rollback method

void rollback(NimbostratusDocumentSnapshot<T> snap)

Rolls back the supplied optimistic snapshot. If the snapshot being rolled back is the current value on the stream, then the snapshot that was chronologically emitted before it becomes the most logical current value for the stream and it is replayed on the stream.

Implementation

void rollback(NimbostratusDocumentSnapshot<T> snap) {
  assert(
    snap.isOptimistic,
    'Attempted to rollback a non-optimistic snap',
  );

  final currentSnap = value;
  final prev = snap.prev;
  final next = snap.next;

  // If the current value is the optimistic snap that is being rolled back, then we need to re-emit the previous value.
  if (currentSnap == snap) {
    // If there is no previous value, meaning that the first snap for this document was an optimistic snap,
    // then create a null non-optimistic snap and replay that.
    if (prev == null) {
      _replay(currentSnap!.withValue(null));
    } else {
      _replay(prev);
    }
    // Otherwise we're rolling back an intermediate optimistic value that has either optimistic or real
    // updates after it. We don't need to re-emit the snap being rolled back since it is already
    // a stale value and just need to remove it from the optimistic updates snapshot linked list.
  } else {
    if (prev != null && prev.isOptimistic) {
      prev.next = next;
    }
    if (next != null && next.isOptimistic) {
      next.prev = prev;
    }
  }
}