class Change<T> {
  /// Value before change
  final T $old;

  /// Value after change
  final $new;

  final ListChangeOp op;

  final int pos;

  final DateTime time;
  final int batch;
  Change({this.$new, this.$old, this.batch, this.op, this.pos, DateTime time})
      : time = time ?? DateTime.now();
  String toString() => 'Change(new: ${$new}, old: ${$old})';

  Change.insert({this.$new, this.$old, this.batch, this.pos, DateTime time})
      : op = ListChangeOp.add,
        time = time ?? new DateTime.now();

  Change.set({this.$new, this.$old, this.batch, this.pos, DateTime time})
      : op = ListChangeOp.set,
        time = time ?? new DateTime.now();

  Change.remove({this.$new, this.$old, this.batch, this.pos, DateTime time})
      : op = ListChangeOp.remove,
        time = time ?? new DateTime.now();

  Change.clear({this.$new, this.$old, this.batch, DateTime time})
      : op = ListChangeOp.clear,
        pos = null,
        time = time ?? new DateTime.now();
}

typedef bool Condition();

/// Change operation
enum ListChangeOp { add, remove, clear, set }