@Override
            protected void transfer(Pipe pipe, Input input, Output output)
                    throws IOException
            {
                for (int number = input.readFieldNumber(wrappedSchema);; number = input
                        .readFieldNumber(wrappedSchema))
                {
                    switch (number)
                    {
                        case 0:
                            return;
                        case 1:
                            output.writeObject(number, pipe, ENTRY_PIPE_SCHEMA, true);
                            break;
                        default:
                            throw new ProtostuffException("The map was incorrectly serialized.");
                    }
                }
            }
This base class handles all the IO for reading and writing polymorphic fields. When a field's type is
polymorphic/dynamic (e.g interface/abstract/object), the type (id) needs to be written (ahead) before its
value/content to be able to deserialize it correctly.
<p>
The underlying impl will determine how the type (id) should be written.
<p>
An {@link IdStrategy} is standalone if the {@link #primaryGroup} is not set.

@author Leo Romanoff
@author David Yu
protostuff-runtime/src/main/java/io/protostuff/runtime/IdStrategy.java