Implement aggressive caching

Author: DavidS

README.md

@@ -44,6 +44,12 @@ If you want an inline display of your photos, I recommend [glightbox](https://gi
 
 at the bottom of the layout.
 
+## Caching
+
+This plugin uses aggressive caching to keep render times short.
+If you need to re-render images for any reason, remove the `.jekyll-cache` folder or change the `_config.yml` file.
+See the [Cache API tutorial](https://jekyllrb.com/tutorials/cache-api/) for some background.
+
 ## Development
 
 After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake spec` to run the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment. There is also a test site in `spec/fixtures/test_site` that you can use to try out changes.

lib/cheesy-gallery/base_image_file.rb

@@ -8,6 +8,8 @@
 class CheesyGallery::BaseImageFile < Jekyll::StaticFile
   extend T::Sig
 
+  @@render_cache = T.let(Jekyll::Cache.new('CheesyGallery::Render'), Jekyll::Cache) # don't need to worry about inheritance here # rubocop:disable Style/ClassVars
+
   sig { params(site: Jekyll::Site, collection: Jekyll::Collection, file: Jekyll::StaticFile, dest_path: T.nilable(String)).void }
   def initialize(site, collection, file, dest_path = nil)
     @source_file = T.let(file, Jekyll::StaticFile)
@@ -27,6 +29,25 @@ def process_and_write(img, path)
     img.write(path) {}
   end
 
+  # Inject cache here to override default delete-before-copy behaviour
+  # See jekyll:lib/jekyll/static_file.rb for source
+  def write(dest)
+    dest_path = destination(dest)
+    return false if File.exist?(dest_path) && !modified?
+
+    self.class.mtimes[path] = mtime
+
+    return if @@render_cache.key?("#{dest_path}-rendered") && File.exist?(dest_path)
+
+    FileUtils.mkdir_p(File.dirname(dest_path))
+    FileUtils.rm(dest_path) if File.exist?(dest_path)
+    copy_file(dest_path)
+
+    @@render_cache["#{dest_path}-rendered"] = true
+
+    true
+  end
+
   private
 
   # instead of copying, allow rmagick processing