How to handle Objective-C protocols that contain properties?

Question

I've seen usage of Objective-C protocols get used in a fashion such as the following:

@protocol MyProtocol <NSObject>

@required

@property (readonly) NSString *title;

@optional

- (void) someMethod;

@end

I've seen this format used instead of writing a concrete superclass that subclasses extend. The question is, if you conform to this protocol, do you need to synthesize the properties yourself? If you're extending a superclass, the answer is obviously no, you do not need to. But how does one deal with properties that a protocol requires to conform to?

To my understanding, you still need to declare the instance variables in the header file of an object that conforms to a protocol that requires these properties. In that case, can we assume that they're just a guiding principle? CLearly the same isn't the case for a required method. The compiler will slap your wrist for excluding a required method that a protocol lists. What's the story behind properties though?

Here's an example that generates a compile error (Note: I've trimmed the code which doesn't reflect upon the problem at hand):

MyProtocol.h

@protocol MyProtocol <NSObject>

@required
@property (nonatomic, retain) id anObject;

@optional

TestProtocolsViewController.h

- (void)iDoCoolStuff;

@end

#import <MyProtocol.h>

@interface TestProtocolsViewController : UIViewController <MyProtocol> {

}

@end

TestProtocolsViewController.m

#import "TestProtocolsViewController.h"

@implementation TestProtocolsViewController
@synthesize anObject; // anObject doesn't exist, even though we conform to MyProtocol.

- (void)dealloc {
    [anObject release]; //anObject doesn't exist, even though we conform to MyProtocol.
    [super dealloc];
}

@end     

Answer 1

The protocol is just telling everyone that knows about your class through the protocol, that the property anObject will be there. Protocols are not real, they have no variables or methods themselves - they only describe a specific set of attributes that is true about your class so that objects holding references to them can use them in specific ways.

That means in your class that conforms to your protocol, you have to do everything to make sure anObject works.

@property and @synthesize are at heart two mechanisms that generate code for you. @property is just saying there will be a getter (and/or setter) method for that property name. These days @property alone is enough to also have methods and a storage variable created for you by the system (you used to have to add @sythesize). But you have to have something to access and store the variable.