allow inspectors to be registered for different objects

lib/rspec/support/object_formatter.rb

@@ -9,6 +9,7 @@ class ObjectFormatter # rubocop:disable Metrics/ClassLength
       ELLIPSIS = "..."
 
       attr_accessor :max_formatted_output_length
+      attr_reader :inspectors
 
       # Methods are deferred to a default instance of the class to maintain the interface
       # For example, calling ObjectFormatter.format is still possible
@@ -24,9 +25,14 @@ def self.prepare_for_inspection(object)
         default_instance.prepare_for_inspection(object)
       end
 
+      def self.inspectors
+        default_instance.inspectors
+      end
+
       def initialize(max_formatted_output_length=200)
         @max_formatted_output_length = max_formatted_output_length
         @current_structure_stack = []
+        @inspectors = Inspectors.new(INSPECTOR_CLASSES.dup)
       end
 
       def format(object)
@@ -60,7 +66,7 @@ def prepare_for_inspection(object)
         when Hash
           prepare_hash(object)
         else
-          inspector_class = INSPECTOR_CLASSES.find { |inspector| inspector.can_inspect?(object) }
+          inspector_class = inspectors.find_for(object)
           inspector_class.new(object, self)
         end
       end
@@ -257,6 +263,27 @@ def inspect
         classes.delete(BigDecimalInspector) if RUBY_VERSION >= '2.4'
       end
 
+      # A simple container for inspector classes, so that inspector classes can be registered.
+      # Inspectors registered later will take precedence over previously registered ones.
+      # @api private
+      class Inspectors
+        def initialize(inspectors)
+          @inspectors = inspectors
+        end
+
+        def register(inspector)
+          unless inspector.respond_to?(:can_inspect?)
+            raise ArgumentError, 'provided inspector does not respond to can_inspect?'
+          end
+
+          @inspectors.unshift(inspector)
+        end
+
+        def find_for(object)
+          @inspectors.find { |inspector| inspector.can_inspect?(object) }
+        end
+      end
+
     private
 
       # Returns the substring defined by the start_index and end_index

spec/rspec/support/object_formatter_spec.rb

@@ -370,6 +370,64 @@ def inspect
           expect(formatter.format('Test String Of A Longer Length')).to eq('"Test String Of A Longer Length"')
         end
       end
+
+      context 'with a custom registered inspector' do
+        class TimeObjectTest
+          SECONDS_PER_MINUTE = 60
+
+          def initialize(minutes, seconds)
+            @minutes = minutes
+            @seconds = seconds
+          end
+
+          def to_i
+            (@minutes * SECONDS_PER_MINUTE) + @seconds
+          end
+
+          def inspect
+            "#{@minutes} minutes and #{@seconds} seconds"
+          end
+
+          def ==(other)
+            self.to_i == other.to_i
+          end
+
+          alias_method :eql?, :==
+
+          def hash
+            [@minutes, @seconds].hash
+          end
+        end
+
+        class CustomInspector < ObjectFormatter::BaseInspector
+          def self.can_inspect?(object)
+            TimeObjectTest === object
+          end
+
+          def inspect
+            object.to_i.inspect
+          end
+        end
+
+        before do
+          ObjectFormatter.inspectors.register(CustomInspector)
+        end
+
+        subject(:output) do
+          ObjectFormatter.format(input)
+        end
+
+        let(:input) do
+          [
+            { :key => TimeObjectTest.new(1, 30) },
+            { :key => TimeObjectTest.new(0, 90) }
+          ]
+        end
+
+        it "uses the custom inspector" do
+          expect(output).to eq('[{:key=>90}, {:key=>90}]')
+        end
+      end
     end
   end
 end