Add basic reducer class Obredux::Reducer

lib/obredux.rb

@@ -20,4 +20,24 @@ module Obredux
       @state = reducer_klass.new(state, action).call
     end
   end
+
+  class Reducer
+    def initialize(state, action)
+      @state = state
+      @action = action
+    end
+
+    def call
+      @state = initial_state if state.equal? UNDEFINED
+      reduce
+    end
+
+  private
+
+    attr_reader :state, :action
+
+    def initial_state
+      raise NotImplementedError, "#{self.class}#initial_state"
+    end
+  end
 end

lib/reducer.rb

@@ -1,12 +1,7 @@
 # frozen_string_literal: true
 
-class Reducer
-  attr_reader :state, :action
-
-  def initialize(state, action)
-    @state = state
-    @action = action
-  end
+class Reducer < Obredux::Reducer
+private
 
   def initial_state
     {
@@ -83,9 +78,7 @@ class Reducer
     }.freeze
   end
 
-  def call
-    @state = initial_state if state == Obredux::UNDEFINED
-
+  def reduce
     case action
     when Actions::LoadFriends
       load_friends