Enforce presence of view component preview files

src/api/.rubocop.yml

@@ -5,6 +5,7 @@ require:
   - ./lib/rubocop/cop/view_component/avoid_global_state.rb
   - ./lib/rubocop/cop/view_component/file_name.rb
   - ./lib/rubocop/cop/view_component/class_name.rb
+  - ./lib/rubocop/cop/view_component/missing_preview_file.rb
 
 inherit_from: .rubocop_todo.yml
 

src/api/lib/rubocop/cop/view_component/missing_preview_file.rb

@@ -0,0 +1,30 @@
+module RuboCop
+  module Cop
+    module ViewComponent
+      class MissingPreviewFile < RuboCop::Cop::Base
+        include RuboCop::Cop::RangeHelp # for source_range
+
+        def on_new_investigation
+          super
+
+          component_filename = File.basename(processed_source.file_path)
+
+          # Only consider Ruby files, anything else is ignored, even Haml templates ending with `.html.haml.rb`
+          return unless component_filename.end_with?('_component.rb')
+          # The ApplicationComponent should not have a preview file
+          return if component_filename == 'application_component.rb'
+
+          preview_filename = component_filename.gsub(/\.rb$/, '_preview.rb')
+          # Relative path from `Rails.root`, but we cannot use that method since it is not available inside a RuboCop cop
+          preview_file_relative_path = "spec/components/previews/#{preview_filename}"
+          preview_file = File.join(Dir.pwd, preview_file_relative_path)
+
+          return if File.exist?(preview_file)
+
+          add_offense(source_range(processed_source.buffer, 1, 0),
+                      message: "This view component should have a preview file (`src/api/#{preview_file_relative_path}`)")
+        end
+      end
+    end
+  end
+end

src/api/spec/lib/rubocop/cop/view_component/missing_preview_file_spec.rb

@@ -0,0 +1,57 @@
+require 'rails_helper'
+require './lib/rubocop/cop/view_component/missing_preview_file'
+
+RSpec.describe RuboCop::Cop::ViewComponent::MissingPreviewFile, :config do
+  let(:config) do
+    RuboCop::Config.new({ 'ViewComponent' => { 'Include' => ['app/components/**/*.rb'] } }, '/some/.rubocop.yml')
+  end
+
+  context 'when a view component does not have a preview file' do
+    it 'registers an offense' do
+      expect_offense(<<~RUBY, 'app/components/user_component.rb')
+        class UserComponent < ApplicationComponent
+        ^ This view component should have a preview file (`src/api/spec/components/previews/user_component_preview.rb`)
+          def initialize
+            @abc = 'abc'
+          end
+        end
+      RUBY
+    end
+  end
+
+  context 'when a view component has a preview file' do
+    before do
+      allow(File).to receive(:exist?).with(Rails.root.join('spec/components/previews/user_component_preview.rb').to_s).and_return(true)
+    end
+
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY, 'app/components/user_component.rb')
+        class UserComponent < ApplicationComponent
+          def initialize
+            @abc = 'abc'
+          end
+        end
+      RUBY
+    end
+  end
+
+  context 'when the ApplicationComponent does not have a preview file' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY, 'app/components/application_component.rb')
+        class ApplicationComponent
+          def initialize
+            @abc = 'abc'
+          end
+        end
+      RUBY
+    end
+  end
+
+  context 'when a file which is not a view component class' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~HAML, 'app/components/user_component.html.haml.rb')
+        Hello world!
+      HAML
+    end
+  end
+end