GitHub User: JiriChara
ActiveSupport Concern is nice way to extend some class with module mixins, but to keep my controllers clean I rather use kindergarten. It is more convenient way to deal with controllers cleanup. Kindergarten additionally use CanCan for easier authorization (no more Ability.rb) and some more useful features.
I've dealt with the same issue on titanium appcelerator mobile platfom. We need a high security in our app, that's why we cannot apply password authorization type. So I have set redirect uri to localhost and I've extracted grant code directly from redirect URL. You can check https://github.com/OrganisedMinds/titanium-oauth2-client for your inspiration, but be aware - this is still in development. Anyway there are already some oauth 2.0 libraries from google developers written in java if you consider to native android development.