FEATURE: backend support for user-selectable components

* FEATURE: backend support for user-selectable components

* fix problems with previewing default theme

* rename preview_key => preview_theme_id

* omit default theme from child themes dropdown and try a different fix

* cache & freeze stylesheets arrays

spec/requests/application_controller_spec.rb

@@ -33,4 +33,74 @@ RSpec.describe ApplicationController do
     end
   end
 
+  describe "#handle_theme" do
+    let(:theme) { Fabricate(:theme, user_selectable: true) }
+    let(:theme2) { Fabricate(:theme, user_selectable: true) }
+    let(:user) { Fabricate(:user) }
+    let(:admin) { Fabricate(:admin) }
+
+    before do
+      sign_in(user)
+    end
+
+    it "selects the theme the user has selected" do
+      user.user_option.update_columns(theme_ids: [theme.id])
+
+      get "/"
+      expect(response.status).to eq(200)
+      expect(controller.theme_ids).to eq([theme.id])
+
+      theme.update_attribute(:user_selectable, false)
+
+      get "/"
+      expect(response.status).to eq(200)
+      expect(controller.theme_ids).to eq([SiteSetting.default_theme_id])
+    end
+
+    it "can be overridden with a cookie" do
+      user.user_option.update_columns(theme_ids: [theme.id])
+
+      cookies['theme_ids'] = "#{theme2.id}|#{user.user_option.theme_key_seq}"
+
+      get "/"
+      expect(response.status).to eq(200)
+      expect(controller.theme_ids).to eq([theme2.id])
+
+      theme2.update!(user_selectable: false)
+      theme.add_child_theme!(theme2)
+      cookies['theme_ids'] = "#{theme.id},#{theme2.id}|#{user.user_option.theme_key_seq}"
+
+      get "/"
+      expect(response.status).to eq(200)
+      expect(controller.theme_ids).to eq([theme.id, theme2.id])
+    end
+
+    it "falls back to the default theme when the user has no cookies or preferences" do
+      user.user_option.update_columns(theme_ids: [])
+      cookies["theme_ids"] = nil
+      theme2.set_default!
+
+      get "/"
+      expect(response.status).to eq(200)
+      expect(controller.theme_ids).to eq([theme2.id])
+    end
+
+    it "can be overridden with preview_theme_id param" do
+      sign_in(admin)
+      cookies['theme_ids'] = "#{theme.id},#{theme2.id}|#{admin.user_option.theme_key_seq}"
+
+      get "/", params: { preview_theme_id: theme2.id }
+      expect(response.status).to eq(200)
+      expect(controller.theme_ids).to eq([theme2.id])
+    end
+
+    it "cookie can fail back to user if out of sync" do
+      user.user_option.update_columns(theme_ids: [theme.id])
+      cookies['theme_ids'] = "#{theme2.id}|#{user.user_option.theme_key_seq - 1}"
+
+      get "/"
+      expect(response.status).to eq(200)
+      expect(controller.theme_ids).to eq([theme.id])
+    end
+  end
 end