Inreoduces UUID types

This introduces a UUID typecasting option, which effectively functions as a string, but coerced to nil unless the string exactly matches the UUID pattern.

This fixes issue cgriego#174

lib/active_attr/typecasting.rb

@@ -7,6 +7,7 @@
 require "active_attr/typecasting/integer_typecaster"
 require "active_attr/typecasting/object_typecaster"
 require "active_attr/typecasting/string_typecaster"
+require "active_attr/typecasting/uuid_typecaster"
 require "active_attr/typecasting/unknown_typecaster_error"
 
 module ActiveAttr
@@ -34,6 +35,7 @@ module Typecasting
       Integer    => IntegerTypecaster,
       Object     => ObjectTypecaster,
       String     => StringTypecaster,
+      UUID       => UUIDTypecaster,
     }.freeze
 
     # Typecasts a value using a Class

lib/active_attr/typecasting/uuid_typecaster.rb

@@ -0,0 +1,32 @@
+module ActiveAttr
+  class UUID
+  end
+
+  module Typecasting
+    # Typecasts an Object to a v4 UUID
+    #
+    # @example Usage
+    #   UUIDTypecaster.new.call("6ed806a8-60ec-4930-ae63-9075bf37ca41") #=> "6ed806a8-60ec-4930-ae63-9075bf37ca41"
+    #   UUIDTypecaster.new.call("6ed806a8-60ec") #=> nil
+    #
+    # @since 0.15.0
+    class UUIDTypecaster
+      # Typecasts an object to a v4 UUID
+      #
+      # Attempts to convert using SecureRandom
+      #
+      # @example Typecast a partial UUID
+      #   typecaster.call("6ed806a8-60ec") #=> nil
+      #
+      # @param [Object, #to_s] value The object to typecast
+      #
+      # @return [String, nil] The result of typecasting
+      #
+      # @since 0.5.0
+      def call(value)
+        regex = /\b[0-9a-f]{8}\b-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-\b[0-9a-f]{12}\b/
+        value.to_s.match?(regex) ? value : nil
+      end
+    end
+  end
+end

spec/unit/active_attr/typecasting/uuid_typecaster_spec.rb

@@ -0,0 +1,25 @@
+require "spec_helper"
+require "active_attr/typecasting/uuid_typecaster"
+
+module ActiveAttr
+  module Typecasting
+    describe UUIDTypecaster do
+      subject(:typecaster) { described_class.new }
+
+      describe "#call" do
+        it "returns the original string for a UUID" do
+          value = "6ed806a8-60ec-4930-ae63-9075bf37ca41"
+          typecaster.call(value).should equal value
+        end
+
+        it "returns nil for an invalid UUID" do
+          typecaster.call("6ed806a8-60ec-4930-ae63-9075bf37").should be_nil
+        end
+
+        it "returns nil for nil" do
+          typecaster.call(nil).should be_nil
+        end
+      end
+    end
+  end
+end