Observer

I subscribe to newspapers/TvChannels
Newspapers/TVchannels subscribe to celebriities.
So we know of events.



Whenever I want time I check my watch but when I am sleeping, I subscribe to an alarm.
Best possible I pull info but (if it is inefficient for an event ) I prefer to be informed.

In general observer pattern is story of a passive observer
that dont want to observe. "Let me know if it changes" attitude.
oh sorry, If too many components need to observe samething?

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

So as I told earlier, A lazy observer subscribes.
Publishers will inform the subscribers when something changes.


So the publisher will have a list of subscribers in a list.

Whenever the publishers property changes...he will inform all subscribers

class Publisher
{
ArrayList mysubscribers;

public MyProperty
{
get {return myProp;}
set
{
myProp = value;
foreach (subscriber s in mysubscribers)
{
inform(s);
}
}
}
}



More than software engineering, this pattern is very well used in real life.

In software Engineering, this is a most used pattern.

1) Even triggers in DB are meant for same purpose.
2) "SQL notification services" is a product to demonstrate the need of this pattern.
3) Do you use gadgets/browse webpages/ subscribe to RSS feeds? -- This is not observer pattern. If you subscribe to an event to get an email, that is observer pattern.