In a [GUI] application written without a ToolKit the actual program is a long loop that continuously calls a function to poll incoming events and then feeds them into a huge switch construct that decides how to react to each event. Every single facet of the application's behaviour has to be reflected here. Needless to say, writing non-trivial applications this way is tiresome at best. 

  

 A [GUI] ToolKit's main responsibility is to abstract away this event loop in a way that it can be maintained and extended easily. To this end, the different "widgets" (such as buttons, labels, menus etc) of a [GUI] are treated as black boxes. An EventModel is then specified, which defines how events "propagate" across these blackboxes. The EventModel is a large factor in the design of an application's architecture. Different [ToolKit]s tend to use very different [EventModel]s, which can cause a great deal of confusion.