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Design Patterns: The Observer Pattern

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This post is part of a series called Design Patterns in PHP.
Design Patterns: The Command Pattern
Design Patterns: The Factory Method Pattern

Now here I come with one more behavioral design pattern in this series, which is the Observer Pattern. Observer means that someone is looking at your activity, and it may be possible that the observer takes some action depending on your activity. 

The same concept is applicable to the design pattern as well. We should implement this pattern when we have a one-to-many dependency between our object and one object needs to be changed/notified when any changes are made to any other object.

Wikipedia says the same thing in the words below:

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. It is mainly used to implement distributed event handling systems. The Observer pattern is also a key part in the familiar model–view–controller (MVC) architectural pattern.

To elaborate more about this design pattern, I am taking the example of a simulator which shows the value of different currencies against the US dollar. We are assuming that the simulator shows the price and also updates the price at regular intervals.

Before we start, let's define the responsibilities of the main simulator class (which is behaving as the observer in this example). 

  1. This observer should have the ability to add a new currency, so that the client can add any as many currencies as they want.
  2. This observer should keep the reference to all the added currencies.
  3. This observer should show the status/value of each registered currency.

In the next section, we will implement our observer:

If you look at the above code, you can see that it has the ability to perform all the responsibilities which we have mentioned in the previous section.

Now we have our observer ready. What we need now is some different currencies which we can add to this observer and then we are good to go. Let's implement our currency classes now.

We are all set now to put everything together and run our observer. 

Putting It All Together

The above code will output:

Here you can see that we have updated the prices for all registered currencies and they're being displayed in our simulator. Now we will consider how we can add new currencies to this simulator with just a minor modification. 

This modification includes the registering of currencies to the simulator only. As such, our client code remains untouched whenever it invokes the price updater in our simulator.

Adding a New Currency

Currency Class

It was easy and straightforward to add a new currency. Now all we need to do is register this new currency in our observer and we are all done!

Conclusion

The state of any object is very important in Object Oriented Programming, because everything is running between objects and their interaction with each other. It is frequently the case that a few objects need to be notified when changes occur in other objects. The Observer design pattern can be utilized when changes in a subject need to be observed by one or more observers.

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