In industrial practice, GUI (Graphic User Interface) testing of mobile apps still inevitably relies on huge manual efforts. The major efforts are those on understanding the GUIs, so that testing scripts can be written accordingly. Quality assurance could therefore be very labor-intensive, especially for modern commercial mobile apps, where one may include tremendous, diverse, and complex GUIs, e.g., those for placing orders of different commercial items. To reduce such human efforts, we propose Appaction, a learning-based automatic GUI interaction approach we developed for Meituan, one of the largest E-commerce providers with over 600 million users. Appaction can automatically analyze the target GUI and understand what each input of the GUI is about, so that corresponding valid inputs can be entered accordingly. To this end, Appaction adopts a multi-modal model to learn from human experiences in perceiving a GUI. This allows it to infer corresponding valid input events that can properly interact with the GUI. In this way, the target app can be effectively exercised. We present our experiences in Meituan on applying Appaction to popular commercial apps. We demonstrate the effectiveness of Appaction in GUI analysis, and it can perform correct interactions for numerous form pages.