The Observer Pattern is one of the heavily used ones and well, the name says it : It’s purpose is to observe other classes for any updates. To be a bit more exact: you register the Observer classes at the Subject class and when the Subject changes, then all Observers get notified.
So, to be fair, the name is a bit misleading, it should be better called: Notifier Pattern or Subscriber Pattern.
Freeman and Robson are comparing the behavior of this Pattern to a subscription of a newspaper (or a magazine if you want): The subscribers register themselves at the publisher – which means that they give him their addresses. Whenever there is a new edition out, every subscriber gets it delivered.
That’s way more convenient than going to the store every some days, hours or minutes to see if a new one arrived (which is usually called polling).
Unglücklicherweise ist dieser Eintrag noch nicht ganz fertig, bitte habt ein wenig Geduld.
Unfortunately this entry is not yet finished – stay tuned.