Accessing Property Values of a Protocol?

Question

Hey guys, I have a protocol which defines a number of ObjC-2.0 properties, like so:

@protocol Repeatable <NSCoding>
@required
@property (nonatomic, retain) Date *startDate;
@property (nonatomic, retain) Date *endDate;
@end

I have another class that implements the protocol:

@interface AbstractRepeatable : NSObject <Repeatable>

And finally, in AbstractRepeatable, I am implementing a method defined by the protocol:

- (BOOL)isEqualToRepeatable:(Repeatable *)r {
    if (r.startDate != startDate) // Compiler error here
        return NO;
    return YES;
}

Bits and pieces have been excluded for example-sake, but when compiling, I receive the familiar "request for member 'startDate' in something not a structure or union" on the line noted above. The AbstractRepeatable obviously includes the Repeatable header, otherwise the protocol would not be visible, so I don't know which part I'm missing.

Answer 1

@property ... startDate only declares two methods -startDate and -setStartDate:.

You have to actually implement them. In a protocol, this cannot be done with instance variables.

Answer 2

That should work and do what you expect. But it doesn't because the compiler is broken.

Sorry about that.

As a workaround, you'll need to declare the methods in your protocol:

@protocol Repeatable <NSCoding>
@required
@property (nonatomic, retain) Date *startDate;
- (Date *) startDate;
- (void) setStartDate: (Date *) aDate;

@property (nonatomic, retain) Date *endDate;
- (Date *) endDate;
- (void) setEndDate: (Date *) aDate;
@end

Skipping the @property, if you want. Once the compiler is fixed, the above still won't generate any warnings.

(Yes, you still can @synthesize the methods. I believe that @synthesize will work in this case. If it doesn't -- which it might not -- then you'll need to redeclare the properties in the class -- potentially in a class extension -- until the compiler is fixed. If this second issue proves to be the case, please file a bug via http://bugreport.apple.com/)

Answer 3

In your isEqualToRepeatable: method you are treating Repeatable as a class, not a protocol. Try using the following. Everything should work fine after that. I tested it. (Fingers crossed that I won't look stupid.)

- (BOOL)isEqualToRepeatable:(id<Repeatable>)r {
    if (r.startDate != startDate) // Compiler error here
        return NO;
    return YES;
}