Question

I’ve had some experience developing for the iPhone but am entirely self taught and am trying to improve my practices. This is a question that is probably pretty introductory to programming.

What is the best way (or is it possible) to maintain instance variables with values that are common to all instances of an object? Is it possible to have the creation, modification, and checking of an object and its attributes happen in different places?

Say I have an object that keeps track of the number of times a user swipes the screen. Is it possible to, say, allocate that object in my AppDelegate, and then somehow increment the count variable of that object from different view controllers?

Is it possible to define an instance variable such that it is shared across all instances of the object to which it belongs? That would solve the problem.

So far, I’ve just made IVARs in my AppDelegate for the things like this that I’ve needed to keep track of and then just accessed them like this:

((MyAppDelegate *)[UIApplication sharedApplication].delegate).instanceVariable

but I don’t know if that’s a good practice or not. Probably not.

Maybe this is related to global variables or singletons. Or the prefix “shared” that appears above and I’ve seen in other places too. I’m just looking for advice or a direction to look. I hope this question makes sense and isn’t too general.

Answer 1

I think the question(s) you are asking involve different practices for different situations. To share data among viewcontrollers is one question with a number of solutions, one of the better ones being to use a delegate for all the viewcontrollers.

If you want to have a Class manage all its own objects, even if the objects come and go, you could do one of two things:

1. Make the Class have a singleton, and keep track of allocations and deallocations in order to know when it should create and then dispose of itself (if needed). This is not a bad way to do it, although it may be more work.

2. Create a controller class that manages your other class of objects. A good example may be a class of views, controlled by a viewcontroller class.

Let’s see what other people suggest.