attach method Null safety

Attaches this finalizer to value.

When value is no longer accessible to the program, while still having an attachement to this finalizer, the callback of this finalizer may be called with finalizationToken as argument. The callback may be called at most once per active attachment, ones which have not been detached by calling Finalizer.detach.

If a non-null detach value is provided, that object can be passed to Finalizer.detach to remove the attachment again.

The value and detach arguments do not count towards those objects being accessible to the program. Both must be objects supported as an Expando key. They may be the same object.

Example:

class Database {
  // Keeps the finalizer itself reachable, otherwise it might be disposed
  // before the finalizer callback gets a chance to run.
  static final Finalizer<DBConnection> _finalizer =
      Finalizer((connection) => connection.close());

  factory Database.connect() {
    // Wraps the connection in a nice user API,
    // *and* closes connection if the user forgets to.
    final connection = DBConnection.connect();
    final wrapper = Database._fromConnection();
    // Get finalizer callback when `wrapper` is no longer reachable.
    _finalizer.attach(wrapper, connection, detach: wrapper);
    return wrapper;
  }

  Database._fromConnection();

  // Some useful methods.
}

Multiple objects may be attached using the same finalization token, and the finalizer can be attached multiple times to the same object with different, or the same, finalization token.