Observer Design Pattern - Specific Topics and Observers

Question

The links (GOF design patterns, chapters of First Design Patterns, http://www.dofactory.com/Patterns/PatternObserver.aspx ) that I read about the observer design pattern provide that a particular subject contains a link to a specific observer. Like that:

class ConcreteObserver : IObserver
{
    ConcreteSubject concreteSubjectInstance;
    //other code, etc.
} 

Now, if a particular object itself implements the Subject interface (or is derived from some abstract Subject class), why not make the type in ConcreteObserver such an abstract / interface? I.e.

class ConcreteObserver : IObserver
{
    ISubject concreteSubjectInstance;
    //other code, etc.
} 

Also, why not just make this field in an interface (like IObserver)?

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Answer 1

"update()" Subject, , Observer ( ), ConcreteSubject, GetState() ( ISubject).

( ConcreteSubject) "update()".

ConcreteSubject ISubject , ConcreteSubject - (, ISubject).

Answer 2

, subject holds a reference to the concrete observer , .

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, IObserver IObservable. , , , , , , .

Answer 3

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