```ruby
# frozen_string_literal: true

require "gir_ffi/builders/base_type_builder"
require "gir_ffi/builders/mapping_method_builder"
require "gir_ffi/callback_base"

module GirFFI
  module Builders
    # Implements the creation of a callback type. The type will be
    # attached to the appropriate namespace module, and will be defined
    # as a callback for FFI.
    class CallbackBuilder < BaseTypeBuilder
      def setup_class
        setup_callback
        setup_constants
        klass.class_eval mapping_method_definition, __FILE__, __LINE__
      end

      def setup_callback
        optionally_define_constant klass, :Callback do
          lib.callback callback_sym, argument_ffi_types, return_ffi_type
        end
      end

      def klass
        @klass ||= get_or_define_class container_module, @classname, CallbackBase
      end

      def container_module
        @container_module ||=
          if @info.container
            type_info = @info.container
            field_info = type_info.container
            container_class_info = field_info.container
            namespace_module.const_get container_class_info.safe_name
          else
            namespace_module
          end
      end

      def mapping_method_definition
        MappingMethodBuilder.for_callback(info).method_definition
      end

      def callback_sym
        @classname.to_sym
      end

      def argument_ffi_types
        @argument_ffi_types ||= @info.argument_ffi_types
      end

      def return_ffi_type
        @return_ffi_type ||= @info.return_ffi_type
      end
    end
  end
end
```

Make CallbackBuilder methods private where possible
user
```ruby
# frozen_string_literal: true

require "gir_ffi/builders/base_type_builder"
require "gir_ffi/builders/mapping_method_builder"
require "gir_ffi/callback_base"

module GirFFI
  module Builders
    # Implements the creation of a callback type. The type will be
    # attached to the appropriate namespace module, and will be defined
    # as a callback for FFI.
    class CallbackBuilder < BaseTypeBuilder
      def setup_class
        setup_callback
        setup_constants
        klass.class_eval mapping_method_definition, __FILE__, __LINE__
      end

      def mapping_method_definition
        MappingMethodBuilder.for_callback(info).method_definition
      end

      def argument_ffi_types
        @argument_ffi_types ||= @info.argument_ffi_types
      end

      def return_ffi_type
        @return_ffi_type ||= @info.return_ffi_type
      end

      private

      def setup_callback
        optionally_define_constant klass, :Callback do
          lib.callback callback_sym, argument_ffi_types, return_ffi_type
        end
      end

      def klass
        @klass ||= get_or_define_class container_module, @classname, CallbackBase
      end

      def container_module
        @container_module ||=
          if @info.container
            type_info = @info.container
            field_info = type_info.container
            container_class_info = field_info.container
            namespace_module.const_get container_class_info.safe_name
          else
            namespace_module
          end
      end

      def callback_sym
        @classname.to_sym
      end
    end
  end
end
```
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