Add callback so v4 extensions can see when other extensions are activated

middleman-core/lib/middleman-core/core_extensions/extensions.rb

@@ -142,6 +142,7 @@ module Middleman
           # Search the root of the project for required files
           $LOAD_PATH.unshift(root)
 
+          ::Middleman::Extension.clear_after_extension_callbacks
           run_hook :initialized
 
           if config[:autoload_sprockets]
@@ -178,7 +179,7 @@ module Middleman
           run_hook :after_configuration
 
           logger.debug "Loaded extensions:"
-          self.extensions.each do |ext,_|
+          self.extensions.each do |ext, klass|
             if ext.is_a?(Hash)
               ext.each do |k,_|
                 logger.debug "== Extension: #{k}"
@@ -186,6 +187,10 @@ module Middleman
             else
               logger.debug "== Extension: #{ext}"
             end
+
+            if klass.is_a?(::Middleman::Extension)
+              ::Middleman::Extension.activated_extension(klass)
+            end
           end
         end
       end

middleman-core/lib/middleman-core/extensions.rb

@@ -1,4 +1,5 @@
 require "active_support/core_ext/class/attribute"
+require "active_support/core_ext/module/delegation"
 
 module Middleman
 
@@ -131,11 +132,31 @@ module Middleman
       def activate
         new(::Middleman::Application)
       end
+
+      def clear_after_extension_callbacks
+        @_extension_activation_callbacks = {}
+      end
+
+      def after_extension_activated(name, &block)
+        @_extension_activation_callbacks ||= {}
+        @_extension_activation_callbacks[name] ||= []
+        @_extension_activation_callbacks[name] << block if block_given?
+      end
+
+      def activated_extension(instance)
+        name = instance.class.extension_name
+        return unless @_extension_activation_callbacks && @_extension_activation_callbacks[name]
+        @_extension_activation_callbacks[name].each do |block|
+          block.arity == 1 ? block.call(instance) : block.call()
+        end
+      end
     end
 
     attr_accessor :options
     attr_reader :app
 
+    delegate :after_extension_activated, :to => :"::Middleman::Extension"
+
     def initialize(klass, options_hash={}, &block)
       @_helpers = []
       @klass = klass