Smart home systems have developed rapidly and entered the daily lives of many people. However, it is difficult for products from different manufacturers to work together due to the interoperability barriers posed by divergent hardware, communication protocols and APIs. In this paper, we introduce HolonCraft as a tool to help the regular IoT user in building complex systems. HolonCraft builds on the Holon ontology which enables systems to share their descriptions so that devices can recognize and understand each other and their respective functions, and work together through composing a system of systems (SoS) at runtime to fulfill a given user workflow. HolonCraft extends the machine-readable information provided by the Holon ontology and uses open-source editors (namely Google Blockly and Microsoft Monaco) to implement a cross-platform graphical programming editor for smart home automation. HolonCraft automatically transforms device capabilities into visual programming elements, enabling users without programming backgrounds to design smart home automation through simple drag-And-drop operations while also supporting advanced programming features such as branches, loops and functions. It then type-checks the composed workflow and, accordingly, generates code to actualize it. In consequence, HolonCraft dramatically improves the abstraction and expressiveness of automation as indicated by our user experiments.