Question

I have an app that was extremely simple until today. It had a tab bar view controller with 3 tabs. The middle tab was a camera, and the other 2 were table views. The tab bar view controller was the central hub for all the data in the app. So from there, I would set a table’s data array as:

(PLEListViewController*)[self.viewControllers objectAtIndex:0] setList:newList];

Obviously, PLEListViewController is my UITableView subclass.

So now, I want to wrap the table views in a UINavigationController, which is fairly simple. But now, that line of code turns into:

[(PLEListViewController*)((UINavigationController*)[self.viewControllers objectAtIndex:0]).topViewController setList:newList];

There are 15 lines in the code that do this, which is not pleasant.

So my question: what is a more elegant way of doing this that I’m missing?

Answer 1

It’s good that you’re asking this and seeing the issue now. Your problem is can be found in your question. The answer to “the correct way to communicate between several different view controllers in Objective C” is “don’t.” Specifically, your mistake is here:

The tab bar view controller was the central hub for all the data in the app.

A view controller should never hold any of the data in the app. Your data should live in your model classes. All the view controllers should talk to the model classes. They should very seldom talk to each other. That’s the heart of MVC.

So, you move your “list” (whatever that is, doesn’t matter) into some model object that all the view controllers know about. That model object can be a singleton, or often better, it can be passed to the view controllers when they are created. When things change, you change the model. And in viewWillAppear: you update your view controller to match the current state of the model.

Never assume that a view controller exists when it is not currently on screen. If your design requires that a non-active view controller exist, then your design needs fixing.