Extensions let you set extra fields for previously defined messages(even for messages in other packages) without changing the original message.

To load extensions you should call Protobuf.load_extensions/0 when your application starts:

def start(_type, _args) do
  Protobuf.load_extensions()
  Supervisor.start_link([], strategy: :one_for_one)
end

Examples

# protoc should be used to generate the code instead of writing by hand.
defmodule Foo do
  use Protobuf, syntax: :proto2

  extensions([{100, 101}, {1000, 536_870_912}])
end

# This module is generated for all "extend" calls in one file.
# This module is needed in `*_extension` function because the field name is scoped
# in the proto file.
defmodule Ext.PbExtension do
  use Protobuf, syntax: :proto2

  extend Foo, :my_custom, 1047, optional: true, type: :string
end

foo = %Foo{}
Foo.put_extension(foo, Ext.PbExtension, :my_custom, "Custom field")
Foo.get_extension(foo, Ext.PbExtension, :my_custom)