Add ERB template support for email content

lib/courrier/email.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "erb"
+
 require "courrier/email/address"
 require "courrier/jobs/email_delivery_job" if defined?(Rails)
 require "courrier/email/layouts"
@@ -133,7 +135,25 @@ def delivery_disabled?
       ENV["COURRIER_EMAIL_DISABLED"] == "true" || ENV["COURRIER_EMAIL_ENABLED"] == "false"
     end
 
-    def method_missing(name, *) = @context_options[name]
+    def method_missing(name, *)
+      if name == :text || name == :html
+        render_template(name.to_s)
+      else
+        @context_options[name]
+      end
+    end
+
+    def render_template(format)
+      template_path = template_file_path(format)
+
+      File.exist?(template_path) ? ERB.new(File.read(template_path)).result(binding) : nil
+    end
+
+    def template_file_path(format)
+      class_path = self.class.name.gsub(/([a-z\d])([A-Z])/, '\1_\2').downcase
+
+      File.join(Courrier.configuration&.email_path, "#{class_path}.#{format}.erb")
+    end
 
     def respond_to_missing?(name, include_private = false) = true
   end

test/courrier/email_test.rb

@@ -98,4 +98,50 @@ def test_url_generation_with_missing_host
 
     assert_equal "Click here: ", email.text
   end
+
+  def test_template_rendering_with_files
+  email_path = "tmp/test_emails"
+
+  FileUtils.mkdir_p(email_path)
+  File.write("#{email_path}/test_email_with_templates.text.erb", "Hello <%= name %>!")
+  File.write("#{email_path}/test_email_with_templates.html.erb", "<p>Hello <strong><%= name %></strong>!</p>")
+
+  Courrier.configure do |config|
+    config.email_path = email_path
+  end
+
+  email = TestEmailWithTemplates.new(
+    to: "recipient@railsdesigner.com",
+    from: "devs@railsdesigner.com",
+    name: "World"
+  )
+
+  assert_equal "Hello World!", email.text
+  assert_equal "<p>Hello <strong>World</strong>!</p>", email.html
+
+  FileUtils.rm_rf(email_path)
+end
+
+def test_method_takes_precedence_over_template
+  email_path = "tmp/test_emails"
+
+  FileUtils.mkdir_p(email_path)
+  File.write("#{email_path}/test_email_with_mixed_content.text.erb", "Template text")
+  File.write("#{email_path}/test_email_with_mixed_content.html.erb", "<p>Template HTML</p>")
+
+  Courrier.configure do |config|
+    config.email_path = email_path
+  end
+
+  email = TestEmailWithMixedContent.new(
+    to: "recipient@railsdesigner.com",
+    from: "devs@railsdesigner.com"
+  )
+
+  assert_equal "Method text", email.text
+  assert_equal "<p>Template HTML</p>", email.html
+
+  FileUtils.rm_rf(email_path)
+end
+
 end

test/fixtures/test_email_with_mixed_content.rb

@@ -0,0 +1,8 @@
+require "courrier/email"
+
+class TestEmailWithMixedContent < Courrier::Email
+  def subject = "Mixed Content Test"
+
+  def text = "Method text"
+  # html is defined in template
+end

test/fixtures/test_email_with_templates.rb

@@ -0,0 +1,5 @@
+require "courrier/email"
+
+class TestEmailWithTemplates < Courrier::Email
+  def subject = "Template Test"
+end