1. A system comprising: one or more computers; and a data storage device coupled to the one or more computers having instructions stored thereon which, when executed by the one or more computers, cause the one or more computers to perform operations comprising: altering code of a mobile device application that is under development in a visual programming integrated development environment, to replace a portion of the code which triggers an occurrence of an event based on a receipt of sensor data, with a different portion of code which triggers the occurrence of the event based on a receipt of a user input entered through a visual programming interface of the visual programming integrated development environment; receiving the user input entered through the visual programming interface of the visual programming integrated development environment, invoking the different portion of code, thereby advancing the mobile device application that is under development in the visual programming integrated development environment to a state in which the sensor data is assumed to have been received, and outputting a mock-up of the mobile device application in the advanced state, through the visual programming interface.