1 files changed, 117 insertions, 0 deletions
diff --git a/actionpack/lib/action_controller/renderer.rb b/actionpack/lib/action_controller/renderer.rb
new file mode 100644
index 0000000000..49c5b782f0
--- /dev/null
+++ b/actionpack/lib/action_controller/renderer.rb
@@ -0,0 +1,117 @@
+# frozen_string_literal: true
+
+require "active_support/core_ext/hash/keys"
+
+module ActionController
+  # ActionController::Renderer allows you to render arbitrary templates
+  # without requirement of being in controller actions.
+  #
+  # You get a concrete renderer class by invoking ActionController::Base#renderer.
+  # For example:
+  #
+  #   ApplicationController.renderer
+  #
+  # It allows you to call method #render directly.
+  #
+  #   ApplicationController.renderer.render template: '...'
+  #
+  # You can use this shortcut in a controller, instead of the previous example:
+  #
+  #   ApplicationController.render template: '...'
+  #
+  # #render allows you to use the same options that you can use when rendering in a controller.
+  # For example:
+  #
+  #   FooController.render :action, locals: { ... }, assigns: { ... }
+  #
+  # The template will be rendered in a Rack environment which is accessible through
+  # ActionController::Renderer#env. You can set it up in two ways:
+  #
+  # *  by changing renderer defaults, like
+  #
+  #       ApplicationController.renderer.defaults # => hash with default Rack environment
+  #
+  # *  by initializing an instance of renderer by passing it a custom environment.
+  #
+  #       ApplicationController.renderer.new(method: 'post', https: true)
+  #
+  class Renderer
+    attr_reader :defaults, :controller
+
+    DEFAULTS = {
+      http_host: "example.org",
+      https: false,
+      method: "get",
+      script_name: "",
+      input: ""
+    }.freeze
+
+    # Create a new renderer instance for a specific controller class.
+    def self.for(controller, env = {}, defaults = DEFAULTS.dup)
+      new(controller, env, defaults)
+    end
+
+    # Create a new renderer for the same controller but with a new env.
+    def new(env = {})
+      self.class.new controller, env, defaults
+    end
+
+    # Create a new renderer for the same controller but with new defaults.
+    def with_defaults(defaults)
+      self.class.new controller, @env, self.defaults.merge(defaults)
+    end
+
+    # Accepts a custom Rack environment to render templates in.
+    # It will be merged with the default Rack environment defined by
+    # +ActionController::Renderer::DEFAULTS+.
+    def initialize(controller, env, defaults)
+      @controller = controller
+      @defaults = defaults
+      @env = normalize_keys defaults.merge(env)
+    end
+
+    # Render templates with any options from ActionController::Base#render_to_string.
+    def render(*args)
+      raise "missing controller" unless controller
+
+      request = ActionDispatch::Request.new @env
+      request.routes = controller._routes
+
+      instance = controller.new
+      instance.set_request! request
+      instance.set_response! controller.make_response!(request)
+      instance.render_to_string(*args)
+    end
+
+    private
+      def normalize_keys(env)
+        new_env = {}
+        env.each_pair { |k, v| new_env[rack_key_for(k)] = rack_value_for(k, v) }
+        new_env["rack.url_scheme"] = new_env["HTTPS"] == "on" ? "https" : "http"
+        new_env
+      end
+
+      RACK_KEY_TRANSLATION = {
+        http_host:   "HTTP_HOST",
+        https:       "HTTPS",
+        method:      "REQUEST_METHOD",
+        script_name: "SCRIPT_NAME",
+        input:       "rack.input"
+      }
+
+      IDENTITY = ->(_) { _ }
+
+      RACK_VALUE_TRANSLATION = {
+        https: ->(v) { v ? "on" : "off" },
+        method: ->(v) { v.upcase },
+      }
+
+      def rack_key_for(key)
+        RACK_KEY_TRANSLATION.fetch(key, key.to_s)
+      end
+
+      def rack_value_for(key, value)
+        RACK_VALUE_TRANSLATION.fetch(key, IDENTITY).call value
+      end
+  end
+end