add example to mimic #42

README.markdown

@@ -182,6 +182,25 @@ null_io.foobar                  # =>
 #  <null:IO>:NullIO (NoMethodError)
 ```
 
+It is also possible to give `mimic` the `example` option to stub methods of a specific instance.
+It can be useful when the instance define its own methods.
+
+```ruby
+require "naught"
+require "logging"
+
+log = Logging.logger["test"]
+log.info
+
+NullLog = Naught.build do |config|
+  config.mimic log.class, example: log
+end
+
+null_log = NullLog.new
+null_log.info                  # => nil
+
+```
+
 There is also `impersonate` which takes `mimic` one step further. The
 generated null class will be derived from the impersonated class. This
 is handy when refitting legacy code that contains type checks.

lib/naught/null_class_builder/commands/mimic.rb

@@ -5,13 +5,14 @@ module Naught
   class NullClassBuilder
     module Commands
       class Mimic < Naught::NullClassBuilder::Command
-        attr_reader :class_to_mimic, :include_super
+        attr_reader :class_to_mimic, :include_super, :instance_to_mimic
 
         def initialize(builder, class_to_mimic, options = {})
           super(builder)
 
           @class_to_mimic = class_to_mimic
           @include_super = options.fetch(:include_super) { true }
+          @instance_to_mimic = options.fetch(:example) { nil }
 
           builder.base_class   = root_class_of(class_to_mimic)
           builder.inspect_proc = lambda { "<null:#{class_to_mimic}>" }
@@ -33,7 +34,13 @@ def root_class_of(klass)
         end
 
         def methods_to_stub
-          class_to_mimic.instance_methods(include_super) - Object.instance_methods
+          methods_to_mimic = if instance_to_mimic
+            instance_to_mimic.public_methods(include_super)
+          else
+            class_to_mimic.instance_methods(include_super)
+          end
+
+          methods_to_mimic - Object.instance_methods
         end
       end
     end

spec/mimic_spec.rb

@@ -78,6 +78,20 @@ def notify_of_overdue_books(titles)
       expect(null).to_not respond_to(:login)
     end
   end
+
+  describe 'with an instance as example' do
+
+    let(:mimic_class) do
+      Naught.build do |b|
+        b.mimic User, :example => LibraryPatron.new
+      end
+    end
+
+    it 'responds to method defined on the example instance' do
+      expect(null).to respond_to(:member?)
+    end
+  end
+
 end
 
 describe 'using mimic with black_hole' do