Developing interactive systems often involves a large set of callback functions for handling user interaction, which makes it challenging to manage UI behaviors, create descriptive documentation, and track revisions. We developed Doppio, a tool that automatically tracks and visualizes UI flows and their changes based on source code elements and their revisions. For each input event listener of a widget, e.g., onClick of an Android View class, Doppio captures and associates its UI output from an execution of the program with its code snippet from the source code. It automatically generates a screenflow diagram that is organized by the callback methods and interaction flow, where developers can review the code and UI revisions interactively. Doppio, implemented as an IDE plugin, is seamlessly integrated into a common development workflow. Our experiments show that Doppio was able to generate quality visual documentation and helped participants understand unfamiliar source code and track changes.