Adding before build hook

middleman-core/lib/middleman-core/application.rb

@@ -40,6 +40,9 @@ module Middleman
     # Ready (all loading and parsing of extensions complete) hook
     define_hook :ready
 
+    # Runs before the build is started
+    define_hook :before_build
+
     # Runs after the build is finished
     define_hook :after_build
 

middleman-core/lib/middleman-core/cli/build.rb

@@ -65,6 +65,8 @@ module Middleman::Cli
       opts[:glob]  = options['glob'] if options.has_key?('glob')
       opts[:clean] = options['clean']
 
+      self.class.shared_instance.run_hook :before_build, self
+
       action BuildAction.new(self, opts)
 
       self.class.shared_instance.run_hook :after_build, self

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

@@ -175,6 +175,7 @@ module Middleman
       # Bind app hooks to local methods
       bind_before_configuration
       bind_after_configuration
+      bind_before_build
       bind_after_build
     end
 
@@ -233,6 +234,19 @@ module Middleman
       end
     end
 
+    def bind_before_build
+      ext = self
+      if ext.respond_to?(:before_build)
+        @klass.before_build do |builder|
+          if ext.method(:before_build).arity === 1
+            ext.before_build(builder)
+          else
+            ext.before_build
+          end
+        end
+      end
+    end
+
     def bind_after_build
       ext = self
       if ext.respond_to?(:after_build)