Methods, systems, and apparatus, including computer programs encoded on computer storage media, for creating mobile device applications using a visual programming integrated development environment (IDE). In one aspect, a method includes receiving a user input entered through a visual programming interface of a visual programming IDE, advancing a mobile device application that is under development in the visual programming IDE to a state in which one or more sensor events that are associated with the user input are assumed to have been received, and outputting a mock-up of the mobile device application in the advanced state, through the visual programming interface.