Behaviour for transformer modules.

By defining a transform_module/0 function on your protobuf message module you can add custom encoding and decoding logic for your message.

The Protobuf.new/1 function will not be called for structs that have a transform module, if you still want to emulate this behavior you can use Protobuf.TransformModule.InferFieldsFromEnum.

As an example we can use this to implement a message that will be decoded as a string value:

defmodule StringMessage do
  use Protobuf, syntax: :proto3

  field :value, 1, type: :string

  def transform_module(), do: MyTransformModule
end

The transformer behaviour implementation:

defmodule MyTransformModule do
  @behaviour Protobuf.TransformModule

  defmacro typespec(_default_ast) do
    quote do
      @type t() :: String.t()
    end
  end

  @impl true
  def encode(string, StringMessage) when is_binary(string), do: struct(StringMessage, value: string)

  @impl true
  def decode(%{value: string}, StringMessage), do: string
end

Notice that since the typespec/1 macro was introduced, transform modules can't depend on the types that they transform anymore in compile time, meaning struct syntax can't be used.