This paper presents a wearable assistance system for service and maintenance personnel in industrial facilities, when used together with mobile smart devices. It provides a practical interaction metaphor designed to fully utilize the user's physical activities, such as touch and hand gestures. As a result of a user activity-driven design approach, the presented system frees user from various constraints during various working operations. Especially, our new developed gesture input method enables the user to immediately interact with services using one-handed motions in an arbitrary postures. Furthermore, an event-driven architecture is proposed to solve the power management problem. We also developed a wristwatch-like wearable device to evaluate the effectiveness of the proposed method. The results presented in this paper demonstrate that the proposed system not only allows for effective and satisfactory service but also cuts down on errors and oversights. Finally, potential applications of this research that include design guidelines for future interface configurations are discussed.