The Observer Pattern:
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
- Strive for loosely coupled designs between objects that interact.
when will I need it?
So there is a smart “NewsGetter Machine”. It fetches news from different sources on its own (these machines will one day start coding as well :cry:). This news is being consumed by two (for now) “Online Newspapers”. You are given the task to implement a way for these newspapers to get the latest news as it comes and display it on their sites. The super-intelligent AI guys who built the NewsGetter also implemented a
news_flash() method. This method gets called every time the NewsGetter gets some new news.
Oh, also, there is a 100% chance of more newspapers starting to use the NewsGetter Machine as their primary source of news (because AI is the future).
You, being you, coded the implementation in under 5 minutes:
Genius! Every time there is a news flash,
news_flash() gets called, which gets the latest news and you update the newspapers. Simple and sweet. And extensible. No. No? No.
What about the stuff we learnt about in our Strategy Pattern post? Looks like we coded concrete implementations inside the
news_flash() method. Now every time we get another newspaper that wants to use the
NewsGetter, we will have to modify our code. That is bad. We should encapsulate what we know will vary.
Observer Pattern to the rescue!
Time to use some Design Principles!
In the Observer Pattern we have two entities, Subject and Observer. As is not clear by the names, Subject is the thing that holds state (information, like news in our case) and Observer is the thing that wants to get notified when the state of the Subject changes, because the Observer needs to do some crazy things based on the change. It is the Subject’s responsibility to notify Observer’s whenever it’s state changes.
Implementing this for our case, we will define two classes:
Subjectclass will implement methods to
Observerclass implements the
updatemethod that is called whenever the Subject notifies the observer.
While we are at it, we will also define a
NewsPaper class that implements a
display_news method that is called whenever the observer’s
update method is called. This is done in order to standardize the APIs that newspapers will create. Any newspaper that needs to integrate with the
NewsGetterMachine will need to inherit from
Newspaper classes and implement the unimplemented methods.
Here is our definition of the superclasses and its usage:
This was the Observer pattern. Notice how we were Pushing the news when we notified observers? This is sometimes not desirable. In that case we can go with a Pulling implementation. The call to
notify_observers is made without any extra information. The observers, when they receive the notification, can call
get_news() method of the
NewsGetterMachine to fetch latest news if they want to.
how did we benefit?
Now we can add any number of newspapers and just register them with the
NewsGetterMachine and they will get the updated news! We can also register or remove observers at runtime.
Also, the implementation of the newspapers, and how they display the news, is all abstracted away from the
NewsGetterMachine. All this machine knows is that there are some observers that it has to notify by calling the
update method on them.
For an excellent, pro-level, implementation of the Observer pattern, take a look at Django’s Signals.