observeProperty<T> method
observe a property for changes
will be added to event for property changes
Implementation
Future<T> observeProperty<T>(String propertyKey) {
// create the id assigned with this property
// +1 because time-pos (which has the id 0) is not included in this object
int prop_id = observedProperties.length + 1;
// store the id into the hash map, such that it can be retrieved later if
// the property should be unobserved
observedProperties[propertyKey] = prop_id;
return command<T>('observe_property', [prop_id, propertyKey]);
}