Test the helper

app/helpers/google_sign_in/button_helper.rb

@@ -0,0 +1,7 @@
+module GoogleSignIn::ButtonHelper
+  def google_sign_in_button(text = nil, proceed_to:, **options, &block)
+    form_with url: google_sign_in.authorization_path do
+      hidden_field_tag(:proceed_to, proceed_to, id: nil) + button_tag(text, name: nil, **options, &block)
+    end
+  end
+end

lib/google_sign_in/engine.rb

@@ -13,10 +13,9 @@ class Engine < ::Rails::Engine
       end
     end
 
-    initializer 'google_sign_in.helper' do
+    initializer 'google_sign_in.helpers' do
       ActiveSupport.on_load :action_controller do
-        require 'google_sign_in/helper'
-        ActionController::Base.helper GoogleSignIn::Helper
+        ActionController::Base.helper GoogleSignIn::Engine.helpers
       end
     end
 

test/helpers/button_helper_test.rb

@@ -0,0 +1,23 @@
+require 'test_helper'
+
+class GoogleSignIn::ButtonHelperTest < ActionView::TestCase
+  test "generating a login button with text content" do
+    assert_dom_equal <<-HTML, google_sign_in_button("Log in with Google", proceed_to: 'https://www.example.com/login')
+      <form action="/google_sign_in/authorization" accept-charset="UTF-8" data-remote="true" method="post">
+        <input name="utf8" type="hidden" value="&#x2713;" />
+        <input name="proceed_to" type="hidden" value="https://www.example.com/login" />
+        <button type="submit">Log in with Google</button>
+      </form>
+    HTML
+  end
+
+  test "generating a login button with HTML content" do
+    assert_dom_equal <<-HTML, google_sign_in_button(proceed_to: 'https://www.example.com/login') { image_tag('google.png') }
+      <form action="/google_sign_in/authorization" accept-charset="UTF-8" data-remote="true" method="post">
+        <input name="utf8" type="hidden" value="&#x2713;" />
+        <input name="proceed_to" type="hidden" value="https://www.example.com/login" />
+        <button type="submit"><img src="/images/google.png"></button>
+      </form>
+    HTML
+  end
+end

test/test_helper.rb

@@ -8,3 +8,10 @@
 require 'rails/test_help'
 require 'webmock/minitest'
 require 'byebug'
+
+class ActionView::TestCase
+  private
+    def assert_dom_equal(expected, actual, message = nil)
+      super expected.remove(/(\A|\n)\s*/), actual, message
+    end
+end