  Reactive applications (rapps) are of interest because of the explosion of
mobile, tablet and web-based platforms. The complexity and proliferation of
implementation technologies makes it attractive to use model-driven techniques
to develop rapp systems. This article proposes a domain specific language for
rapps consisting of stereotyped class models for the structure of the
application and state machine models for the application behaviour. The models
are given a semantics in terms of a transformation to a calculus called Widget.
The languages are introduced using an example application for mobile phones.
