add possibility to just use a parser on a per call basis

lib/multi_json.rb

@@ -70,19 +70,23 @@ def engine=(new_engine)
   # * <tt>:yajl</tt>
   # * <tt>:nsjsonserialization</tt> (MacRuby only)
   def use(new_adapter)
+    @adapter = load_adapter(new_adapter)
+  end
+  alias :adapter= :use
+
+  def load_adapter(new_adapter)
     case new_adapter
     when String, Symbol
       require "multi_json/adapters/#{new_adapter}"
-      @adapter = MultiJson::Adapters.const_get("#{new_adapter.to_s.split('_').map{|s| s.capitalize}.join('')}")
+      MultiJson::Adapters.const_get("#{new_adapter.to_s.split('_').map{|s| s.capitalize}.join('')}")
     when NilClass
-      @adapter = nil
+      nil
     when Class
-      @adapter = new_adapter
+      new_adapter
     else
       raise "Did not recognize your adapter specification. Please specify either a symbol or a class."
     end
   end
-  alias :adapter= :use
 
   # TODO: Remove for 2.0 release (but no sooner)
   def decode(string, options={})
@@ -95,12 +99,22 @@ def decode(string, options={})
   # <b>Options</b>
   #
   # <tt>:symbolize_keys</tt> :: If true, will use symbols instead of strings for the keys.
+  # <tt>:adapter</tt> :: If set, the selected engine will be used just for the call.
   def load(string, options={})
+    adapter = current_adapter(options)
     adapter.load(string, options)
   rescue adapter::ParseError => exception
     raise DecodeError.new(exception.message, exception.backtrace, string)
   end
 
+  def current_adapter(options)
+    if new_adapter = (options || {}).delete(:adapter)
+      load_adapter(new_adapter)
+    else
+      adapter
+    end
+  end
+
   # TODO: Remove for 2.0 release (but no sooner)
   def encode(object, options={})
     deprecate("MultiJson.encode is deprecated and will be removed in the next major version. Use MultiJson.dump instead.")
@@ -109,6 +123,7 @@ def encode(object, options={})
 
   # Encodes a Ruby object as JSON.
   def dump(object, options={})
+    adapter = current_adapter(options)
     adapter.dump(object, options)
   end
 

spec/multi_json_spec.rb

@@ -59,6 +59,21 @@
       MultiJson.use MockDecoder
       MultiJson.adapter.name.should == 'MockDecoder'
     end
+
+    context "using one-shot parser" do
+      before(:each) do
+        require 'multi_json/adapters/json_pure'
+        MultiJson::Adapters::JsonPure.should_receive(:dump).exactly(1).times.and_return('dump_something')
+        MultiJson::Adapters::JsonPure.should_receive(:load).exactly(1).times.and_return('load_something')
+      end
+
+      it "should use the defined parser just for the call" do
+        MultiJson.use :yajl
+        MultiJson.dump('', :adapter => :json_pure).should eql('dump_something')
+        MultiJson.load('', :adapter => :json_pure).should eql('load_something')
+        MultiJson.adapter.to_s.should eql("MultiJson::Adapters::Yajl")
+      end
+    end
   end
 
   %w(json_gem json_pure nsjsonserialization oj ok_json yajl).each do |adapter|