An essential step of UI design is sending your files off for development. The more organized the process is, the better the design will be implemented. There are several programs available whose sole purpose is to ensure a seamless hand-off of files.
Adobe XD - Design Specs
Design Specs is a web-based platform used to hand-off design files to development and is integrated with Adobe XD. This recent update opens up the possibility of using one program to design/prototype/deliver an app. Read more here