Add I18n.interpolation_keys

lib/i18n.rb

@@ -231,6 +231,30 @@ def translate!(key, **options)
     end
     alias :t! :translate!
 
+    # Returns an array of interpolation keys for the given translation key
+    #
+    # Suppose we have the following:
+    #   I18n.t 'example.zero' == 'Zero interpolations'
+    #   I18n.t 'example.one' == 'One interpolation %{foo}'
+    #   I18n.t 'example.two' == 'Two interpolations %{foo} %{bar}'
+    #   I18n.t 'example.one', locale: :other == 'One interpolation %{baz} %{bar}'
+    #
+    # Then we can expect the following results:
+    #   I18n.interpolation_keys('example.zero') #=> []
+    #   I18n.interpolation_keys('example.one') #=> ['foo']
+    #   I18n.interpolation_keys('example.two') #=> ['foo', 'bar']
+    #   I18n.interpolation_keys('one', scope: 'example', locale: :other) #=> ['baz']
+    #   I18n.interpolation_keys('does-not-exist') #=> []
+    def interpolation_keys(key, **options)
+      raise I18n::ArgumentError if !key.is_a?(String) || key.empty?
+
+      return [] unless exists?(key, **options.slice(:locale, :scope))
+
+      translation = translate(key, **options.slice(:locale, :scope))
+      interpolation_keys_from_translation(translation)
+        .flatten.compact
+    end
+
     # Returns true if a translation exists for a given key, otherwise returns false.
     def exists?(key, _locale = nil, locale: _locale, **options)
       locale ||= config.locale
@@ -429,6 +453,17 @@ def normalize_key(key, separator)
           keys
         end
     end
+
+    def interpolation_keys_from_translation(translation)
+      case translation
+      when ::String
+        translation.scan(Regexp.union(I18n.config.interpolation_patterns))
+      when ::Array
+        translation.map { |element| interpolation_keys_from_translation(element) }
+      else
+        []
+      end
+    end
   end
 
   extend Base

test/i18n_test.rb

@@ -287,6 +287,34 @@ def setup
     assert_equal I18n.config.available_locales.size * 2, I18n.config.available_locales_set.size
   end
 
+  test "interpolation_keys returns an array of keys" do
+    store_translations(:en, "example_two" => "Two interpolations %{foo} %{bar}")
+    assert_equal ["foo", "bar"], I18n.interpolation_keys("example_two")
+  end
+
+  test "interpolation_keys returns an empty array when no interpolations " do
+    store_translations(:en, "example_zero" => "Zero interpolations")
+    assert_equal [], I18n.interpolation_keys("example_zero")
+  end
+
+  test "interpolation_keys returns an empty array when missing translation " do
+    assert_equal [], I18n.interpolation_keys("does-not-exist")
+  end
+
+  test "interpolation_keys returns an empty array when nested translation" do
+    store_translations(:en, "example_nested" => { "one" => "One %{foo}", "two" => "Two %{bar}" })
+    assert_equal [], I18n.interpolation_keys("example_nested")
+  end
+
+  test "interpolation_keys returns an array of keys when translation is an Array" do
+    store_translations(:en, "example_array" => ["One %{foo}", ["Two %{bar}", ["Three %{baz}"]]])
+    assert_equal ["foo", "bar", "baz"], I18n.interpolation_keys("example_array")
+  end
+
+  test "interpolation_keys raises I18n::ArgumentError when non-string argument" do
+    assert_raises(I18n::ArgumentError) { I18n.interpolation_keys(["bad-argument"]) }
+  end
+
   test "exists? given an existing key will return true" do
     assert_equal true, I18n.exists?(:currency)
   end