ways to reference a model from a view controller

Question

All the examples I've come across so far ( including Stanford podcasts ) reference a Model by declaring it as a property of a View Controller and using it from there:

#import "myClass.h"    // assume it carries a single NSString property

@interface
@property (nonatomic,strong) myClass *myobject;
@end

@implementation ViewController

-(void)viewDidLoad {
self.myObject = [[myClass alloc] init]
.
.   
.
-(void)someMethod{
displayLabel = self.myObject.myString; 

seems like that's more self.C-V than M-V-C.
After messin' about on my own this works:

#import "myClass.h"

@implementation ViewController {
MyClass *myObject;
}

-(void)viewDidLoad {
myObject = [[myClass alloc] init]
}
.
.
.
-(void)someMethod{
displayLabel = myObject.myString;

my question is; is there any danger in using second example? or to ask differently, does it give the compiler an easier task of keeping MODEL separate from VIEW and CONTROLLER?

Answer 1

There are a couple of implementation details that are different between your two examples but they are essentially doing the exact same thing.

In both cases you are declaring a backing ivar. This line @property (nonatomic,strong) myClass *myobject; will implicitly @synthesize myObject = _myObject;, which is similar to you manually writing:

@interface ViewController : UIViewController {
  MyClass *_myObject;
}

// or

@implementation ViewController {
  MyClass *_myObject;
}

The only other difference is that @property (nonatomic,strong) myClass *myobject; will also create the accessor methods for you

- (void)setMyObject:(MyClass *)myObject;
- (MyObject *)myObject;

---

This is indeed still MVC but controllers that are subclasses of UIViewController always manage at least one view. The M component is your myObject instance. As in most diagrams the Controller sits there managing the communications between the V and M both of which the controller owns

Answer 2

Using singletons or holding the model as a property of the controller is also a question of lifetime. I prefer using singletons for helper classes (getting data from game center or fetching data from core data). Singletons are useful for things you need to access from different places within your app (where lifetime of your singleton is longer than your view controller's lifetime). But for view controller dependent models you can of course use them as properties of the view controller. Let's say you have ten view controllers in your app each displaying completely different content it does absolutely not make sense to keep all data for all possible view controllers in memory (in a singleton) just to have data ready in case the user wants to see any of the view controllers. In this case it's no shame to load your model's data from within your view controller implementation and hold it as a property. This guarantees that data is auto released when the view controller's lifetime ends and avoids conflicts. Holding data in singleton would make sense when you display data loaded from a server that does not need to be refreshed everytime you present your data to reduce the amount of traffic generated by loading the data. Using singletons might be dangerous regarding thread safety e.g. when data is mutated from a background thread while iterating over your datasource object to refresh a table view's content. Singletons can also lead to tigh coupling which should be avoided. Using an instance variable instead of a property is still a good choice if you want to hold a weak reference to an object as it is automatically set to nil if the referenced object gets auto released. A weak property would in this case lead to bad access.