Notifications: pattern or anti-pattern?

Notifications are well-known communication pattern used in one or another way almost in every program. Apple system frameworks widely utilize notifications to communicate with their clients. Does it necessary mean that it’s a good technique? Lets clear up the misconceptions and answer the question: are the notifications considered to be a pattern or anti-pattern?

Cons of Using Notifications and Notification Center

You are making your way through the cobwebs of hundreds lines of code. You set dozens of breakpoints and your head is overflowing because of focusing on too many things simultaneously. Does it sound familiar to you? If you have seen one application that heavily utilizes Notifications API, you’ve seen them all. Lets identify the core Notifications problems which lead to such consequences.

1. Hard to understand what the system actually does

Every notification-based flow consists of the following steps:

1. Publish
2. Subscribe
3. Handle
4. Unsubscribe

The steps are usually located in different files, classes and functions and to follow the flow of execution, you must keep all of them in your mind simultaneously. Most of the time it’s impossible to do just by reading the code and you will have to set lots of breakpoints and conduct live debugging.

2. Creates one-to-many and many-to-many relationships

In a project that heavily utilizes Notifications its inevitable to have multiple subscribers or publishers, or even both, for a single Notification. This creates lots of one-to-many and many-to-many relationships in your object graph which are way more complicated to manage compared to one-to-one ones.

3. Couples subscribers

Subscribers are indirectly coupled with each other through the Notification’s interface. The backwards force is applied by clients upon interfaces. Thus, some subscribers might demand changes in the Notification’s interface which will result in a cascade of changes in the rest of the subscribers.

4. Breaks encapsulation

Notifications often carry some extra information which results in a global knowledge of private data available for any subscriber.

5. Non-deterministic behavior

Chances high to end up with non-deterministic bugs due to synchronization issues, because NotificationCenter does not define an order of notifying subscribers.

6. Leaves lots of room for mistake

Subscribing / unsubscribing for Notifications often needs to be tied to ViewControllers life cycle and is a very common source of bugs. What is more, adding first Notification often leads to a so-called “notifications explosion”, when dozens of others are added blazing fast.

A look on the bright side

Lets discuss advantages of Notifications and see what outweighs.

1. Lowers coupling

Many direct references can be removed from the object graph which reduces overall coupling between classes and modules.

2. Fast on early stages

Notifications pass data globally from any place of the program disregarding the existing object graph. On early development stages it’s usually faster than modifying the existing graph and chaining calls.

Verdict: pattern or anti-pattern?

By this time it must be clear that Notifications are anti-pattern and should be avoided for the majority of the cases. Lets see what options do we have in regard to notifications usage.

1. Use alternative communication patterns:
   • Callbacks
   • Delegates
   • Target-actions
   • Custom observers with explicit contracts and deterministic order of notifying subscribers
2. Sometimes, when you deal with iOS standard frameworks, 3rd parties or legacy code, Notifications are inevitable. Use adapters and facades to wrap them up and do not let them leak outside.

Wrapping up

All OOP design principles and patterns target single goal: deal with code complexity. Notifications most of the time lend themselves to the opposite. Projects that heavily use Notification and NotificationCenter API usually end up having spaghetti code that lacks clarity even for original developers.

Follow the suggested rules of thumb if you have to deal with Notifications. Consider alternative communication patterns for your application, because for the majority of the cases the best choice is to simply opt out of Notifications usage.