Extract current text formatting helper into separate helper

app/helpers/application_helper.rb

@@ -42,9 +42,6 @@ module ApplicationHelper
   include IconsHelper
   include AdditionalUrlHelpers
 
-  extend Forwardable
-  def_delegators :wiki_helper, :text_formatting_js_includes, :wikitoolbar_for, :heads_for_wiki_formatter
-
   # Return true if user is authorized for controller/action, otherwise false
   def authorize_for(controller, action, project: @project)
     User.current.allowed_to?({ controller: controller, action: action }, project)
@@ -599,11 +596,6 @@ module ApplicationHelper
 
   private
 
-  def wiki_helper
-    helper = OpenProject::TextFormatting::Formatters.helper_for(Setting.text_formatting)
-    extend helper
-    self
-  end
 
   def link_to_content_update(text, url_params = {}, html_options = {})
     link_to(text, url_params, html_options)

app/helpers/text_formatting_helper.rb

@@ -27,8 +27,17 @@
 # See docs/COPYRIGHT.rdoc for more details.
 #++
 
-module PreviewsHelper
+module TextFormattingHelper
+  extend Forwardable
+  def_delegators :current_formatting_helper,
+                 :text_formatting_has_preview?,
+                 :text_formatting_js_includes,
+                 :wikitoolbar_for,
+                 :heads_for_wiki_formatter
+
   def preview_link(path, link_id, options = {})
+    return '' unless text_formatting_has_preview?
+
     options = {
       accesskey: accesskey(:preview),
       id: link_id,
@@ -42,4 +51,12 @@ module PreviewsHelper
       l(:label_preview)
     end
   end
+
+  private
+
+  def current_formatting_helper
+    helper = OpenProject::TextFormatting::Formatters.helper_for(Setting.text_formatting)
+    extend helper
+    self
+  end
 end

app/views/wiki/edit.html.erb

@@ -48,6 +48,7 @@ See docs/COPYRIGHT.rdoc for more details.
   <%= link_to t(:button_cancel),
       { controller: '/wiki', action: 'show', project_id: @project, id: @page },
       class: 'button' %>
+  <%= preview_link preview_project_wiki_path(@project, @page), 'wiki_form-preview' %>
 <% end %>
 <div id="preview"></div>
 <% content_for :header_tags do %>

lib/open_project/text_formatting/formatters/markdown/helper.rb

@@ -34,6 +34,10 @@ module OpenProject::TextFormatting::Formatters
         javascript_include_tag 'vendor/ckeditor/openproject-ckeditor.js', async: true
       end
 
+      def text_formatting_has_preview?
+        false
+      end
+
       def wikitoolbar_for(field_id)
         tag 'op-ckeditor-form', 'textarea-selector': "##{field_id}"
       end

lib/open_project/text_formatting/formatters/plain/helper.rb

@@ -33,6 +33,13 @@ module OpenProject::TextFormatting::Formatters
       def wikitoolbar_for(_field_id)
       end
 
+      def text_formatting_has_preview?
+        false
+      end
+
+      def text_formatting_js_includes
+      end
+
       def heads_for_wiki_formatter
       end
 

lib/open_project/text_formatting/formatters/textile/helper.rb

@@ -34,6 +34,10 @@ module OpenProject::TextFormatting::Formatters
         # TODO Nothing to do here yet, since the js_toolbar is still part of application
       end
 
+      def text_formatting_has_preview?
+        true
+      end
+
       def wikitoolbar_for(field_id)
         heads_for_wiki_formatter
         url = url_for(controller: '/help', action: 'wiki_syntax')