1 files changed, 82 insertions, 0 deletions
diff --git a/activesupport/lib/active_support/lazy_load_hooks.rb b/activesupport/lib/active_support/lazy_load_hooks.rb
new file mode 100644
index 0000000000..a6b096a973
--- /dev/null
+++ b/activesupport/lib/active_support/lazy_load_hooks.rb
@@ -0,0 +1,82 @@
+# frozen_string_literal: true
+
+module ActiveSupport
+  # lazy_load_hooks allows Rails to lazily load a lot of components and thus
+  # making the app boot faster. Because of this feature now there is no need to
+  # require <tt>ActiveRecord::Base</tt> at boot time purely to apply
+  # configuration. Instead a hook is registered that applies configuration once
+  # <tt>ActiveRecord::Base</tt> is loaded. Here <tt>ActiveRecord::Base</tt> is
+  # used as example but this feature can be applied elsewhere too.
+  #
+  # Here is an example where +on_load+ method is called to register a hook.
+  #
+  #   initializer 'active_record.initialize_timezone' do
+  #     ActiveSupport.on_load(:active_record) do
+  #       self.time_zone_aware_attributes = true
+  #       self.default_timezone = :utc
+  #     end
+  #   end
+  #
+  # When the entirety of +ActiveRecord::Base+ has been
+  # evaluated then +run_load_hooks+ is invoked. The very last line of
+  # +ActiveRecord::Base+ is:
+  #
+  #   ActiveSupport.run_load_hooks(:active_record, ActiveRecord::Base)
+  module LazyLoadHooks
+    def self.extended(base) # :nodoc:
+      base.class_eval do
+        @load_hooks = Hash.new { |h, k| h[k] = [] }
+        @loaded     = Hash.new { |h, k| h[k] = [] }
+        @run_once   = Hash.new { |h, k| h[k] = [] }
+      end
+    end
+
+    # Declares a block that will be executed when a Rails component is fully
+    # loaded.
+    #
+    # Options:
+    #
+    # * <tt>:yield</tt> - Yields the object that run_load_hooks to +block+.
+    # * <tt>:run_once</tt> - Given +block+ will run only once.
+    def on_load(name, options = {}, &block)
+      @loaded[name].each do |base|
+        execute_hook(name, base, options, block)
+      end
+
+      @load_hooks[name] << [block, options]
+    end
+
+    def run_load_hooks(name, base = Object)
+      @loaded[name] << base
+      @load_hooks[name].each do |hook, options|
+        execute_hook(name, base, options, hook)
+      end
+    end
+
+    private
+
+      def with_execution_control(name, block, once)
+        unless @run_once[name].include?(block)
+          @run_once[name] << block if once
+
+          yield
+        end
+      end
+
+      def execute_hook(name, base, options, block)
+        with_execution_control(name, block, options[:run_once]) do
+          if options[:yield]
+            block.call(base)
+          else
+            if base.is_a?(Module)
+              base.class_eval(&block)
+            else
+              base.instance_eval(&block)
+            end
+          end
+        end
+      end
+  end
+
+  extend LazyLoadHooks
+end