Don't create rails engine unless user opts in

lib/mini_profiler/config.rb

@@ -60,8 +60,7 @@ def self.default
         :base_url_path, :disable_caching, :enabled,
         :flamegraph_sample_rate, :logger, :pre_authorize_cb, :skip_paths,
         :skip_schema_queries, :storage, :storage_failure, :storage_instance,
-        :storage_options, :user_provider, :enable_advanced_debugging_tools,
-        :assets_url
+        :storage_options, :user_provider, :enable_advanced_debugging_tools
       attr_accessor :skip_sql_param_names, :suppress_encoding, :max_sql_param_length
 
       # ui accessors
@@ -72,6 +71,15 @@ def self.default
       # Deprecated options
       attr_accessor :use_existing_jquery
 
+      attr_reader :assets_url
+
+      def assets_url=(lmbda)
+        if defined?(Rack::MiniProfilerRails)
+          Rack::MiniProfilerRails.create_engine
+        end
+        @assets_url = lmbda
+      end
+
       def vertical_position
         position.include?('bottom') ? 'bottom' : 'top'
       end

lib/mini_profiler_rails/railtie.rb

@@ -6,13 +6,6 @@
 module Rack::MiniProfilerRails
   extend Rack::MiniProfilerRailsMethods
 
-  class Engine < ::Rails::Engine
-    engine_name 'rack-mini-profiler'
-    config.assets.paths << File.expand_path('../../html', __FILE__)
-    config.assets.precompile << 'rack-mini-profiler.js'
-    config.assets.precompile << 'rack-mini-profiler.css'
-  end
-
   # call direct if needed to do a defer init
   def self.initialize!(app)
 
@@ -125,6 +118,17 @@ def self.initialize!(app)
     @already_initialized = true
   end
 
+  def self.create_engine
+    return if defined?(Rack::MiniProfilerRails::Engine)
+    klass = Class.new(::Rails::Engine) do
+      engine_name 'rack-mini-profiler'
+      config.assets.paths << File.expand_path('../../html', __FILE__)
+      config.assets.precompile << 'rack-mini-profiler.js'
+      config.assets.precompile << 'rack-mini-profiler.css'
+    end
+    Rack::MiniProfilerRails.const_set("Engine", klass)
+  end
+
   def self.subscribe(event, &blk)
     if ActiveSupport::Notifications.respond_to?(:monotonic_subscribe)
       ActiveSupport::Notifications.monotonic_subscribe(event) { |*args| blk.call(*args) }