A control is a gadget which is linked to an underlying model by having its model slot set to a model instance.

A model is an observable value. Changing a model's value notifies other objects which depend on the model automatically, ...

set-model ( value model -- ) Changes the value of a model and calls model-changed on all observers registered with add-connection.

add-connection ( observer model -- ) Registers an object interested in being notified of changes to the model's value. When the value is changed as a result of a call to set-model, the model-changed word is called on the observer.

model-changed ( model observer -- ) Called to notify observers of a model that the model value has changed as a result of a call to set-model. Observers can be registered with add-connection.