[json_engine] Add comments and more specific rescue and add tests

lib/rabl/json_engine.rb

@@ -1,3 +1,10 @@
+# Defines the default JSON engine for RABL when rendering JSON is invoked on a template.
+# You can define your own json engine by creating an object that responds to the `encode` method
+# and setting the corresponding configuration option:
+#
+#     config.json_engine = ActiveSupport::JSON
+#
+
 require 'multi_json'
 require 'singleton'
 
@@ -13,10 +20,14 @@ def initialize
 
     def set(engine_name_or_class)
       @current_engine = begin
-        MultiJson.respond_to?(:use) ? MultiJson.use(engine_name_or_class) : MultiJson.engine = engine_name_or_class
-      rescue
+        MultiJson.respond_to?(:use) ?
+          MultiJson.use(engine_name_or_class) :
+          MultiJson.engine = engine_name_or_class
+      rescue RuntimeError => e #
+        # Re-raise if engine_name_or_class is invalid
+        raise e unless engine_name_or_class.respond_to?(:encode)
         engine_name_or_class
       end
     end
-  end
-end
+  end # JsonEngine
+end # Rabl

test/configuration_test.rb

@@ -1,5 +1,4 @@
 require File.expand_path('../teststrap', __FILE__)
-require File.expand_path('../../lib/rabl', __FILE__)
 
 context 'Rabl::Configuration' do
   context 'defaults' do
@@ -24,7 +23,7 @@
     end
 
     asserts('uses a custom JSON engine') { topic.json_engine.to_s =~ /MultiJson.*::Yajl/ }
-  end
+  end # custom json, symbol
 
   context 'custom JSON engine configured as Class' do
     setup do
@@ -34,5 +33,13 @@
     end
 
     asserts('uses a custom JSON engine') { topic.json_engine.to_s == 'ActiveSupport::JSON' }
-  end
+  end # custom JSON, class
+
+  context 'invalid JSON engine configured' do
+    asserts {
+      Rabl.configure do |c|
+        c.json_engine = Kernel
+      end
+    }.raises(RuntimeError)
+  end # invalid
 end