Add language switching mechanism

Continuing to add support for #233, this commit adds a language switcher
element to the secondary navigation bar. The base application controller
now inspects the user's language choice, whether by URL parameter or by
HTTP header, and sets the `I18n.locale` accordingly.

Gemfile

@@ -32,6 +32,7 @@ gem 'sentry-raven'
 
 gem 'faraday'
 gem 'faraday_middleware'
+gem 'http_accept_language'
 gem 'puma'
 gem 'yajl-ruby', require: 'yajl'
 

Gemfile.lock

@@ -134,6 +134,7 @@ GEM
       haml (>= 4.0, < 6)
       nokogiri (>= 1.6.0)
       ruby_parser (~> 3.5)
+    http_accept_language (2.1.1)
     i18n (1.8.5)
       concurrent-ruby (~> 1.0)
     js-routes (1.4.9)
@@ -322,6 +323,7 @@ DEPENDENCIES
   govuk_template
   haml-lint
   haml-rails
+  http_accept_language
   js-routes
   json_expressions
   m

app/assets/stylesheets/_ukhpi-header.scss

@@ -40,3 +40,9 @@
 .u-active {
   border-bottom: 2px solid $black;
 }
+
+.o-secondary-banner {
+  display: flex;
+  flex-direction: row;
+  justify-content: space-between;
+}

app/controllers/application_controller.rb

@@ -5,4 +5,19 @@ class ApplicationController < ActionController::Base
   # Prevent CSRF attacks by raising an exception.
   # For APIs, you may want to use :null_session instead.
   protect_from_forgery with: :exception
+
+  before_action :set_locale
+
+  private
+
+  # Set the user's preferred locale. An explicit locale set via
+  # the URL param `lang` is preeminent, otherwise we look to the
+  # user's preferred language specified via browser headers
+  def set_locale
+    user_locale =
+      params['lang'] ||
+      http_accept_language.compatible_language_from(I18n.available_locales)
+
+    I18n.locale = user_locale if user_locale
+  end
 end

app/models/concerns/user_language.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+# Shared code for working with user lanuage choices
+module UserLanguage
+  def english?
+    I18n.locale != :cy
+  end
+
+  def welsh?
+    I18n.locale == :cy
+  end
+
+  def alternative_language_params
+    with('lang', english? ? 'cy' : 'en')
+  end
+end

app/models/user_compare_selections.rb

@@ -4,6 +4,7 @@
 # each other
 class UserCompareSelections
   include UserChoices
+  include UserLanguage
 
   DEFAULT_INDICATOR = 'hpi'
   DEFAULT_STATISTIC = 'all'
@@ -16,6 +17,7 @@ class UserCompareSelections
     'to' => Struct::UserParam.new(Time.zone.today, false, nil),
     'st' => Struct::UserParam.new(DEFAULT_STATISTIC, false, nil),
     'in' => Struct::UserParam.new(DEFAULT_INDICATOR, false, nil),
+    'lang' => Struct::UserParam.new(:en, false, nil),
 
     # used by selections update form
     'form-action' => Struct::UserParam.new(nil, false, nil),

app/models/user_selections.rb

@@ -11,13 +11,15 @@
 class UserSelections # rubocop:disable Metrics/ClassLength
   include UserChoices
   include UserSelectionValidations
+  include UserLanguage
 
   DEFAULT_INDICATORS = %w[hpi avg pmc pac].freeze
   DEFAULT_STATISTICS = %w[all].freeze
   DEFAULT_NON_PT_INDICATORS = %w[salesVolume].freeze
   DEFAULT_REGION = 'http://landregistry.data.gov.uk/id/region/united-kingdom'
   DEFAULT_REGION_TYPE = 'country'
   DEFAULT_THEMES = %w[property_type volume].freeze
+  DEFAULT_LANGUAGE = 'en'
 
   USER_PARAMS_MODEL = {
     'location' => Struct::UserParam.new(DEFAULT_REGION, false, nil),
@@ -31,6 +33,7 @@ class UserSelections # rubocop:disable Metrics/ClassLength
     'st' => Struct::UserParam.new(DEFAULT_STATISTICS, true, nil),
     'in' => Struct::UserParam.new(DEFAULT_INDICATORS, true, nil),
     'thm' => Struct::UserParam.new(DEFAULT_THEMES, true, nil),
+    'lang' => Struct::UserParam.new(DEFAULT_LANGUAGE, false, nil),
 
     # used by selections update form
     'form-action' => Struct::UserParam.new(nil, false, nil),

app/presenters/compare_locations_presenter.rb

@@ -9,6 +9,7 @@ class CompareLocationsPresenter # rubocop:disable Metrics/ClassLength
   include LocationsTable
 
   attr_reader :user_compare_selections, :query_results
+  alias user_selections user_compare_selections
 
   def initialize(user_compare_selections, query_results)
     @user_compare_selections = user_compare_selections

app/services/language_selector.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+# Shared service for managing user language selection
+class LanguageSelector
+end

app/views/common/_header.html.haml

@@ -33,9 +33,20 @@
 .o-lr-top-bar
 
 .container.o-container
-  .o-beta-banner
-    %p
+  .o-secondary-banner
+    .o-secondary-banner__phase-tag
       %strong.o-beta-tag
         = t('common.header.beta_tag')
       %span.text-muted
-        = t('common.header.feedback_request').html_safe
+        = t('common.header.feedback_request').html_safe
+
+    .o-secondary-banner__lang-select
+      - if @view_state.user_selections.english?
+        English
+      - else
+        = link_to('English', @view_state.user_selections.alternative_language_params.params)
+      |
+      - if @view_state.user_selections.welsh?
+        Cymraeg
+      - else
+        = link_to('Cymraeg', @view_state.user_selections.alternative_language_params.params)

app/views/layouts/application.html.haml

@@ -38,7 +38,7 @@
     = render partial: 'common/flash_message'
 
     %main.container.o-container
-      .o-beta-banner
+      .o-secondary-banner
         %p
           %strong.o-beta-tag BETA
           %span.text-muted

config/locales/cy.yml

@@ -0,0 +1,6 @@
+cy:
+  common:
+    header:
+      home_link: "!!! Go to the UK HPI homepage"
+      app_title: "!!! UK House Price Index"
+      beta_tag: "!!! BETA"

test/models/user_selections_test.rb

@@ -230,5 +230,52 @@ class UserSelectionsTest < ActiveSupport::TestCase
         _(selections.valid?).must_equal(false)
       end
     end
+
+    describe 'language handling' do
+      it 'should return English as the default' do
+        selections = user_selections({})
+        assert selections.english?
+        assert_not selections.welsh?
+      end
+
+      it 'should return Welsh when that language is selected' do
+        selections = user_selections('lang' => 'cy')
+        assert_not selections.english?
+        assert selections.welsh?
+      end
+
+      it 'should return English when that language is selected' do
+        selections = user_selections('lang' => 'en')
+        assert selections.english?
+        assert_not selections.welsh?
+      end
+
+      it 'should ignore other languages' do
+        selections = user_selections('lang' => 'fr')
+        assert selections.english?
+        assert_not selections.welsh?
+      end
+
+      it 'should generate the correct Welsh language options' do
+        selections = user_selections(
+          'from' => '2017-01'
+        )
+
+        alt_params = selections.alternative_language_params
+        _(alt_params.params['from']).must_equal('2017-01')
+        _(alt_params.params['lang']).must_equal('cy')
+      end
+
+      it 'should generate the correct English language options' do
+        selections = user_selections(
+          'from' => '2017-01',
+          'lang' => 'cy'
+        )
+
+        alt_params = selections.alternative_language_params
+        _(alt_params.params['from']).must_equal('2017-01')
+        _(alt_params.params['lang']).must_equal('en')
+      end
+    end
   end
 end