Add configuration to enable mail previews

Adds `config.action_mailer.preview_enabled`

This allows mail previewing to be enabled easily in non-development
environments such as staging. The default is set to true for development
so no changes should be required to existing Rails applications.

The mail preview path can still be configured using the existing
`config.action_mailer.preview_path` configuration option.

Adding this avoids devs from having to do stuff like:
https://gist.github.com/lengarvey/fa2c9bd6cdbeba96526a

Update actionmailer/CHANGELOG with new configuration.
Update configuring guide with new configuratation.
Add `config.action_mailer.preview_path` to configuring guide.

2 files changed, 21 insertions, 1 deletions

diff --git a/actionmailer/lib/action_mailer/preview.rb b/actionmailer/lib/action_mailer/preview.rb
index 33a9faa7e7..f27fab1a0d 100644
--- a/actionmailer/lib/action_mailer/preview.rb
+++ b/actionmailer/lib/action_mailer/preview.rb
@@ -11,6 +11,14 @@ module ActionMailer
       #
       mattr_accessor :preview_path, instance_writer: false
 
+      # Enable or disable mailer previews through app configuration:
+      #
+      #     config.action_mailer.preview_enabled = true
+      #
+      # Defaults to true for development environment
+      #
+      mattr_accessor :preview_enabled, instance_writer: false
+
       # :nodoc:
       mattr_accessor :preview_interceptors, instance_writer: false
       self.preview_interceptors = []
@@ -94,6 +102,10 @@ module ActionMailer
           Base.preview_path
         end
 
+        def preview_enabled #:nodoc:
+          Base.preview_enabled
+        end
+
         def inform_preview_interceptors(message) #:nodoc:
           Base.preview_interceptors.each do |interceptor|
             interceptor.previewing_email(message)
diff --git a/actionmailer/lib/action_mailer/railtie.rb b/actionmailer/lib/action_mailer/railtie.rb
index 671551fa53..409a7a897a 100644
--- a/actionmailer/lib/action_mailer/railtie.rb
+++ b/actionmailer/lib/action_mailer/railtie.rb
@@ -18,8 +18,9 @@ module ActionMailer
       options.assets_dir      ||= paths["public"].first
       options.javascripts_dir ||= paths["public/javascripts"].first
       options.stylesheets_dir ||= paths["public/stylesheets"].first
+      options.preview_enabled = Rails.env.development? if options.preview_enabled.nil?
 
-      if Rails.env.development?
+      if options.preview_enabled
         options.preview_path  ||= defined?(Rails.root) ? "#{Rails.root}/test/mailers/previews" : nil
       end
 
@@ -37,6 +38,13 @@ module ActionMailer
         register_observers(options.delete(:observers))
 
         options.each { |k,v| send("#{k}=", v) }
+
+        if options.preview_enabled
+          app.routes.append do
+            get '/rails/mailers'         => "rails/mailers#index"
+            get '/rails/mailers/*path'   => "rails/mailers#preview"
+          end
+        end
       end
     end