resolves #32 add support for gtag variant of tracking code

features/google_analytics.feature

@@ -1,6 +1,6 @@
 Feature: Google Analytics tag helper
 
-  Scenario: Basic tracking code
+  Scenario: Analytics tracking code
     Given the Server is running at "basic-app"
     When I go to "/google-analytics.html"
     Then I should see:
@@ -15,6 +15,22 @@ Feature: Google Analytics tag helper
       </script>
       """
 
+  Scenario: gtag tracking code
+    Given the Server is running at "gtag-app"
+    When I go to "/google-analytics.html"
+    Then I should see:
+      """
+      <script async src="//www.googletagmanager.com/gtag/js?id=UA-123456-78"></script>
+      <script>
+      window.dataLayer = window.dataLayer || [];
+      function gtag () {
+        dataLayer.push(arguments);
+      }
+      gtag('js', new Date());
+      gtag('config', 'UA-123456-78');
+      </script>
+      """
+
   Scenario: Tracking across a domain and its subdomains
     Given the Server is running at "across-a-domain-app"
     When I go to "/google-analytics.html"

fixtures/gtag-app/config.rb

@@ -0,0 +1,4 @@
+activate :google_analytics do |ga|
+  ga.tracking_id = 'UA-123456-78'
+  ga.variant = 'gtag'
+end

fixtures/gtag-app/source/google-analytics.html.erb

@@ -0,0 +1 @@
+<%= google_analytics_tag %>

lib/middleman-google-analytics/extension.rb

@@ -14,6 +14,7 @@ class GoogleAnalyticsExtension < Extension
     option :test, false, 'Testing your implementation without sending hits'
     option :minify, false, 'Compress the JavaScript code'
     option :output, :html, 'Output style - :html includes <script> tag'
+    option :variant, :analytics, 'Tracking code variant'
 
     def after_configuration
       options.test = true if legacy_development?
@@ -28,6 +29,11 @@ def after_configuration
         raise ArgumentError, 'No domain_name given' if app.build?
       end
 
+      unless [:analytics, :gtag].include?(options.variant.try(:to_sym))
+        $stderr.puts 'Google Analytics: Please specify a valid variant type (analytics|gtag).'
+        raise ArgumentError, 'Only "analytics" or "gtag" allowed' if app.build?
+      end
+
       unless [:html, :js].include?(options.output.try(:to_sym))
         $stderr.puts 'Google Analytics: Please specify a valid output type (html|js).'
         raise ArgumentError, 'Only "html" or "js" allowed' if app.build?

lib/middleman-google-analytics/gtag.js.erb

@@ -0,0 +1,9 @@
+<script async src="//www.googletagmanager.com/gtag/js?id=<%= @options.tracking_id %>"></script>
+<script>
+window.dataLayer = window.dataLayer || [];
+function gtag () {
+  dataLayer.push(arguments);
+}
+gtag('js', new Date());
+gtag('config', '<%= @options.tracking_id %>');
+</script>

lib/middleman-google-analytics/helpers.rb

@@ -11,16 +11,25 @@ def google_analytics_tag
 
         return nil if options.disable
 
-        file = File.join(File.dirname(__FILE__), 'analytics.js.erb')
+        file = File.join(File.dirname(__FILE__), "#{options.variant}.js.erb")
         context = { options: options }
         content = Erubis::FastEruby.new(File.read(file)).evaluate(context)
-        content = Uglifier.compile(content) if options.minify
 
-        if options.output.to_sym == :html
-          content = indent(content) unless options.minify
-          content_tag(:script, content, type: 'text/javascript')
+        if options.variant.to_sym == :analytics
+          if options.output.to_sym == :html
+            content = options.minify ? Uglifier.compile(content) : indent(content)
+            content_tag(:script, content, type: 'text/javascript')
+          else
+            options.minify ? Uglifier.compile(content) : content
+          end
         else
-          content
+          if options.output.to_sym == :html
+            content = content.gsub(/(?<=<script>)(.+)(?=<\/script>)/) do
+              options.minify ? Uglifier.compile($1) : index($1)
+            end
+          else
+            options.minify ? content.gsub(/.*<script>(.+)<\/script>/) { Uglifier.compile($1) } : content
+          end
         end
       end