming language of choice, pass a pointer to a function (A) to another function (b), then call A from B.

pointer_demo.dart

void main() {
  b(() => print('I am function a passed to function b'));
}

/// Calling a function a from b
void b(Function a) {
  print('This is the beginning of function b');
  print('before calling function a from b');
  a();
  print('after calling function a');
}