Add config.define_derived_metadata.

Fixes #969 and supercedes #1089.

Changelog.md

@@ -97,6 +97,9 @@ Enhancements:
 * Add `disable_monkey_patching!` config option that disables all monkey
   patching from whatever pieces of RSpec you use. (Alexey Fedorov)
 * Add `Pathname` support for setting all output streams. (Aaron Kromer)
+* Add `config.define_derived_metadata`, which can be used to apply
+  additional metadata to all groups or examples that match a given
+  filter. (Myron Marston)
 
 Bug Fixes:
 

lib/rspec/core/configuration.rb

@@ -300,6 +300,7 @@ def initialize
         @profile_examples = false
         @requires = []
         @libs = []
+        @derived_metadata_blocks = []
       end
 
       # @private
@@ -1209,6 +1210,32 @@ def disable_monkey_patching!
       # @private
       attr_accessor :disable_monkey_patching
 
+      # Defines a callback that can assign derived metadata values.
+      #
+      # @param filters [Array<Symbol>, Hash] metadata filters that determine which example
+      #   or group metadata hashes the callback will be triggered for. If none are given,
+      #   the callback will be run against the metadata hashes of all groups and examples.
+      # @yieldparam metadata [Hash] original metadata hash from an example or group. Mutate this in
+      #   your block as needed.
+      #
+      # @example
+      #   RSpec.configure do |config|
+      #     # Tag all groups and examples in the spec/unit directory with :type => :unit
+      #     config.define_derived_metadata(:file_path => %r{/spec/unit/}) do |metadata|
+      #       metadata[:type] = :unit
+      #     end
+      #   end
+      def define_derived_metadata(*filters, &block)
+        @derived_metadata_blocks << [Metadata.build_hash_from(filters), block]
+      end
+
+      # @private
+      def apply_derived_metadata_to(metadata)
+        @derived_metadata_blocks.each do |filter, block|
+          block.call(metadata) if filter.empty? || MetadataFilter.any_apply?(filter, metadata)
+        end
+      end
+
     private
 
       def get_files_to_run(paths)

lib/rspec/core/metadata.rb

@@ -84,6 +84,7 @@ def populate
 
           populate_location_attributes
           metadata.update(user_metadata)
+          RSpec.configuration.apply_derived_metadata_to(metadata)
         end
 
       private

spec/rspec/core/configuration_spec.rb

@@ -1077,6 +1077,107 @@ def metadata_hash(*args)
       end
     end
 
+    describe "#define_derived_metadata" do
+      it 'allows the provided block to mutate example group metadata' do
+        RSpec.configuration.define_derived_metadata do |metadata|
+          metadata[:reverse_description] = metadata[:description].reverse
+        end
+
+        group = RSpec.describe("My group")
+        expect(group.metadata).to include(:description => "My group", :reverse_description => "puorg yM")
+      end
+
+      it 'allows the provided block to mutate example metadata' do
+        RSpec.configuration.define_derived_metadata do |metadata|
+          metadata[:reverse_description] = metadata[:description].reverse
+        end
+
+        ex = RSpec.describe("My group").example("foo")
+        expect(ex.metadata).to include(:description => "foo", :reverse_description => "oof")
+      end
+
+      it 'allows multiple configured blocks to be applied, in order of definition' do
+        RSpec.configure do |c|
+          c.define_derived_metadata { |m| m[:b1_desc] = m[:description] + " (block 1)" }
+          c.define_derived_metadata { |m| m[:b2_desc] = m[:b1_desc]     + " (block 2)" }
+        end
+
+        group = RSpec.describe("bar")
+        expect(group.metadata).to include(:b1_desc => "bar (block 1)", :b2_desc => "bar (block 1) (block 2)")
+      end
+
+      it "derives metadata before the group or example blocks are eval'd so their logic can depend on the derived metadata" do
+        RSpec.configure do |c|
+          c.define_derived_metadata(:foo) do |metadata|
+            metadata[:bar] = "bar"
+          end
+        end
+
+        group_bar_value = example_bar_value = nil
+
+        RSpec.describe "Group", :foo do
+          group_bar_value = metadata[:bar]
+          example_bar_value = example("ex", :foo).metadata[:bar]
+        end
+
+        expect(group_bar_value).to eq("bar")
+        expect(example_bar_value).to eq("bar")
+      end
+
+      context "when passed a metadata filter" do
+        it 'only applies to the groups and examples that match that filter' do
+          RSpec.configure do |c|
+            c.define_derived_metadata(:apply => true) do |metadata|
+              metadata[:reverse_description] = metadata[:description].reverse
+            end
+          end
+
+          g1 = RSpec.describe("G1", :apply)
+          g2 = RSpec.describe("G2")
+          e1 = g1.example("E1")
+          e2 = g2.example("E2", :apply)
+          e3 = g2.example("E3")
+
+          expect(g1.metadata).to include(:reverse_description => "1G")
+          expect(g2.metadata).not_to include(:reverse_description)
+
+          expect(e1.metadata).to include(:reverse_description => "1E")
+          expect(e2.metadata).to include(:reverse_description => "2E")
+          expect(e3.metadata).not_to include(:reverse_description)
+        end
+
+        it 'applies if any of multiple filters apply (to align with module inclusion semantics)' do
+          RSpec.configure do |c|
+            c.define_derived_metadata(:a => 1, :b => 2) do |metadata|
+              metadata[:reverse_description] = metadata[:description].reverse
+            end
+          end
+
+          g1 = RSpec.describe("G1", :a => 1)
+          g2 = RSpec.describe("G2", :b => 2)
+          g3 = RSpec.describe("G3", :c => 3)
+
+          expect(g1.metadata).to include(:reverse_description => "1G")
+          expect(g2.metadata).to include(:reverse_description => "2G")
+          expect(g3.metadata).not_to include(:reverse_description)
+        end
+
+        it 'allows a metadata filter to be passed as a raw symbol' do
+          RSpec.configure do |c|
+            c.define_derived_metadata(:apply) do |metadata|
+              metadata[:reverse_description] = metadata[:description].reverse
+            end
+          end
+
+          g1 = RSpec.describe("G1", :apply)
+          g2 = RSpec.describe("G2")
+
+          expect(g1.metadata).to include(:reverse_description => "1G")
+          expect(g2.metadata).not_to include(:reverse_description)
+        end
+      end
+    end
+
     describe "#add_setting" do
       describe "with no modifiers" do
         context "with no additional options" do