Add new GitlabDanger class

This class encapsulates our use of the Danger gem.

lib/gitlab_danger.rb

@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+class GitlabDanger
+  LOCAL_RULES ||= %w[
+    changes_size
+    gemfile
+    documentation
+    frozen_string
+    duplicate_yarn_dependencies
+    prettier
+    eslint
+    database
+  ].freeze
+
+  CI_ONLY_RULES ||= %w[
+    metadata
+    changelog
+    specs
+    commit_messages
+    roulette
+    single_codebase
+    gitlab_ui_wg
+    ce_ee_vue_templates
+    only_documentation
+  ].freeze
+
+  MESSAGE_PREFIX = '==>'.freeze
+
+  attr_reader :gitlab_danger_helper
+
+  def initialize(gitlab_danger_helper)
+    @gitlab_danger_helper = gitlab_danger_helper
+  end
+
+  def self.local_warning_message
+    "#{MESSAGE_PREFIX} Only the following Danger rules can be run locally: #{LOCAL_RULES.join(', ')}"
+  end
+
+  def self.success_message
+    "#{MESSAGE_PREFIX} No Danger rule violations!"
+  end
+
+  def rule_names
+    ci? ? LOCAL_RULES | CI_ONLY_RULES : LOCAL_RULES
+  end
+
+  def html_link(str)
+    self.ci? ? gitlab_danger_helper.html_link(str) : str
+  end
+
+  def ci?
+    !gitlab_danger_helper.nil?
+  end
+end

spec/lib/gitlab_danger_spec.rb

@@ -0,0 +1,76 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe GitlabDanger do
+  let(:gitlab_danger_helper) { nil }
+
+  subject { described_class.new(gitlab_danger_helper) }
+
+  describe '.local_warning_message' do
+    it 'returns an informational message with rules that can run' do
+      expect(described_class.local_warning_message).to eq('==> Only the following Danger rules can be run locally: changes_size, gemfile, documentation, frozen_string, duplicate_yarn_dependencies, prettier, eslint, database')
+    end
+  end
+
+  describe '.success_message' do
+    it 'returns an informational success message' do
+      expect(described_class.success_message).to eq('==> No Danger rule violations!')
+    end
+  end
+
+  describe '#rule_names' do
+    context 'when running locally' do
+      it 'returns local only rules' do
+        expect(subject.rule_names).to eq(described_class::LOCAL_RULES)
+      end
+    end
+
+    context 'when running under CI' do
+      let(:gitlab_danger_helper) { double('danger_gitlab_helper') }
+
+      it 'returns all rules' do
+        expect(subject.rule_names).to eq(described_class::LOCAL_RULES | described_class::CI_ONLY_RULES)
+      end
+    end
+  end
+
+  describe '#html_link' do
+    context 'when running locally' do
+      it 'returns the same string' do
+        str = 'something'
+
+        expect(subject.html_link(str)).to eq(str)
+      end
+    end
+
+    context 'when running under CI' do
+      let(:gitlab_danger_helper) { double('danger_gitlab_helper') }
+
+      it 'returns a HTML link formatted version of the string' do
+        str = 'something'
+        html_formatted_str = %Q{<a href="#{str}">#{str}</a>}
+
+        expect(gitlab_danger_helper).to receive(:html_link).with(str).and_return(html_formatted_str)
+
+        expect(subject.html_link(str)).to eq(html_formatted_str)
+      end
+    end
+  end
+
+  describe '#ci?' do
+    context 'when gitlab_danger_helper is not available' do
+      it 'returns false' do
+        expect(subject.ci?).to be_falsey
+      end
+    end
+
+    context 'when gitlab_danger_helper is available' do
+      let(:gitlab_danger_helper) { double('danger_gitlab_helper') }
+
+      it 'returns true' do
+        expect(subject.ci?).to be_truthy
+      end
+    end
+  end
+end