What is SharedInstance actually?

sharedInstance could be used for several ways.

For example you can access an object from a static context. Actually it is used most ways for supporting the Singleton-pattern. That means that just one object of the class is used in your whole program code, just one instance at all.

Interface can look like:

@interface ARViewController
{
}
@property (nonatomic, retain) NSString *ARName;

+ (ARViewController *) sharedInstance;

Implementation ARViewController:

@implementation ARViewController
static id _instance
@synthesize ARName;
...
- (id) init
{
    if (_instance == nil)
    {
        _instance = [[super allocWithZone:nil] init];
    }
    return _instance;
}

+ (ARViewController *) sharedInstance
{
    if (!_instance)
    {
        return [[ARViewController alloc] init];
    }
    return _instance;
}

And to access it, use the following in class CustomARFunction:

#import "ARViewController.h"

@implementation CustomARFunction.m

- (void) yourMethod
{
    [ARViewController sharedInstance].ARName = @"New Name";
}

Shared Instance is a process by which you can access the same instance or object of a class anywhere in the project. The main idea behind this is to return the same object each time a method is called so that the values/properties stored in the instance can be used anywhere in the application.

This can be done in 2 simple process as follows:-

1) Using a static variable initialised only once

@implementation SharedInstanceClass

static SharedInstanceClass *sharedInstance = nil;

+ (id)sharedInstanceMethod
{
    @synchronized(self) {
        if (sharedInstance == nil) {
            sharedInstance = [SharedInstanceClass new];
        }
    }
    return sharedInstance;
}

@end

2) Using GCD's :-

+ (id)sharedInstance{
    static dispatch_once_t onceToken;
    static SharedInstanceClass *sharedInstance = nil;
    dispatch_once(&onceToken, ^{
        sharedInstance = [SharedInstanceClass new];
    });
    return sharedInstance;
}

These have to be called as:-

SharedInstanceClass *instance = [SharedInstanceClass sharedInstance];

Thus everytime the same instance will be returned from the function and the values set to the properties will be retained and can be used anywhere in the application.

Regards,

A sharedInstance is often implemented with the singleton pattern. Like in [UIApplication sharedApplication] -> There is only one application which you access through this singleton.

The idea is to have one instance of a class which can be accessed by calling a class method, in objective-c commonly named sharedXXX.

To solve your problem you could actually implement a singleton of a model class and write and access date to and from one existing instance which can be accessed with a static method, let's call it sharedModel.

The next step to improve on your model and updating the views would be KVO: Key Value Observing. You add an observer in your viewController to 'watch' changes made to your model. If such a change occurs your viewController is informed and you can update the view then.

You can read more about KVO in Apple's documentation or over at mindsizzlers for a small and easy tutorial.