layout | title | folder | permalink | pumlid | categories | tags | ||||
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pattern |
Observer |
observer |
/patterns/observer/ |
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Behavioral |
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Dependents, Publish-Subscribe
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
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
- changing in one object leads to a change in other objects