1 files changed, 114 insertions, 0 deletions
diff --git a/activemodel/lib/active_model/attribute_mutation_tracker.rb b/activemodel/lib/active_model/attribute_mutation_tracker.rb
new file mode 100644
index 0000000000..9072460124
--- /dev/null
+++ b/activemodel/lib/active_model/attribute_mutation_tracker.rb
@@ -0,0 +1,114 @@
+# frozen_string_literal: true
+
+module ActiveModel
+  class AttributeMutationTracker # :nodoc:
+    OPTION_NOT_GIVEN = Object.new
+
+    def initialize(attributes)
+      @attributes = attributes
+      @forced_changes = Set.new
+    end
+
+    def changed_values
+      attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
+        if changed?(attr_name)
+          result[attr_name] = attributes[attr_name].original_value
+        end
+      end
+    end
+
+    def changes
+      attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
+        change = change_to_attribute(attr_name)
+        if change
+          result[attr_name] = change
+        end
+      end
+    end
+
+    def change_to_attribute(attr_name)
+      attr_name = attr_name.to_s
+      if changed?(attr_name)
+        [attributes[attr_name].original_value, attributes.fetch_value(attr_name)]
+      end
+    end
+
+    def any_changes?
+      attr_names.any? { |attr| changed?(attr) }
+    end
+
+    def changed?(attr_name, from: OPTION_NOT_GIVEN, to: OPTION_NOT_GIVEN)
+      attr_name = attr_name.to_s
+      forced_changes.include?(attr_name) ||
+        attributes[attr_name].changed? &&
+        (OPTION_NOT_GIVEN == from || attributes[attr_name].original_value == from) &&
+        (OPTION_NOT_GIVEN == to || attributes[attr_name].value == to)
+    end
+
+    def changed_in_place?(attr_name)
+      attributes[attr_name.to_s].changed_in_place?
+    end
+
+    def forget_change(attr_name)
+      attr_name = attr_name.to_s
+      attributes[attr_name] = attributes[attr_name].forgetting_assignment
+      forced_changes.delete(attr_name)
+    end
+
+    def original_value(attr_name)
+      attributes[attr_name.to_s].original_value
+    end
+
+    def force_change(attr_name)
+      forced_changes << attr_name.to_s
+    end
+
+    # TODO Change this to private once we've dropped Ruby 2.2 support.
+    # Workaround for Ruby 2.2 "private attribute?" warning.
+    protected
+
+      attr_reader :attributes, :forced_changes
+
+    private
+
+      def attr_names
+        attributes.keys
+      end
+  end
+
+  class NullMutationTracker # :nodoc:
+    include Singleton
+
+    def changed_values(*)
+      {}
+    end
+
+    def changes(*)
+      {}
+    end
+
+    def change_to_attribute(attr_name)
+    end
+
+    def any_changes?(*)
+      false
+    end
+
+    def changed?(*)
+      false
+    end
+
+    def changed_in_place?(*)
+      false
+    end
+
+    def forget_change(*)
+    end
+
+    def original_value(*)
+    end
+
+    def force_change(*)
+    end
+  end
+end