Prevent adding duplicates to autoload_paths

docs/CHANGELOG.md

@@ -9,6 +9,10 @@ title: Changelog
 
 ## main
 
+* Prevent adding duplicates to `autoload_paths`.
+
+    *Thomas Hutterer*
+
 * Include polymorphic slots in `ViewComponent::Base` by default.
 * Add per-component config option for stripping newlines from templates before compilation.
 

lib/view_component/engine.rb

@@ -77,7 +77,8 @@ class Engine < Rails::Engine # :nodoc:
       options = app.config.view_component
 
       if options.show_previews && !options.preview_paths.empty?
-        ActiveSupport::Dependencies.autoload_paths.concat(options.preview_paths)
+        paths_to_add = options.preview_paths - ActiveSupport::Dependencies.autoload_paths
+        ActiveSupport::Dependencies.autoload_paths.concat(paths_to_add) if paths_to_add.any?
       end
     end
 

test/sandbox/config/application.rb

@@ -17,6 +17,10 @@
 module Sandbox
   class Application < Rails::Application
     config.action_controller.asset_host = "http://assets.example.com"
+
+    # Prepare test_set_no_duplicate_autoload_paths
+    config.autoload_paths.push("#{config.root}/my/components/previews")
+    config.view_component.preview_paths << "#{config.root}/my/components/previews"
   end
 end
 

test/sandbox/test/initializers/engine_test.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+class ViewComponent::Base::UnitTest < Minitest::Test
+  def test_set_no_duplicate_autoload_paths
+    test_path = "#{Rails.root}/my/components/previews" # set in sandbox/config/application.rb
+    assert_equal 1, ActiveSupport::Dependencies.autoload_paths.count(test_path)
+  end
+end