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Ed is Taking Note
Ed is Taking Note

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[Design Pattern] Observer Pattern

The observer pattern is a one to many relationship dependency, and when one object(Observable object) changes its status, all its dependencies(Observer objects) will be notified and updated accordingly.

Scenario Problem
Now we have a publisher object and many subscriber objects, and subscriber objects poll new status from the publisher. However, all these subscribers don't know when the publisher will update a new status, so they just keep polling new status from the publisher(like polling every 1 minute or something).


Now there comes a bad smell... What if there are like 1000 subscribers and each of them tries to get status every 10 seconds!?
As a result, the requesting traffic must explode and obviously there are lots of unnecessary calling... Also, subscribers might not get the most recent status - there is still up to 10 seconds delay before getting the status.

With the observable pattern, every time the publisher(Observable object) has a new status, it notifies all its subscribers. This way we can avoid lots of unnecessary callings, and all subscribers can get the most recent status immediately without delay.


A very simple class diagram of the pattern is like the following:


We have register() and unregister() methods to add or remove subscriber to the subscribers list of the publisher class.

The publisher's notify() method will be like:

public void notify() {

    this.status = // Do whatever to get the latest status...; -> {
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The subscriber's update() method will be like:

public void update(Object status) {
     // Do whatever the subscriber want to do with the received status.

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This is a very basic and simple observer pattern. There are actually many different kinds of implementation for observer pattern, but the core concept is generally the same.

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