[Chore] Move transform_key method to Alba module

The main purpose of this commit is to prepare for type export feature.
Also this method is generic enough, so it's safe to extract it for
smaller `Alba::Resource`.

lib/alba.rb

@@ -144,6 +144,26 @@ def regularize_key(key)
       @symbolize_keys ? key.to_sym : key.to_s
     end
 
+    # Transform a key with given transform_type
+    #
+    # @param key [String] a target key
+    # @param transform_type [Symbol] a transform type, either one of `camel`, `lower_camel`, `dash` or `snake`
+    # @return [String]
+    def transform_key(key, transform_type:)
+      raise Alba::Error, 'Inflector is nil. You must set inflector before transforming keys.' unless inflector
+
+      key = key.to_s
+
+      k = case transform_type
+          when :camel then inflector.camelize(key)
+          when :lower_camel then inflector.camelize_lower(key)
+          when :dash then inflector.dasherize(key)
+          when :snake then inflector.underscore(key)
+          else raise Alba::Error, "Unknown transform type: #{transform_type}"
+          end
+      regularize_key(k)
+    end
+
     # Register types, used for both builtin and custom types
     #
     # @see Alba::Type

lib/alba/resource.rb

@@ -236,23 +236,7 @@ def handle_error(error, obj, key, attribute, hash)
       def transform_key(key)
         return Alba.regularize_key(key) if @_transform_type == :none || key.nil? || key.empty? # We can skip transformation
 
-        inflector = Alba.inflector
-        raise Alba::Error, 'Inflector is nil. You must set inflector before transforming keys.' unless inflector
-
-        Alba.regularize_key(_transform_key(inflector, key.to_s))
-      end
-
-      def _transform_key(inflector, key)
-        case @_transform_type
-        when :camel then inflector.camelize(key)
-        when :lower_camel then inflector.camelize_lower(key)
-        when :dash then inflector.dasherize(key)
-        when :snake then inflector.underscore(key)
-        else
-          # :nocov:
-          raise Alba::Error, "Unknown transform type: #{@_transform_type}"
-          # :nocov:
-        end
+        Alba.transform_key(key, transform_type: @_transform_type)
       end
 
       def fetch_attribute(obj, key, attribute) # rubocop:disable Metrics/CyclomaticComplexity

test/alba_test.rb

@@ -290,4 +290,48 @@ def test_inline_serialization_for_multiple_root_keys
       end
     )
   end
+
+  def test_transform_key_with_camel
+    Alba.inflector = :active_support
+    assert_equal(
+      'FooBar',
+      Alba.transform_key(:foo_bar, transform_type: :camel)
+    )
+    Alba.inflector = nil
+  end
+
+  def test_transform_key_with_lower_camel
+    Alba.inflector = :active_support
+    assert_equal(
+      'fooBar',
+      Alba.transform_key('foo_bar', transform_type: :lower_camel)
+    )
+    Alba.inflector = nil
+  end
+
+  def test_transform_key_with_dash
+    Alba.inflector = :active_support
+    assert_equal(
+      'foo-bar',
+      Alba.transform_key(:foo_bar, transform_type: :dash)
+    )
+    Alba.inflector = nil
+  end
+
+  def test_transform_key_with_snake
+    Alba.inflector = :active_support
+    assert_equal(
+      'foo_bar',
+      Alba.transform_key('FooBar', transform_type: :snake)
+    )
+    Alba.inflector = nil
+  end
+
+  def test_transform_key_with_unknown
+    Alba.inflector = :active_support
+    assert_raises(Alba::Error) do
+      Alba.transform_key(:foo_bar, transform_type: :this_is_an_error)
+    end
+    Alba.inflector = nil
+  end
 end