i was just figuring out how to have private variable with class extension. And what i noticed was that there is no default code for class extension inside a normal class. And when i try to declare variable with @property, it does not allow me but to type @protected and @private while inside ViewController, it only allow me to have @property @optional @required.
SO, my question is that is there any reason for that..? What is the difference and what makes it different..?
You didn't post any code, leaving us to infer what you're doing. I'm going to assume it's something like this:
@interface ViewController ()
@property id someProperty; // This works
@private
id someVar; // This does not work
@end
and:
@implementation ModelClass
{
@property id someProperty; // This does not work
@private
id someVar; // This works
}
@end
Assuming that's true, it seems you're confused about the nature of properties vs. instance variables. @property is essentially a way to declare accessor methods, for which the compiler (by default) automatically synthesizes method implementations and a corresponding backing instance variable. Instance variables on the other hand, are simply per-instance variables, not methods at all.
The first block of code above is a class extension on ViewController. Class extensions allow you to declare additional methods -- and thereby @properties too -- separately from the main/public interface for a class.
The braces after @implementation in the second block denote a place to declare additional instance variables (only).
@protected and @private are visibility modifiers for instance variable declarations, and control whether an instance variable is visible only to instances of the class itself (@private), instances of the class and its subclasses (@protected), or publicly (@public). These modifiers cannot be used for methods (of which @properties are a special case). Afterall, in Objective-C, methods are actually always public in the sense that it's only at runtime that a message send is turned into a method call, and the compiler can't truly enforce a limitation on calls to "private" methods.
Finally, to answer what I think is the heart of your question, you most certainly can add a class extension to your model class in order to declare additional "private" @properties and other methods. Xcode may not include one in the default new file template for non-view controllers, but that doesn't mean you can't add one yourself:
@interface ModelClass ()
@property id somePrivateProperty; // Works just fine
@end
