1 files changed, 63 insertions, 0 deletions
diff --git a/activesupport/lib/active_support/descendants_tracker.rb b/activesupport/lib/active_support/descendants_tracker.rb
new file mode 100644
index 0000000000..05236d3162
--- /dev/null
+++ b/activesupport/lib/active_support/descendants_tracker.rb
@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+module ActiveSupport
+  # This module provides an internal implementation to track descendants
+  # which is faster than iterating through ObjectSpace.
+  module DescendantsTracker
+    @@direct_descendants = {}
+
+    class << self
+      def direct_descendants(klass)
+        @@direct_descendants[klass] || []
+      end
+
+      def descendants(klass)
+        arr = []
+        accumulate_descendants(klass, arr)
+        arr
+      end
+
+      def clear
+        if defined? ActiveSupport::Dependencies
+          @@direct_descendants.each do |klass, descendants|
+            if ActiveSupport::Dependencies.autoloaded?(klass)
+              @@direct_descendants.delete(klass)
+            else
+              descendants.reject! { |v| ActiveSupport::Dependencies.autoloaded?(v) }
+            end
+          end
+        else
+          @@direct_descendants.clear
+        end
+      end
+
+      # This is the only method that is not thread safe, but is only ever called
+      # during the eager loading phase.
+      def store_inherited(klass, descendant)
+        (@@direct_descendants[klass] ||= []) << descendant
+      end
+
+      private
+
+        def accumulate_descendants(klass, acc)
+          if direct_descendants = @@direct_descendants[klass]
+            acc.concat(direct_descendants)
+            direct_descendants.each { |direct_descendant| accumulate_descendants(direct_descendant, acc) }
+          end
+        end
+    end
+
+    def inherited(base)
+      DescendantsTracker.store_inherited(self, base)
+      super
+    end
+
+    def direct_descendants
+      DescendantsTracker.direct_descendants(self)
+    end
+
+    def descendants
+      DescendantsTracker.descendants(self)
+    end
+  end
+end