create_adapter_manually.md

Create adapter manually

Sometimes it might be necessary to create a custom TypeAdapter. You can do that by extending the TypeAdapter class. Make sure to specify the generic argument.

!> Test your custom TypeAdapter thoroughly. If it does not work correctly, it may corrupt your box.

It is very easy to implement a TypeAdapter. Keep in mind that TypeAdapters have to be immutable! Here is the DateTimeAdapter used by Hive internally:

class DateTimeAdapter extends TypeAdapter<DateTime> {
  @override
  final typeId = 16;

  @override
  DateTime read(BinaryReader reader) {
    final micros = reader.readInt();
    return DateTime.fromMicrosecondsSinceEpoch(micros);
  }

  @override
  void write(BinaryWriter writer, DateTime obj) {
    writer.writeInt(obj.microsecondsSinceEpoch);
  }
}

!> As of Hive 1.3.0, all adapters require a typeId instance variable!

The typeId instance variable assigns the number to be registered to that adapter. It has to be unique between all adapters. The read() method is called when your object has to be read from the disk. Use the BinaryReader to read all the properties of your object. In the above sample, it is only an int containing microsecondsSinceEpoch.
The write() method is the same just for writing the object to the disk.

?> Make sure, you read properties in the same order you have written them before.