iOS category conforms to a protocol with properties: how to implement

Question

I have a category on a class I made, and that category conforms to a protocol, which requires a property in its implementation. However, because I'm in a category, I cannot synthesize the property in the implementation of the category. Because of this, I'm stumped on how to implement the setter method (when I keep the protocol's property readonly it works fine, since all I need is an accessor method).

This is my protocol:

@protocol SomeProtocol <NSObject>
@property (nonatomic) BOOL didDisplayRecommendation;
@end

I know if I do this I'll get an infinite loop:

- (void)setDidDisplayRecommendation:(BOOL)didDisplayRecommendation
{
    self.didDisplayRecommendation = didDisplayRecommendation;
}

But when I try this I get a compiler error:

- (void)setDidDisplayRecommendation:(BOOL)didDisplayRecommendation
{
    _didDisplayRecommendation = didDisplayRecommendation;
}

Note that didDisplayRecommendation is the property in the protocol. What's the best way of getting around this? Thanks in advance!

possible duplicate of [Objective-C Category and new iVar](http://stackoverflow.com/questions/10502539/objective-c-category-and-new-ivar) — Martin R, Dec 23 '13 at 21:50
Not a duplicate, doesn't address the issue of conforming to protocols — Mason, Dec 24 '13 at 02:03
You are allowed to add "special" properties in a categories with (at)property (nonatomic,retain) MyClass *anObject - but in the implementation file all you can do is (at)dynamic anObject, you can then use Associative References cfunction to masquerade other objects in place of that property. It works and it's not a workaround, you are allowed to do this in commercial apps, also it passes implements protocol checks. — unom, Apr 30 '14 at 09:04

score 2 · Accepted Answer · edited May 23 '17 at 12:25

2

You are not allowed to add instance variables to a class through categories, see https://stackoverflow.com/a/13000930/171933

Since you need a variable to hold the value of didDisplayRecommendation, you are out of luck of doing this with a category (unless you want to get dirty https://developer.apple.com/library/mac/documentation/Cocoa/Reference/ObjCRuntimeRef/Reference/reference.html#//apple_ref/doc/uid/TP40001418-CH3g-SW5).

I'd recommend re-thinking your architecture to see if you really need to use categories. Subclasses or even Mixins might be the better way to go.

edited May 23 '17 at 12:25

Community

1
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answered Dec 23 '13 at 21:50

Johannes Fahrenkrug

42,912
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You are allowed to add "special" properties in a categories with (at)property (nonatomic,retain) MyClass *anObject - but in the implementation file all you can do is (at)dynamic anObject, you can then use Associative References cfunction to masquerade other objects in place of that property. It works and it's not a workaround, you are allowed to do this in commercial apps, also it passes implements protocol checks. – unom Mar 25 '14 at 19:34
@unmircea Yes, it is definitely **technically possible**. However, I think you should have a very compelling reason for resorting to associative references. – Johannes Fahrenkrug Mar 25 '14 at 20:18

score 0 · Answer 2 · answered Dec 23 '13 at 22:01

0

When you add properties in a protocol you need to @synthesize them.

answered Dec 23 '13 at 22:01

0xFADE

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5
7

score 0 · Answer 3 · answered Feb 12 '14 at 06:53

I have a macro that lets you declare "properties" in categories like this:

@implementation NSObject (AwesomeUtils)

JESynthesize(assign, NSInteger, index, setIndex);
JESynthesize(strong, NSString *, name, setName);
JESynthesize(copy, void(^)(void), completion, setCompletion);
JESynthesize(unsafe_unretained, id, unsafeObject, setUnsafeObject);
JESynthesize(weak, id<UITableViewDelegate>, delegate, setDelegate);
JESynthesize(strong, NSString *, readonlyID, changeReadonlyID);

// …
@end

I say "properties" with quotes because you can use them even without the @property declaration. The macro also works around to support weak.

You can check the implementation here (the header files are at the bottom): http://nspicks.com/2013/12/15/cleaner-properties-implementation-in-categories/

iOS category conforms to a protocol with properties: how to implement

3 Answers3