Evaluate host block only if the return value is used

lib/rack/canonical_host.rb

@@ -17,8 +17,7 @@ def call(env)
 
       return request.bad_request_response unless request.valid?
 
-      host = evaluate_host(env)
-      redirect = Redirect.new(env, host, options)
+      redirect = Redirect.new(env, host, options, &block)
 
       if redirect.canonical?
         app.call(env)
@@ -33,11 +32,5 @@ def call(env)
     attr_accessor :host
     attr_accessor :options
     attr_accessor :block
-
-  private
-
-    def evaluate_host(env)
-      block and block.call(env) or host
-    end
   end
 end

lib/rack/canonical_host/redirect.rb

@@ -15,12 +15,13 @@ class Redirect
         </html>
       HTML
 
-      def initialize(env, host, options={})
+      def initialize(env, host, options={}, &block)
         self.env = env
         self.host = host
         self.ignore = Array(options[:ignore])
         self.conditions = Array(options[:if])
         self.cache_control = options[:cache_control]
+        self.block = block
       end
 
       def canonical?
@@ -39,6 +40,7 @@ def response
       attr_accessor :ignore
       attr_accessor :conditions
       attr_accessor :cache_control
+      attr_accessor :block
 
     private
 
@@ -74,15 +76,19 @@ def ignored?
       end
 
       def known?
-        host.nil? || request_uri.host == host
+        evaluated_host.nil? || request_uri.host == evaluated_host
       end
 
       def new_url
         uri = request_uri.dup
-        uri.host = host
+        uri.host = evaluated_host
         uri.normalize.to_s
       end
 
+      def evaluated_host
+        @evaluated_host ||= block and block.call(env) or host
+      end
+
       def request_uri
         @request_uri ||= Addressable::URI.parse(Rack::Request.new(env).url)
       end