Provide payload to initialize StaticSiteEditor

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/212560

* Add `Gitlab::StaticSiteEditor::Config` entity to generate payload
* Populate `data` attribute with required attributes

app/controllers/projects/static_site_editor_controller.rb

 # frozen_string_literal: true
 
 class Projects::StaticSiteEditorController < Projects::ApplicationController
+  include ExtractsPath
   layout 'fullscreen'
 
   prepend_before_action :authenticate_user!, only: [:show]
+  before_action :assign_ref_and_path, only: [:show]
 
   def show
+    @config = Gitlab::StaticSiteEditor::Config.new(@repository, @ref, @path, params[:return_url])
+  end
+
+  private
+
+  def assign_ref_and_path
+    @ref, @path = extract_ref(params[:id])
+
+    render_404 if @ref.blank? || @path.blank?
   end
 end

app/views/projects/static_site_editor/show.html.haml

-#static-site-editor{ data: { project_id: '8', path: 'README.md' } }
+#static-site-editor{ data: @config.payload }

changelogs/unreleased/212560_initialize_sse_frontend.yml

+---
+title: Provide configuration options for Static Site Editor
+merge_request: 29058
+author:
+type: added

lib/gitlab/static_site_editor/config.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module StaticSiteEditor
+    class Config
+      def initialize(repository, ref, file_path, return_url)
+        @repository = repository
+        @ref = ref
+        @file_path = file_path
+        @return_url = return_url
+      end
+
+      def payload
+        {
+          branch: ref,
+          path: file_path,
+          commit: commit.id,
+          project_id: project.id,
+          project: project.path,
+          namespace: project.namespace.path,
+          return_url: return_url
+        }
+      end
+
+      private
+
+      attr_reader :repository, :ref, :file_path, :return_url
+
+      delegate :project, to: :repository
+
+      def commit
+        repository.commit(ref)
+      end
+    end
+  end
+end

spec/controllers/projects/static_site_editor_controller_spec.rb

@@ -10,7 +10,8 @@ describe Projects::StaticSiteEditorController do
       {
         namespace_id: project.namespace,
         project_id: project,
-        id: 'master/README.md'
+        id: 'master/README.md',
+        return_url: 'http://example.com'
       }
     end
 
@@ -38,6 +39,18 @@ describe Projects::StaticSiteEditorController do
           it 'renders the edit page' do
             expect(response).to render_template(:show)
           end
+
+          it 'assigns a config variable' do
+            expect(assigns(:config)).to be_a(Gitlab::StaticSiteEditor::Config)
+          end
+
+          context 'when combination of ref and file path is incorrect' do
+            let(:default_params) { super().merge(id: 'unknown') }
+
+            it 'responds with 404 page' do
+              expect(response).to have_gitlab_http_status(:not_found)
+            end
+          end
         end
       end
     end

spec/lib/gitlab/static_site_editor/config_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::StaticSiteEditor::Config do
+  subject(:config) { described_class.new(repository, ref, file_path, return_url) }
+
+  let(:project) { create(:project, :public, :repository, name: 'project', namespace: namespace) }
+  let(:namespace) { create(:namespace, name: 'namespace') }
+  let(:repository) { project.repository }
+  let(:ref) { 'master' }
+  let(:file_path) { 'README.md' }
+  let(:return_url) { 'http://example.com' }
+
+  describe '#payload' do
+    subject { config.payload }
+
+    it 'returns data for the frontend component' do
+      is_expected.to eq(
+        branch: 'master',
+        commit: repository.commit.id,
+        namespace: 'namespace',
+        path: 'README.md',
+        project: 'project',
+        project_id: project.id,
+        return_url: 'http://example.com'
+      )
+    end
+  end
+end