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basically Observer pattern says that there is an object that is a subject of interest of other objects. Let's say for example, we have a <code>Kid</code> object, who has properties such as <code>MathGrade</code>, <code>GeographyGrade</code> etc.. Now, objects <code>Mother</code>, <code>Father</code>, <code>Principal</code> are all interested in changes of the <code>Kid</code> object's properties. They have basically 2 options- <ol> <li>coming up to the <code>Kid</code> every day and ask 'has your <code>MathGrade</code> changed?' 'has your <code>GeographyGrade</code> changed?' etc. that approach is not very efficient. </li> <li>they can just say to the <code>Kid</code>- 'OK, here's my number- call me if your <code>MathGrade</code> changes'. that is essentially the Observer pattern: whoever is interested in changes to the object registers themselves with that object, and is guaranteed to be notified whenever a change occurs. </li> </ol> And if to get back to the programming world- you would have an <code>IGradeObserver</code> interface, which will declare a <code>MathGradeChanged(int newGrade)</code>, <code>GeorgraphyGradeChanged(int newGrade)</code> functions. The classes <code>Mother</code>, <code>Father</code>, and <code>Principal</code> would implement <code>IGradeObserver</code>. Now, the <code>Kid</code> class will have a list of <code>IGradeObservers</code>, and a <code>RegisterObserver(IGradeObserver observer)</code> method. Now, whenever a grade changes, the <code>Kid</code> class would iterate through its Observers list, and notify them of that change. callbacks and event listeners are one way of implementing this pattern. I suggest googling this, I've found a couple of interesting guides myself (<a href="http://code.pavankumar.info/archives/85" rel="nofollow">here</a>, and <a href="http://elope.wordpress.com/2006/04/07/tutorial-on-observer-pattern-2/" rel="nofollow">here</a>). I'm sure you can find some others as well, also pertaining to the programming language of your choice. good luck. 
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1. POObserver Pattern - How to handle unexpected changes?
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1. COIt's n that I don't understand the concepts. It's the mechanics that confuse me, specially if you have to handle unexpected data changes and you have no trigger mechanism, like pressing of a btn, querying a server, etc... unless you have a trigger mechanism how do you know that something has changed? Unless you are always checking... or the object is self-conscious of it's state at all times, and this is something that confuses me, hoe can the object be self-conscious? Thanks for the reply though and the links I will study them with more attention. I am rereading HFDP it raises some confusion
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2. CO@Adam yes- the object *is* aware of it's state at all times. If to continue on my example, the `Kid` would know when its properties have changed, right? So that's when it'll notify its Observers. It's like that for all objects, because in order to change the state of an object you have to invoke an operation on it. So this is how the object is 'self conscious', as you put it.
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