Question

Consider the following common situation:

You have some MainView in your Cocoa application, loaded from a NIB, which is controlled by a MainViewController. Your MainView contains some controls, such as a UILabel infoLabel. You also have a delegate MyDelegate class which receives some sort of event.

You would like to make sure that when MyDelegate receives its event, the infoLabel is updated appropriately. However, the problem is that MyDelegate does not have a reference to the MainView or the MainViewController and does not know about the label.

One solution is to pass a MainViewController reference to the delegate object, but this feels wrong because you might find yourself in the undesirable situation where the object has each other’s references.

What is the proper design to solve this problem?

Answer 1

In an unnamed developer forum, someone writes:

So, to make a long story short, I have decided that I will start making use of NSNotifications. The Stanford course online that people have been following is taught by two Apple engineers. They have just now unequivocally said NOT to use the app delegate or global variables, and have said to use NSNotifications, delegates, and K-V observing.

If that is what the Apple engineers say, I am going to move in that direction.

The NSNotifications are pretty ingenious in that they don’t really interfere with encapsulation that much. The listener only listens for the notification and an object – I don’t think it has to know or care who sent it.

So in your example I would consider having the delegate post a notification that the label had changed, or better yet have the controller observe that property if possible.