extract TopicNotifier class from topic

app/models/topic_notifier.rb

@@ -0,0 +1,42 @@
+class TopicNotifier
+  def initialize(topic)
+    @topic = topic
+  end
+
+  { :watch!    => :watching,
+    :tracking! => :tracking,
+    :regular!  => :regular,
+    :muted!    => :muted }.each_pair do |method_name, level|
+
+    define_method method_name do |user_id|
+      change_level user_id, level
+    end
+
+  end
+
+  def created_topic!(user_id)
+    change_level user_id, :watching, :created_topic
+  end
+
+  # Enable/disable the mute on the topic
+  def toggle_mute(user_id)
+    change_level user_id, (muted?(user_id) ? levels[:regular] : levels[:muted])
+  end
+
+  def muted?(user_id)
+    tu = @topic.topic_users.where(user_id: user_id).first
+    tu && tu.notification_level == levels[:muted]
+  end
+
+  private
+
+  def levels
+    @notification_levels ||= TopicUser.notification_levels
+  end
+
+  def change_level(user_id, level, reason=nil)
+    attrs = {notification_level: levels[level]}
+    attrs.merge!(notifications_reason_id: TopicUser.notification_reasons[reason]) if reason
+    TopicUser.change(user_id, @topic.id, attrs)
+  end
+end