4.2.2 Observers

Observers are used to link graphical elements with the model. An observer is notified whenever a model has changed its state, i.e. whenever an event has been executed. In response, the observer will query the model’s state and triggers actions on the linked graphical elements in respect to the new state.

In general, observers are defined in the JavaScript file. BMotionWeb comes with some predefined observers that are described in the following sections. Formula Observer

The formula observer observes a list of formulas (e.g. expressions, predicates or single variables) and triggers a function whenever a state change occurred. The values of the formulas and the origin (the reference to the graphical element that the observer is attached to) are passed to the trigger function. Within the trigger function you can change the attributes of the origin (e.g. its colour or position) according to the values of the formulas in the respective state. The following options can be passed to the formula observer:

Example formula observer:

bms.observe("formula", {
  selector: "#door",
  formulas: ["cur_floor", "door_open"],
  translate: true,
  trigger: function (origin, values) {
    switch (values[0]) {
      case -1: origin.attr("y", "275"); break
      case 0: origin.attr("y", "175"); break
      case 1: origin.attr("y", "60"); break
    if(values[1]) {
      origin.attr("fill", "white");
    } else {
      origin.attr("fill", "lightgray");


  1. http://api.jquery.com/.