Very incomplete first stab

Gemfile

@@ -10,3 +10,4 @@ gem 'httparty'
 
 gem 'aws-sdk', '~> 2', require: false
 gem 'google-cloud-storage', require: false
+gem 'mini_magick'

Gemfile.lock

@@ -99,6 +99,7 @@ GEM
     mime-types (3.1)
       mime-types-data (~> 3.2015)
     mime-types-data (3.2016.0521)
+    mini_magick (4.8.0)
     mini_portile2 (2.1.0)
     minitest (5.10.2)
     multi_json (1.12.1)
@@ -143,6 +144,7 @@ DEPENDENCIES
   byebug
   google-cloud-storage
   httparty
+  mini_magick
   rake
   sqlite3
 

lib/active_storage/controllers/variants_controller.rb

+require "action_controller"
+require "active_storage/blob"
+
+class ActiveStorage::Controllers::VariantsController < ActionController::Base
+  def show
+    if blob_key = decode_verified_key
+      variant = ActiveStorage::Variant.lookup(blob_key: blob_key, variation_key: params[:variation_key])
+      redirect_to variant.url
+    else
+      head :not_found
+    end
+  end
+
+  private
+    def decode_verified_key
+      ActiveStorage::VerifiedKeyWithExpiration.decode(params[:encoded_key])
+    end
+
+    def disposition_param
+      params[:disposition].presence_in(%w( inline attachment )) || 'inline'
+    end
+end

lib/active_storage/engine.rb

@@ -33,6 +33,18 @@ class Engine < Rails::Engine # :nodoc:
       end
     end
 
+    initializer "active_storage.verifiers" do
+      require "active_storage/verified_key_with_expiration"
+      require "active_storage/variant"
+
+      config.after_initialize do |app|
+        ActiveStorage::VerifiedKeyWithExpiration.verifier = \
+        ActiveStorage::Variant.verifier = \
+          Rails.application.message_verifier('ActiveStorage')
+      end
+    end
+
+    initializer "active_storage.services" do
       config.after_initialize do |app|
         if config_choice = app.config.active_storage.service
           config_file = Pathname.new(Rails.root.join("config/storage_services.yml"))
@@ -59,4 +71,5 @@ class Engine < Rails::Engine # :nodoc:
         end
       end
     end
+  end
 end

lib/active_storage/routes.rb

 get  "/rails/active_storage/disk/:encoded_key/*filename" => "active_storage/disk#show", as: :rails_disk_blob
+get  "/rails/active_storage/variants/:encoded_key/:encoded_transformation/*filename" => "active_storage/controllers/variants#show", as: :rails_blob_variant
 post "/rails/active_storage/direct_uploads" => "active_storage/direct_uploads#create", as: :rails_direct_uploads

lib/active_storage/variant.rb

+require "active_storage/blob"
+require "mini_magick"
+
+class ActiveStorage::Variant
+  class_attribute :verifier
+
+  attr_reader :blob, :variation
+  delegate :service, to: :blob
+
+  def self.lookup(blob_key:, variation_key:)
+    new ActiveStorage::Blob.find_by!(key: blob_key), variation: verifier.verify(variation_key)
+  end
+
+  def self.encode_key(variation)
+    verifier.generate(variation)
+  end
+
+  def initialize(blob, variation:)
+    @blob, @variation = blob, variation
+  end
+
+  def url(expires_in: 5.minutes, disposition: :inline)
+    perform unless exist?
+    service.url key, expires_in: expires_in, disposition: disposition, filename: blob.filename
+  end
+
+  def key
+    verifier.generate(variation)
+  end
+
+  private
+    def perform
+      upload_variant transform(download_blob)
+    end
+
+    def download_blob
+      service.download(blob.key)
+    end
+
+    def upload_variant(variation)
+      service.upload key, variation
+    end
+
+    def transform(io)
+      # FIXME: Actually do a variant based on the variation
+      File.open MiniMagick::Image.read(io).resize("500x500").path
+    end
+
+    def exist?
+      service.exist?(key)
+    end
+end

lib/active_storage/verified_key_with_expiration.rb

 class ActiveStorage::VerifiedKeyWithExpiration
-  class_attribute :verifier, default: defined?(Rails) ? Rails.application.message_verifier('ActiveStorage') : nil
+  class_attribute :verifier
 
   class << self
     def encode(key, expires_in: nil)

test/test_helper.rb

@@ -24,6 +24,9 @@
 require "active_storage/verified_key_with_expiration"
 ActiveStorage::VerifiedKeyWithExpiration.verifier = ActiveSupport::MessageVerifier.new("Testing")
 
+require "active_storage/variant"
+ActiveStorage::Variant.verifier = ActiveSupport::MessageVerifier.new("Testing")
+
 class ActiveSupport::TestCase
   private
     def create_blob(data: "Hello world!", filename: "hello.txt", content_type: "text/plain")

test/variation_test.rb

+require "test_helper"
+require "database/setup"
+require "active_storage/variant"
+
+class ActiveStorage::VariationTest < ActiveSupport::TestCase
+  test "square variation" do
+    blob = ActiveStorage::Blob.create_after_upload! \
+      io: File.open(File.expand_path("../fixtures/files/racecar.jpg", __FILE__)), filename: "racecar.jpg", content_type: "image/jpeg"
+
+    variation_key = ActiveStorage::Variant.encode_key(resize: "500x500")
+
+    variant = ActiveStorage::Variant.lookup(blob_key: blob.key, variation_key: variation_key)
+
+    assert_match /racecar.jpg/, variant.url
+  end
+end