Observer pattern

The Observer defines a one to many relationship, so that when one object changes state, the others are notified and updated automatically. Some auctions demonstrate this pattern. Each bidder possesses a numbered paddle that is used to indicate a bid. The auctioneer starts the bidding, and “observes” when a paddle is raised to accept the bid. The acceptance of the bid changes the bid price, which is broadcast to all of the bidders in the form of a new bid

Intent

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

Alternative Name:

                Dependents, Publish-Subscribe

Problem:

How to handle a common side-effect (i.e. need to maintain consistency between related object)of partitioning a system into a collection of cooperating classes with out making the classes tightly coupled ,  – which in turn makes classes less reusable.

Solution:

The observer pattern describes how to establish these relationships. The key objects in this pattern are subject and observer.

–        A subject may have any number of dependent observer.

–        All observers are notified whenever the subject undergoes a change in state.

–         In response, each observer will query the subject to synchronize its state with the subject’s state.

Applicability

Use the Observer pattern in any of the following situations:

• When an abstraction has two aspects, one dependent on the other.  Encapsulating these aspects in separate objects lets you vary and reuse them independently.
• When a change to one object requires changing others, and you don’t know how many objects need to be changed.
• When an object should be able to notify other objects without making assumptions about who these objects are.  In other words, you don’t want these objects tightly coupled.

Consequences

• Abstract coupling between Subject and Observer
• Support for broadcast communication.
• Unexpected updates.

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