How do I use the Xcode comparison function？

Question

I succeed before, but what's the problem now? It reminds me "File was added", but how do I add an old version file to the right blank and comparing it with the left? Thanks. picture displayed

Answer 1

The comparison tool is only available for files that are under source control. Clicking the clock symbol will give the timeline with commits to select the versions for each side; you might want to pick one from directly below the editor as well.