Add shared examples for localisation functionality

The with_localisations method allows us to temporarily set the locale
backend and always cleans up after itself.

Refs #68

spec/support/locales/sample.en.yaml

@@ -0,0 +1,12 @@
+helpers:
+  label:
+    person:
+      name: Who goes there?
+  fieldset:
+    person:
+      favourite_colour: To which colours are you most partial?
+  hint:
+    person:
+      name: Check your birth certificate or passport
+      favourite_colour: |-
+        Yes, there are plenty of options but please pick the one you enjoy most

spec/support/localisation.rb

@@ -0,0 +1,7 @@
+def with_localisations(localisations, locale: :en)
+  I18n.backend.store_translations(locale, localisations[locale])
+
+  yield
+ensure
+  I18n.reload!
+end

spec/support/shared/shared_localisation_examples.rb

@@ -0,0 +1,96 @@
+LOCALISATIONS = {
+  en: YAML.load_file('spec/support/locales/sample.en.yaml')
+}.freeze
+
+shared_examples 'a field that supports setting the label via localisation' do
+  let(:localisations) { LOCALISATIONS }
+
+  context 'localising when no text is supplied' do
+    let(:expected_label) { I18n.translate("helpers.label.person.#{attribute}") }
+    subject { builder.send(*args) }
+
+    specify 'should set the label from the locales' do
+      with_localisations(localisations) do
+        expect(subject).to have_tag('label', text: expected_label)
+      end
+    end
+  end
+
+  context 'allowing localised text to be overridden' do
+    let(:expected_label) { "Yeah but, who are you really?" }
+
+    subject { builder.send(*args.push(label: { text: expected_label })) }
+
+    specify 'should use the supplied label text' do
+      with_localisations(localisations) do
+        expect(subject).to have_tag('label', text: expected_label)
+      end
+    end
+  end
+end
+
+shared_examples 'a field that supports setting the hint via localisation' do
+  let(:arbitrary_html_content) { builder.tag.p("a wild paragraph has appeared") }
+  let(:localisations) { LOCALISATIONS }
+
+  context 'localising when no text is supplied' do
+    let(:expected_hint) { I18n.translate("helpers.hint.person.#{attribute}") }
+
+    subject { builder.send(*args) { arbitrary_html_content } }
+
+    specify 'should set the hint from the locales' do
+      with_localisations(localisations) do
+        expect(subject).to have_tag('span', text: expected_hint, with: { class: 'govuk-hint' })
+      end
+    end
+  end
+
+  context 'allowing localised text to be overridden' do
+    let(:expected_hint) { "It's quite a straightforward question!" }
+
+    subject do
+      builder.send(*args.push(hint_text: expected_hint)) { arbitrary_html_content }
+    end
+
+    specify 'should use the supplied hint text' do
+      with_localisations(localisations) do
+        expect(subject).to have_tag('span', text: expected_hint, with: { class: 'govuk-hint' })
+      end
+    end
+  end
+end
+
+shared_examples 'a field that supports setting the legend via localisation' do
+  let(:arbitrary_html_content) { builder.tag.p("a wild paragraph has appeared") }
+  let(:localisations) { LOCALISATIONS }
+
+  context 'localising when no text is supplied' do
+    let(:expected_legend) { I18n.translate("helpers.fieldset.person.#{attribute}") }
+    subject { builder.send(*args) { arbitrary_html_content } }
+
+    specify 'should set the legend from the locales' do
+      with_localisations(localisations) do
+        expect(subject).to have_tag('fieldset') do
+          with_tag('legend', with: { class: 'govuk-fieldset__legend' }) do
+            with_tag('h1', text: expected_legend, with: { class: 'govuk-fieldset__heading' })
+          end
+        end
+      end
+    end
+  end
+
+  context 'allowing localised text to be overridden' do
+    let(:expected_legend) { "It is quite a straightforward question!" }
+    subject { builder.send(*args.push(legend: { text: expected_legend })) { arbitrary_html_content } }
+
+    specify 'should set the legend from the locales' do
+      with_localisations(localisations) do
+        expect(subject).to have_tag('fieldset') do
+          with_tag('legend', with: { class: 'govuk-fieldset__legend' }) do
+            with_tag('h1', text: expected_legend, with: { class: 'govuk-fieldset__heading' })
+          end
+        end
+      end
+    end
+  end
+end