Do not register a reloader when file paths not exist

If all the paths specified in `FactoryBot.definition_file_paths` do not
exist, an empty array is passed to the reloader.

If an empty array is specified, reloader passes it to `Listen.to` as it
is. This causes the application root to be watched and `node_modules` to
become a target of listening.

The behavior when an empty array is passed to reloader depends on the
Rails side. Fixes to not register a reloader when file paths do not exist
consistent behavior regardless of Rails side.

Fixes thoughtbot#328.

lib/factory_bot_rails/definition_file_paths.rb

@@ -19,5 +19,9 @@ def directories
     def files
       @files.select { |file| File.exist?(file) }
     end
+
+    def any?
+      directories.any?
+    end
   end
 end

lib/factory_bot_rails/reloader.rb

@@ -7,9 +7,12 @@ class Reloader
     def initialize(app, config)
       @app = app
       @config = config
+      @paths = DefinitionFilePaths.new(FactoryBot.definition_file_paths)
     end
 
     def run
+      return unless @paths.any?
+
       register_reloader(build_reloader)
     end
 
@@ -18,9 +21,7 @@ def run
     attr_reader :app, :config
 
     def build_reloader
-      paths = DefinitionFilePaths.new(FactoryBot.definition_file_paths)
-
-      reloader_class.new(paths.files, paths.directories) do
+      reloader_class.new(@paths.files, @paths.directories) do
         FactoryBot.reload
       end
     end

spec/factory_bot_rails/definition_file_paths_spec.rb

@@ -30,4 +30,14 @@
       expect(directories).to eq("spec/fixtures/factories" => [:rb])
     end
   end
+
+  describe "#any?" do
+    it "returns true only if definition file paths exist" do
+      definition_file_paths = ["spec/fixtures/factories", "not_exist_directory"]
+      expect(described_class.new(definition_file_paths).any?).to eq true
+
+      definition_file_paths = ["not_exist_directory"]
+      expect(described_class.new(definition_file_paths).any?).to eq false
+    end
+  end
 end

spec/factory_bot_rails/reloader_spec.rb

@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+describe FactoryBotRails::Reloader do
+  describe "#run" do
+    before do
+      @original_definition_file_paths = FactoryBot.definition_file_paths
+    end
+
+    after do
+      FactoryBot.definition_file_paths = @original_definition_file_paths
+    end
+
+    context "when a definition file paths exist" do
+      it "register a reloader" do
+        allow(reloader_class).to receive(:new)
+
+        run_reloader(["spec/fixtures/factories", "not_exist_directory"])
+
+        expect(reloader_class).to have_received(:new)
+      end
+    end
+
+    context "when a definition file paths NOT exist" do
+      it "does NOT register a reloader" do
+        allow(reloader_class).to receive(:new)
+
+        run_reloader(["not_exist_directory"])
+
+        expect(reloader_class).not_to have_received(:new)
+      end
+    end
+
+    def run_reloader(definition_file_paths)
+      FactoryBot.definition_file_paths = definition_file_paths
+      FactoryBotRails::Reloader.
+        new(Rails.application, Rails.application.config).run
+    end
+
+    def reloader_class
+      Rails.application.config.file_watcher
+    end
+  end
+end