Since most software systems provide their users with interactive features, building user interfaces (UI) is one of the core software engineering tasks. It consists in designing, implementing and testing ever more sophisticated and versatile ways for users to interact with software systems, and safely connecting these interactions with commands querying or modifying their state. However, most UI frameworks still rely on a low level model, the bare bone UI event processing model. This model was suitable for the rather simple UIs of the early 80's (menus, buttons, keyboards, mouse clicks), but now exhibits major software engineering flaws for modern, highly interactive UIs. These flaws include lack of separation of concerns, weak modularity and thus low reusability of code for advanced interactions, as well as low testability. To mitigate these flaws, we propose Interacto as a high level user interaction processing model. By reifying the concept of user interaction, Interacto makes it easy to design, implement and test modular and reusable advanced user interactions, and to connect them to commands with built-in undo/redo support. To demonstrate its applicability and generality, we briefly present two open source implementations of Interacto for Java/JavaFX and TypeScript/Angular. We evaluate Interacto interest (1) on a real world case study, where it has been used since 2013, and with (2) a controlled experiment with 44 master students, comparing it with traditionnal UI frameworks.