Add the ability to intercept emails before previewing

To support the ability for tools like CSS style inliners to operate on emails
being previewed this commit adds a hook in a similar fashion to the existing
delivery interceptor hook, e.g:

  class CSSInlineStyler
    def self.previewing_email(message)
      # inline CSS styles
    end
  end

  ActionMailer::Base.register_preview_interceptor CSSInlineStyler

Fixes #13622.

1 files changed, 42 insertions, 3 deletions

diff --git a/actionmailer/test/base_test.rb b/actionmailer/test/base_test.rb
index c1759d9b92..454e1afe97 100644
--- a/actionmailer/test/base_test.rb
+++ b/actionmailer/test/base_test.rb
@@ -539,12 +539,18 @@ class BaseTest < ActiveSupport::TestCase
   end
 
   class MyInterceptor
-    def self.delivering_email(mail)
-    end
+    def self.delivering_email(mail); end
+    def self.previewing_email(mail); end
   end
 
   class MySecondInterceptor
-    def self.delivering_email(mail)
+    def self.delivering_email(mail); end
+    def self.previewing_email(mail); end
+  end
+
+  class BaseMailerPreview < ActionMailer::Preview
+    def welcome
+      BaseMailer.welcome
     end
   end
 
@@ -570,6 +576,39 @@ class BaseTest < ActiveSupport::TestCase
     mail.deliver
   end
 
+  test "you can register a preview interceptor to the mail object that gets passed the mail object before previewing" do
+    ActionMailer::Base.register_preview_interceptor(MyInterceptor)
+    mail = BaseMailer.welcome
+    BaseMailerPreview.stubs(:welcome).returns(mail)
+    MyInterceptor.expects(:previewing_email).with(mail)
+    BaseMailerPreview.call(:welcome)
+  end
+
+  test "you can register a preview interceptor using its stringified name to the mail object that gets passed the mail object before previewing" do
+    ActionMailer::Base.register_preview_interceptor("BaseTest::MyInterceptor")
+    mail = BaseMailer.welcome
+    BaseMailerPreview.stubs(:welcome).returns(mail)
+    MyInterceptor.expects(:previewing_email).with(mail)
+    BaseMailerPreview.call(:welcome)
+  end
+
+  test "you can register an interceptor using its symbolized underscored name to the mail object that gets passed the mail object before previewing" do
+    ActionMailer::Base.register_preview_interceptor(:"base_test/my_interceptor")
+    mail = BaseMailer.welcome
+    BaseMailerPreview.stubs(:welcome).returns(mail)
+    MyInterceptor.expects(:previewing_email).with(mail)
+    BaseMailerPreview.call(:welcome)
+  end
+
+  test "you can register multiple preview interceptors to the mail object that both get passed the mail object before previewing" do
+    ActionMailer::Base.register_preview_interceptors("BaseTest::MyInterceptor", MySecondInterceptor)
+    mail = BaseMailer.welcome
+    BaseMailerPreview.stubs(:welcome).returns(mail)
+    MyInterceptor.expects(:previewing_email).with(mail)
+    MySecondInterceptor.expects(:previewing_email).with(mail)
+    BaseMailerPreview.call(:welcome)
+  end
+
   test "being able to put proc's into the defaults hash and they get evaluated on mail sending" do
     mail1 = ProcMailer.welcome['X-Proc-Method']
     yesterday = 1.day.ago