Add delayed ordering strategy

Author: JonRowe

lib/rspec/core/ordering.rb

@@ -78,6 +78,30 @@ def order(list)
         end
       end
 
+      # @private
+      # A strategy which delays looking up the ordering until needed
+      class Delayed
+        def initialize(registry, name)
+          @registry = registry
+          @name = name
+        end
+
+        def order(list)
+          strategy.order(list)
+        end
+
+        private
+
+        def strategy
+          @strategy ||= lookup_strategy
+        end
+
+        def lookup_strategy
+          raise "Undefined ordering strategy #{@name.inspect}" unless @registry.has_strategy?(@name)
+          @registry.fetch(@name)
+        end
+      end
+
       # @private
       # Stores the different ordering strategies.
       class Registry

spec/rspec/core/ordering_spec.rb

@@ -104,6 +104,18 @@ def order_with(seed)
         end
       end
 
+      RSpec.describe Delayed do
+        let(:registry) { Registry.new(Configuration.new) }
+
+        it 'looks up a strategy to order the list later on' do
+          strategy = Delayed.new(registry, :reverse)
+          expect { strategy.order([1, 2, 3, 4]) }.to raise_error("Undefined ordering strategy :reverse")
+
+          registry.register(:reverse, Custom.new(proc { |list| list.reverse }))
+          expect(strategy.order([1, 2, 3, 4])).to eq([4, 3, 2, 1])
+        end
+      end
+
       RSpec.describe Registry do
         let(:configuration) { Configuration.new }
         subject(:registry) { Registry.new(configuration) }