At NSConference Dave Dribin made the following statement in a slide:

Delegates are preferable to Notifications which are preferable to KVO

Now this caused a bit of a murmur on Twitter, as it was taken as "KVO is bad". As this is an important topic and not one that can easily fit into 140 characters I thought I would write a post to outline my rules for when to use each technology and why use them where I do.

Rule #1: Always use delegates for 1 to 1 relationships

One of the main reasons delegates were listed as the most preferable is that they provide the least 'magic' way of communicating between objects while keeping them loosely coupled. It sends the message directly to the object and you can see what method is being run. As such it makes your code more self explanatory. So whenever you have a 1 to 1 relationship between two classes, opt for delegation over notification.

Rule #2: Use notifications for communicating events to many objects

Notifications should really be used wherever you would use a delegate, but need to communicate with any number of objects. They should really be thought of as 1 to many delegates. Quite often in Cocoa you will see a case where there is a notification and a delegate method for the same event (eg change of selection in a table view). These are cases where 80% of the time you just need a 1 to 1 relationship, but occasionally do need a 1 to many relationship. You will rarely encounter this in your own code, unless you are building a class for use in multiple apps.

Rule #3: Use KVO for communicating data model changes

This is where I have found KVO to be the most powerful. You need to keep various UI elements in sync with your data model. Changing the value of the model from one view should also update the value in other views. KVO was built for exactly this situation and as such requires much less code than delegation and notifications.

To achieve the same effect with delegation or notifications you would have to add delegate methods or post a notification in each setter method of each model object. This obviously is a lot of code that needs to be written. Code that could have bugs and code that makes your source more cluttered. KVO of a value in an object requires just one line of code and the implementation of one method. And if you use bindings then it is actually no code at all.

Rule #4: Use blocks instead of callback delegates

I'll start with the exception to this: if you have multiple callback options then use delegates (eg NSURLConnection). But the vast majority of asynchronous operations need just a single callback. In these cases, if you are using Mac OS X 10.6, then you should be using blocks instead of delegates. Blocks reduce the layer of indirection in your source by putting the call back information inline with the invocation of the method.

The next two rules aren't quite rules of communication, but are relevant to the discussion at hand.

Rule #5: Comment anything that isn't clear

This should be pretty obvious. If something in your code isn't explicitly clear from the code itself then comment it. If you are sending off a notification say why you are sending it off, so you can get an idea of what it is doing elsewhere in the application.

Rule #6: If something seems like magic, learn how it works

There is no such thing as magic in programming, simply things we don't fully understand. If notifications, delegation, KVO or anything else seems like magic, then read up the documentation on it or ask a developer experienced in that area to explain how it works. When you find out how they work you realise that what seems like magic is fundamentally a very simple process.

Ultimately, what Dave said was right for some scenarios. Each technology has its use case and you should only use it where it is needed. You should always aim for the smallest amount of indirection in your code while still retaining loose coupling of classes. KVO and notifications are necessary in some occasions (unless you want to go crazy with delegation) but they should only be used when they are needed.

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