Add PageDrop to provide Liquid templates with data

docs/_docs/pages.md

@@ -34,3 +34,38 @@ If you have a lot of pages, you can organize them into subfolders. The same subf
 ## Changing the output URL
 
 You might want to have a particular folder structure for your source files that changes for the built site. With [permalinks](/docs/permalinks) you have full control of the output URL.
+
+## Liquid Representation
+
+From Jekyll 4.1 onwards, there is a minor change in how instances of `Jekyll::Page` are exposed to layouts and other Liquid
+templates. `Jekyll::Page` instances now use a `Liquid::Drop` instead of a `Hash`. This change results in greater performance
+for a site with numerous *standlone pages not within a collection*.
+
+### For plugin developers
+
+While end-users do not need to take any extra action due to this change, plugin authors depending on the existing behavior *may*
+need to make minor changes to their plugins.
+
+If a `Jekyll::Page` subclass' `to_liquid` method calls `super`, it will have to be slightly modified.
+```ruby
+class Foo::BarPage < Jekyll::Page
+  def to_liquid(*)
+    payload = super        # This needs to be changed to `super.to_h`
+                           # to obtain a Hash as in v4.0.0.
+
+    do_something(payload)  # Logic specific to `Foo::BarPage` objects
+  end
+end
+```
+
+`Jekyll::Page` subclasses won't inherit the optimization automatically until the next major version. However, plugin authors
+can opt-in to the optimization in their subclasses by wrapping the temporary `liquid_drop` method if the subclass doesn't
+override the superclass method:
+```ruby
+class Foo::BarPage < Jekyll::Page
+  def to_liquid(*)
+    liquid_drop    # Returns an instance of `Jekyll::Drops::PageDrop`.
+                   # Will be removed in Jekyll 5.0.
+  end
+end
+```

lib/jekyll/drops/page_drop.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+module Jekyll
+  module Drops
+    class PageDrop < Drop
+      extend Forwardable
+
+      mutable false
+
+      def_delegators :@obj, :content, :dir, :name, :path, :url
+      private def_delegator :@obj, :data, :fallback_data
+    end
+  end
+end

lib/jekyll/page.rb

@@ -70,6 +70,38 @@ def dir
       end
     end
 
+    # For backwards-compatibility in subclasses that do not redefine
+    # the `:to_liquid` method, stash existing definition under a new name
+    #
+    # TODO: Remove in Jekyll 5.0
+    alias_method :legacy_to_liquid, :to_liquid
+    private :legacy_to_liquid
+
+    # Private
+    # Subclasses can choose to optimize their `:to_liquid` method by wrapping
+    # it around this definition.
+    #
+    # TODO: Remove in Jekyll 5.0
+    def liquid_drop
+      @liquid_drop ||= begin
+        defaults = site.frontmatter_defaults.all(relative_path, type)
+        unless defaults.empty?
+          Utils.deep_merge_hashes!(data, Utils.deep_merge_hashes!(defaults, data))
+        end
+        Drops::PageDrop.new(self)
+      end
+    end
+    private :liquid_drop
+
+    # Public
+    #
+    # Liquid representation of current page
+    #
+    # TODO: Remove optional parameter in Jekyll 5.0
+    def to_liquid(attrs = nil)
+      self.class == Jekyll::Page ? liquid_drop : legacy_to_liquid(attrs)
+    end
+
     # The full path and filename of the post. Defined in the YAML of the post
     # body.
     #

test/test_page.rb

@@ -50,6 +50,42 @@ def do_render(page)
         assert_equal "/+/%25%23%20+.html", @page.url
       end
 
+      should "be exposed to Liquid as a Liquid::Drop subclass" do
+        page = setup_page("properties.html")
+        liquid_rep = page.to_liquid
+        refute_equal Hash, liquid_rep.class
+        assert_equal true, liquid_rep.is_a?(Liquid::Drop)
+        assert_equal Jekyll::Drops::PageDrop, liquid_rep.class
+      end
+
+      should "make attributes accessible for use in Liquid templates" do
+        page = setup_page("/contacts", "index.html")
+        template = Liquid::Template.parse(<<~TEXT)
+          Name: {{ page.name }}
+          Path: {{ page.path }}
+          URL:  {{ page.url }}
+        TEXT
+        expected = <<~TEXT
+          Name: index.html
+          Path: contacts/index.html
+          URL:  /contacts/
+        TEXT
+        assert_equal(expected, template.render!("page" => page.to_liquid))
+      end
+
+      should "make front matter data accessible for use in Liquid templates" do
+        page = setup_page("properties.html")
+        template = Liquid::Template.parse(<<~TEXT)
+          TITLE: {{ page.title }}
+          FOO:   {{ page.foo }}
+        TEXT
+        expected = <<~TEXT
+          TITLE: Properties Page
+          FOO:   bar
+        TEXT
+        assert_equal expected, template.render!("page" => page.to_liquid)
+      end
+
       context "in a directory hierarchy" do
         should "create URL based on filename" do
           @page = setup_page("/contacts", "bar.html")

test/test_page_without_a_file.rb

@@ -76,6 +76,12 @@ def render_and_write
           end
         end
       end
+
+      should "be exposed to Liquid as a Hash" do
+        liquid_rep = @page.to_liquid
+        refute_equal Jekyll::Drops::PageDrop, liquid_rep.class
+        assert_equal Hash, liquid_rep.class
+      end
     end
 
     context "with site-wide permalink configuration" do