Add options for passing a string formatter

Author: lukearndt

README.md

@@ -98,6 +98,11 @@ RSpec.configure do |config|
   # Set this value to put all snapshots in a fixed directory
   config.snapshot_dir = "spec/fixtures/snapshots"
 
+  # Defaults to not applying any additional formatting
+  #
+  # Set this value to use a custom formatter for when `expect` receives a string
+  config.snapshot_formatter = MyFavoriteFormatter
+
   # Defaults to using the awesome_print gem to serialize values for snapshots
   #
   # Set this value to use a custom snapshot serializer
@@ -114,18 +119,31 @@ You can pass custom serializers to `rspec-snapshot` if you prefer. Pass a serial
 matcher as a config option:
 
 ```ruby
+# Set a custom string formatter for all tests
+RSpec.configure do |config|
+  config.snapshot_formatter = MyCoolGeneralFormatter
+end
+
 # Set a custom serializer for all tests
 RSpec.configure do |config|
   config.snapshot_serializer = MyCoolGeneralSerializer
 end
 
-# Set a custom serializer for this specific test
+# Set a custom formatter for this specific test
 expect(html_response).to(
-  match_snapshot('html_response', { snapshot_serializer: MyAwesomeHTMLSerializer })
+  match_snapshot('html_response', { snapshot_formatter: MyAwesomeFormatter })
+)
+
+# Set a custom serializer for this specific test
+expect(result_object).to(
+  match_snapshot('result_object', { snapshot_serializer: MyAwesomeSerializer })
 )
 ```
 
-Serializer classes are required to have one instance method `dump` which takes
+Formatters are required to have a single `call` method that takes the raw string value
+and returns a formatted string. If you want, you can use a proc or lambda.
+
+Serializer classes are required to have one instance method `dump`, which takes
 the value to be serialized and returns a string.
 
 ## Migration

lib/rspec/snapshot/configuration.rb

@@ -10,6 +10,8 @@ class Configuration; end
     def self.initialize_configuration(config)
       config.add_setting :snapshot_dir, default: :relative
 
+      config.add_setting :snapshot_formatter, default: nil
+
       config.add_setting :snapshot_serializer, default: nil
     end
 

lib/rspec/snapshot/matchers/match_snapshot.rb

@@ -14,11 +14,21 @@ def initialize(metadata, snapshot_name, config)
           @metadata = metadata
           @snapshot_name = snapshot_name
           @config = config
+          @formatter = formatter
           @serializer = serializer_class.new
           @snapshot_path = File.join(snapshot_dir, "#{@snapshot_name}.snap")
           create_snapshot_dir
         end
 
+        private def formatter
+          if @config[:snapshot_formatter]
+            @config[:snapshot_formatter]
+          elsif RSpec.configuration.snapshot_formatter
+            RSpec.configuration.snapshot_formatter
+          else
+            ->(value) { value }
+        end
+
         private def serializer_class
           if @config[:snapshot_serializer]
             @config[:snapshot_serializer]
@@ -44,7 +54,7 @@ def initialize(metadata, snapshot_name, config)
         end
 
         def matches?(actual)
-          @actual = serialize(actual)
+          @actual = serialize(format(actual))
 
           write_snapshot
 
@@ -56,6 +66,14 @@ def matches?(actual)
         # === is the method called when matching an argument
         alias === matches?
 
+        private def format(value)
+          if value.is_a?(String)
+            @formatter.call(value)
+          else
+            value
+          end
+        end
+
         private def serialize(value)
           if value.is_a?(String)
             value