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observer
subject			fact

observer	can be used to separate the model^2 from the controller in the Model-View-Controller architecture
	has definition A pattern found in class diagrams in which instances of one class are informed of changes to instances of a second class
	has antipatterns connecting an observer directly to an observable so that they both have references to each other making the observers subclasses of the observable
	has context When you create a two-way association between two classes, the code for the classes becomes inseparable When you compile one, the other one has to be available since the first one explicitly refers to it This means if you want to reuse one of the classes, you also have to reuse the other; similarly, if you change one, you probably have to change the other
	has forces You want to maximize the flexibility of the system to the greatest extent possible.
	has problem How do you reduce the interconnection between classes, especially between classes that belong to different modules or subsystems?
	has references one of the Gang of Four patterns.
	has solution Create an abstract class «Observable» that maintains a collection of «Observer» instances. The «Observable» class is very simple; it merely has a mechanism to add and remove observers, as well as a method notifyObservers that sends an update message to each «Observer». Any application classes can declare itself to be a subclass of «Observable». «Observer» is an interface, defining only an abstract update method. Any class can thus be made to observe an Observable by declaring that it implements the interface, and asking to be a member of the observer list of the «Observable». The «Observer» can then expect calls to its update method whenever the «Observable» changes. Using this pattern, the «Observable» neither has to know the nature of the classes that will be interested in receiving the update messages, nor what they will do with this information.
	is a subtopic of 6.6 - The Observer Pattern
	is an instance of design pattern
design pattern	has name
	has related patterns zero or more related design patterns
	should be illustrated using a simple diagram
	should be written using a narrative writing style
pattern	should be as general as possible
	should be described in an easy-to-understand form so that people can determine when and how to use it
	should contain a solution that has been proven to effectively solve the problem in the indicated context

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