Raising an error if generate_string_file would create an empty file.

README.md

@@ -100,7 +100,7 @@ If you would like to enable twine to create language files in another format, cr
 
 #### `generate-string-file`
 
-This command creates an Apple or Android strings file from the master strings data file.
+This command creates an Apple or Android strings file from the master strings data file. If the output file would not contain any translations, twine will exit with an error.
 
 	$ twine generate-string-file /path/to/strings.txt values-ja.xml --tags common,app1
 	$ twine generate-string-file /path/to/strings.txt Localizable.strings --lang ja --tags mytag

lib/twine/runner.rb

@@ -51,6 +51,8 @@ module Twine
       formatter, lang = prepare_read_write(@options[:output_path], lang)
       output = formatter.format_file(lang)
 
+      raise Twine::Error.new "Nothing to generate! The resulting file would not contain any strings." unless output
+
       IO.write(@options[:output_path], output, encoding: encoding)
     end
 

test/test_generate_string_file.rb

@@ -49,6 +49,15 @@ class TestGenerateStringFile < CommandTestCase
     new_runner(nil, "#{random_language}.xml").generate_string_file
   end
 
+  def test_returns_error_if_nothing_written
+    formatter = prepare_mock_formatter Twine::Formatters::Android
+    formatter.expects(:format_file).returns(false)
+
+    assert_raises Twine::Error do
+      new_runner('fr', 'fr.xml').generate_string_file
+    end
+  end
+
   class TestValidate < CommandTestCase
     def new_runner(validate)
       options = {}