Cache JSON generation for individual messages

lib/segment/analytics/message.rb

@@ -1,13 +1,9 @@
-require 'forwardable'
-
 require 'segment/analytics/defaults'
 
 module Segment
   class Analytics
     # Represents a message to be sent to the API
     class Message
-      extend Forwardable
-
       def initialize(hash)
         @hash = hash
       end
@@ -16,8 +12,11 @@ def too_big?
         to_json.bytesize > Defaults::Message::MAX_BYTES
       end
 
-      def_delegators :@hash, :to_json # TODO: Cache and reuse
-      def_delegators :@hash, :[]
+      # Since the hash is expected to not be modified (set at initialization),
+      # the JSON version can be cached after the first computation.
+      def to_json(*args)
+        @json ||= @hash.to_json(*args)
+      end
     end
   end
 end

lib/segment/analytics/message_batch.rb

@@ -19,7 +19,7 @@ def <<(message)
         end
       end
 
-      def_delegators :@messages, :to_json # TODO: Cache and reuse
+      def_delegators :@messages, :to_json
       def_delegators :@messages, :clear
       def_delegators :@messages, :empty?
       def_delegators :@messages, :length

spec/segment/analytics/message_spec.rb

@@ -0,0 +1,35 @@
+require 'spec_helper'
+
+module Segment
+  class Analytics
+    describe Message do
+      describe '#to_json' do
+        it 'caches JSON conversions' do
+          # Keeps track of the number of times to_json was called
+          nested_obj = Class.new do
+            attr_reader :to_json_call_count
+
+            def initialize
+              @to_json_call_count = 0
+            end
+
+            def to_json(*_)
+              @to_json_call_count += 1
+              '{}'
+            end
+          end.new
+
+          message = Message.new('some_key' => nested_obj)
+          expect(nested_obj.to_json_call_count).to eq(0)
+
+          message.to_json
+          expect(nested_obj.to_json_call_count).to eq(1)
+
+          # When called a second time, the call count shouldn't increase
+          message.to_json
+          expect(nested_obj.to_json_call_count).to eq(1)
+        end
+      end
+    end
+  end
+end