Object Model of Objective C

Object Model of objective C is derived from SmallTalk. It has its basis in SmallTalk. Few things to know before understanding the Object Model.

What is Object?

Think of Object as a real world entity. What is a real world entity? It is something which have some data in its possession and perform different kind of functions. e.g. Think of a Tap as an object. It perform different kind of functions like start water flow, stop water flow, control the rate at which water should flow. Objects hold the data and the methods it perform.

IBAction and IBOutlet keywords

(IBAction)selectedCard:(UIButton *)sender {

IBAction is nothing but typedef void. Xcode puts this to keep track that this not a random method that returns void but an Action method that connects view to the controller.

@property (weak, nonatomic) IBOutlet UILabel *selecedLabel;

IBOutlet is typedef of void. Xcode puts this just to keep track that it not some random @property, but it is an Outlet i.e. connection to the view.

Application Delegate

Every iOS application must contain an application delegate object. Application Delegate class is responsible for handling all kind of events happen in the application. The root class is UIApplication. 

We do not subclass the UIApplication class to customize the handing of different kind of events, rather we provide an AppDelegate class that implements UIApplicationDelegate protocol. We provide this delegate class to the UIApplicaiton class which will sendd notifications to AppDelegate class for each of the application level event.

Difference between Weak and Strong keywords

Strong means you want to keep control over the lifetime of the object. If an object is marked as strong mean object remains in the heap until you point that object strongly. If you send the object to nil only than it will be collected by the Garbage collection.

Weak means you don't want to keep control over the lifetime of the object. A weak reference does not extend the lifetime of the object it points to, and automatically becomes nil when there are no strong references to the object. If no one is pointing the object strongly than the object will be collected by the heap.

you can find more details about ARC(Automatic Reference Counting) Click Here...

What is "id" in Objective C

id

'id' is a typedef in objective C. This means "Pointer to an Object". This Object can be any object of Objective C.

typedef struct objc_class *Class;
typedef struct objc_object {
    Class isa;
} *id;


typedef struct objc_selector    *SEL;    
typedef id          (*IMP)(id, SEL, ...);