How do I use forward class declaration for circular dependencies in an iOS iPhone app?

Question

I am working on an iPhone app that requires sequential flow in order for me to capitalize on user-friendly handling. The application is one large loop of multiple view controllers, where a user advances through the various modules of the app. I have an internal loop within the main application loop that consists of just 2 of the view controllers, and concludes whenever the user decides to proceed onward.

The internal 2-controller loop is what I am targeting with my question. It consists of a table view controller (apply change to the user's selection) and a detail view controller (display the changes). Currently, my issue is with increasing memory usage every time the loop proceeds to one of the view controllers. I've heard that forward class declaration is a good way to handle circular dependencies (2-controller loop), but I haven't seen any working example that applies to my situation. Can forward class declaration be used in my case to effectively loop multiple view controllers, and if so how can I do it?

Below is an example of what I'm thinking I need to employ. Is this reasonable? (I am very new to objective-c and the forward declaration concept)

TableViewController.h

#import <UIKit/UIKit.h>

@class detailViewController;

@interface TableViewController : UITableViewController
{
    // Stuff for TableViewController object
}

@end

detailViewController.h

#import <UIKit/UIKit.h>

@class TableViewController

@interface detailViewController : UIViewController
{
    // Stuff for detailViewController object
}

@end

Answer 1

I'm not 100% I understand the structure of your project, but here are some hints I can give you:

You are discussing two different problems.

Retain Cycles

Retain Cycles can cause memory issues in your program. When you are facing a memory issue in your app (and are using ARC) the changes are high that you have created a retain cycle. This means that two objects in your code each reference each other through a strong relationship. That will cause both objects to stay in memory forever. In your application it seems likely that the TableViewController and the DetailViewController are referencing each other strongly. If you only want to pass information from one ViewController to another, you should use a weak reference:

@property (weak) DetailViewController *detailViewController;

Circular Dependencies

Circular Dependencies occur when two Classes try to import each others header. If Class A imports Class B in its header and Class B imports Class A in its header you will have a unresolvable circular dependency because the preprocessor will not know how to import Header files that depend on each other. This is resolved by using forward declarations instead of import statements. But this has nothing to do with memory management. Basically you should be using forward declarations whenever it is possible (when you only need to know that a certain class exists) and only use #import statements when it's actually necessary to know details about the interface of that class (subclassing, accessing properties of other Classes in .m files).

Answer 2

Your .h file is just a public interface. Without inclusion into your .h file, you can still have members of view controllers visible to each other, while oblivious to other classes.

If no public data members are dependent upon another View Controller, you can consider including one view controller .h file inside another one's .m file, instead of it's .h file. That way, circular dependency can be avoided.

As you know, you can declare private data members within .m file as well, after @interface keyword. Like this:

2.m:

#import 1.h

@interface 2
{

}

1.m:

#import 2.h
@interface 1
{

}

Even removing from either .h the reference to another .h file may solve the circular dependency problem.

Having said that, circular dependency is something you need to handle at compile / build time, and it has nothing to do how you navigate between the two view controllers when your app runs.

Assuming that is your question, and you have a storyboard in front of you, you should consider:

1 - embedding your first view controller inside a Navigation controller

2 - create a push segue from first to second view controller. use performSegueUsingIdentifier to navigate from first to second VC.

3 - Similarly, you can use popViewController function to navigate from second to first.

None of these steps require you to deal with circular dependency, and you should read the documentation very thoroughly to understand what you want to achieve.

In short, RTFM.

Answer 3

You know, I really think you're asking for nothing more than how to pop on and pop off a view controller. It's really trivial and it will solve all your problems. Check it out.

Just regarding putting "@class blah;" at the top of a file - yeah, whatever, do that. It's no big deal. If it's "not finding" a class, sure, just put it in.

As everyone has explained, you do not and should not think about "memory management" today in iPhone programming. It's irrelevant.