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 code revisions. We developed Doppio, a tool that automatically tracks and visualizes UI flows and their changes based on source code. For each input event listener of a widget, e.g., onClick of an Android View class, Doppio captures and associates its UI output from a program execution with its code snippet from the codebase. It automatically generates a screenflow diagram organized by the callback methods and interaction flow, where developers can review the code and UI revisions interactively. Doppio, as an IDE plugin, is seamlessly integrated into a common development workflow. Our studies show that our tool is able to generate quality visual documentation and helped participants understand unfamiliar source code and track changes.