Inspired by a post which talks about remind users to update app when they exit from app rather than enter. Obviously, user wants to browse the content in app and update reminder will interrupt the action which is not good for user experience.
The above post shows each side of implement but VersionAgent, Harpy, which is similar one and could be found on Github or Cocoapods. If you feel the library is too large and cannot use all of features, just implement daily version check and combine with code provided by the first post.
If you would like it fancier and differentiate features for banner and alert reminder popup, this post would help.