structurallyShare function

Object? structurallyShare(Object? previous, Object? next, [ int depth = 0 ])

Deeply reuses unchanged JSON-like values from previous.

Implementation

Object? structurallyShare(Object? previous, Object? next, [int depth = 0]) {
  if (identical(previous, next) || previous == next) return previous;
  if (depth > 500) return next;
  if (previous is List && next is List) {
    final shared = List<dynamic>.of(next);
    var equal = previous.length == next.length;
    for (var index = 0; index < next.length; index++) {
      final value = structurallyShare(
        index < previous.length ? previous[index] : null,
        next[index],
        depth + 1,
      );
      shared[index] = value;
      equal =
          equal && index < previous.length && identical(value, previous[index]);
    }
    return equal ? previous : shared;
  }
  if (previous is Map && next is Map) {
    if (previous.keys.any((key) => key is! String) ||
        next.keys.any((key) => key is! String)) {
      return next;
    }
    final shared = <String, Object?>{};
    var equal = previous.length == next.length;
    for (final entry in next.entries) {
      final key = entry.key as String;
      final value = structurallyShare(previous[key], entry.value, depth + 1);
      shared[key] = value;
      equal =
          equal && previous.containsKey(key) && identical(value, previous[key]);
    }
    return equal ? previous : shared;
  }
  return next;
}