Add locale selector to email preview (#31596)

- Add set_locale to detect suitable locale
- Make feature compatible with Rails 5.x

3 files changed, 97 insertions, 18 deletions

diff --git a/railties/lib/rails/mailers_controller.rb b/railties/lib/rails/mailers_controller.rb
index 66636e5d6b..0b0e802358 100644
--- a/railties/lib/rails/mailers_controller.rb
+++ b/railties/lib/rails/mailers_controller.rb
@@ -6,9 +6,9 @@ class Rails::MailersController < Rails::ApplicationController # :nodoc:
   prepend_view_path ActionDispatch::DebugExceptions::RESCUES_TEMPLATE_PATH
 
   before_action :require_local!, unless: :show_previews?
-  before_action :find_preview, only: :preview
+  before_action :find_preview, :set_locale, only: :preview
 
-  helper_method :part_query
+  helper_method :part_query, :locale_query
 
   def index
     @previews = ActionMailer::Preview.all
@@ -84,4 +84,12 @@ class Rails::MailersController < Rails::ApplicationController # :nodoc:
     def part_query(mime_type)
       request.query_parameters.merge(part: mime_type).to_query
     end
+
+    def locale_query(locale)
+      request.query_parameters.merge(locale: locale).to_query
+    end
+
+    def set_locale
+      I18n.locale = params[:locale] || I18n.default_locale
+    end
 end
diff --git a/railties/lib/rails/templates/rails/mailers/email.html.erb b/railties/lib/rails/templates/rails/mailers/email.html.erb
index 89c1129f90..44c400a7e6 100644
--- a/railties/lib/rails/templates/rails/mailers/email.html.erb
+++ b/railties/lib/rails/templates/rails/mailers/email.html.erb
@@ -96,10 +96,22 @@
     <% end %>
 
     <% if @email.multipart? %>
+      <dt>Format:</dt>
       <dd>
-        <select onchange="formatChanged(this);">
-          <option <%= request.format == Mime[:html] ? 'selected' : '' %> value="?<%= part_query('text/html') %>">View as HTML email</option>
-          <option <%= request.format == Mime[:text] ? 'selected' : '' %> value="?<%= part_query('text/plain') %>">View as plain-text email</option>
+        <select id="part" onchange="refreshBody();">
+          <option <%= request.format == Mime[:html] ? 'selected' : '' %> value="<%= part_query('text/html') %>">View as HTML email</option>
+          <option <%= request.format == Mime[:text] ? 'selected' : '' %> value="<%= part_query('text/plain') %>">View as plain-text email</option>
+        </select>
+      </dd>
+    <% end %>
+
+    <% if I18n.available_locales.count > 1 %>
+      <dt>Locale:</dt>
+      <dd>
+        <select id="locale" onchange="refreshBody();">
+          <% I18n.available_locales.each do |locale| %>
+            <option <%= I18n.locale == locale ? 'selected' : '' %> value="<%= locale_query(locale) %>"><%= locale %></option>
+          <% end %>
         </select>
       </dd>
     <% end %>
@@ -116,15 +128,18 @@
 <% end %>
 
 <script>
-  function formatChanged(form) {
-    var part_name = form.options[form.selectedIndex].value
-    var iframe =document.getElementsByName('messageBody')[0];
-    iframe.contentWindow.location.replace(part_name);
+  function refreshBody() {
+    var part_select = document.querySelector('select#part');
+    var locale_select = document.querySelector('select#locale');
+    var part_param = part_select.options[part_select.selectedIndex].value;
+    var locale_param = locale_select.options[locale_select.selectedIndex].value;
+    var iframe = document.getElementsByName('messageBody')[0];
+    iframe.contentWindow.location = '?' + part_param + '&' + locale_param;
 
     if (history.replaceState) {
-      var url    = location.pathname.replace(/\.(txt|html)$/, '');
-      var format = /html/.test(part_name) ? '.html' : '.txt';
-      window.history.replaceState({}, '', url + format);
+      var url = location.pathname.replace(/\.(txt|html)$/, '');
+      var format = /html/.test(part_param) ? '.html' : '.txt';
+      window.history.replaceState({}, '', url + format + '?' + locale_param);
     }
   }
 </script>
diff --git a/railties/test/application/mailer_previews_test.rb b/railties/test/application/mailer_previews_test.rb
index 4e77cece1b..288968484c 100644
--- a/railties/test/application/mailer_previews_test.rb
+++ b/railties/test/application/mailer_previews_test.rb
@@ -450,11 +450,67 @@ module ApplicationTests
 
       get "/rails/mailers/notifier/foo.html"
       assert_equal 200, last_response.status
-      assert_match '<option selected value="?part=text%2Fhtml">View as HTML email</option>', last_response.body
+      assert_match '<option selected value="part=text%2Fhtml">View as HTML email</option>', last_response.body
 
       get "/rails/mailers/notifier/foo.txt"
       assert_equal 200, last_response.status
-      assert_match '<option selected value="?part=text%2Fplain">View as plain-text email</option>', last_response.body
+      assert_match '<option selected value="part=text%2Fplain">View as plain-text email</option>', last_response.body
+    end
+
+    test "locale menu selects correct option" do
+      app_file "config/initializers/available_locales.rb", <<-RUBY
+        Rails.application.configure do
+          config.i18n.available_locales = %i[en ja]
+        end
+      RUBY
+
+      mailer "notifier", <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            mail to: "to@example.org"
+          end
+        end
+      RUBY
+
+      html_template "notifier/foo", <<-RUBY
+        <p>Hello, World!</p>
+      RUBY
+
+      text_template "notifier/foo", <<-RUBY
+        Hello, World!
+      RUBY
+
+      mailer_preview "notifier", <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      app("development")
+
+      get "/rails/mailers/notifier/foo.html"
+      assert_equal 200, last_response.status
+      assert_match '<option selected value="locale=en">en', last_response.body
+      assert_match '<option  value="locale=ja">ja', last_response.body
+
+      get "/rails/mailers/notifier/foo.html?locale=ja"
+      assert_equal 200, last_response.status
+      assert_match '<option  value="locale=en">en', last_response.body
+      assert_match '<option selected value="locale=ja">ja', last_response.body
+
+      get "/rails/mailers/notifier/foo.txt"
+      assert_equal 200, last_response.status
+      assert_match '<option selected value="locale=en">en', last_response.body
+      assert_match '<option  value="locale=ja">ja', last_response.body
+
+      get "/rails/mailers/notifier/foo.txt?locale=ja"
+      assert_equal 200, last_response.status
+      assert_match '<option  value="locale=en">en', last_response.body
+      assert_match '<option selected value="locale=ja">ja', last_response.body
     end
 
     test "mailer previews create correct links when loaded on a subdirectory" do
@@ -520,8 +576,8 @@ module ApplicationTests
       get "/rails/mailers/notifier/foo.txt"
       assert_equal 200, last_response.status
       assert_match '<iframe seamless name="messageBody" src="?part=text%2Fplain">', last_response.body
-      assert_match '<option selected value="?part=text%2Fplain">', last_response.body
-      assert_match '<option  value="?part=text%2Fhtml">', last_response.body
+      assert_match '<option selected value="part=text%2Fplain">', last_response.body
+      assert_match '<option  value="part=text%2Fhtml">', last_response.body
 
       get "/rails/mailers/notifier/foo?part=text%2Fplain"
       assert_equal 200, last_response.status
@@ -530,8 +586,8 @@ module ApplicationTests
       get "/rails/mailers/notifier/foo.html?name=Ruby"
       assert_equal 200, last_response.status
       assert_match '<iframe seamless name="messageBody" src="?name=Ruby&amp;part=text%2Fhtml">', last_response.body
-      assert_match '<option selected value="?name=Ruby&amp;part=text%2Fhtml">', last_response.body
-      assert_match '<option  value="?name=Ruby&amp;part=text%2Fplain">', last_response.body
+      assert_match '<option selected value="name=Ruby&amp;part=text%2Fhtml">', last_response.body
+      assert_match '<option  value="name=Ruby&amp;part=text%2Fplain">', last_response.body
 
       get "/rails/mailers/notifier/foo?name=Ruby&part=text%2Fhtml"
       assert_equal 200, last_response.status