[rackspace] add requirements to service

lib/fog/rackspace.rb

@@ -5,13 +5,9 @@ module Fog
   module Rackspace
 
     def self.authenticate(options)
-      unless @rackspace_api_key = options[:rackspace_api_key]
-        raise ArgumentError.new('rackspace_api_key is required to access rackspace')
-      end
-      unless @rackspace_username = options[:rackspace_username]
-        raise ArgumentError.new('rackspace_username is required to access rackspace')
-      end
       connection = Fog::Connection.new("https://auth.api.rackspacecloud.com")
+      @rackspace_api_key  = options[:rackspace_api_key]
+      @rackspace_username = options[:rackspace_username]
       response = connection.request({
         :expects  => 204,
         :headers  => {

lib/fog/rackspace/files.rb

@@ -3,6 +3,8 @@ module Fog
     module Files
       extend Fog::Service
 
+      requires :rackspace_api_key, :rackspace_username
+
       model_path 'fog/rackspace/models/files'
       model 'directory'
       model 'directories'

lib/fog/rackspace/servers.rb

@@ -3,6 +3,8 @@ module Fog
     module Servers
       extend Fog::Service
 
+      requires :rackspace_api_key, :rackspace_username
+
       model_path 'fog/rackspace/models/servers'
       model 'flavor'
       model 'flavors'

lib/fog/service.rb

@@ -10,6 +10,18 @@ module Fog
         module Collections; end
 
         def self.new(options={})
+          missing = []
+          for requirement in requirements
+            missing << requirement unless options[requirement]
+          end
+          unless missing.empty?
+            if missing.length == 1
+              raise(ArgumentError, [missing.first, "is required for this service"].join(' '))
+            else
+              raise(ArgumentError, [missing[0...-1].join(", "), 'and', missing[-1], 'are required for this service'].join(' '))
+            end
+          end
+
           unless @required
             for model in models
               require [@model_path, model].join('/')
@@ -68,6 +80,14 @@ module Fog
       @requests ||= []
     end
 
+    def requires(*args)
+      requirements.concat(args)
+    end
+
+    def requirements
+      @requirements ||= []
+    end
+
     def reset_data(keys=Mock.data.keys)
       Mock.reset_data(keys)
     end