Objective-C: Property / instance variable in category

.h-file

@interface NSObject (LaserUnicorn)

@property (nonatomic, strong) LaserUnicorn *laserUnicorn;

@end

.m-file

#import <objc/runtime.h>

static void * LaserUnicornPropertyKey = &LaserUnicornPropertyKey;

@implementation NSObject (LaserUnicorn)

- (LaserUnicorn *)laserUnicorn {
    return objc_getAssociatedObject(self, LaserUnicornPropertyKey);
}

- (void)setLaserUnicorn:(LaserUnicorn *)unicorn {
    objc_setAssociatedObject(self, LaserUnicornPropertyKey, unicorn, OBJC_ASSOCIATION_RETAIN_NONATOMIC); 
}

@end

Just like a normal property - accessible with dot-notation

NSObject *myObject = [NSObject new];
myObject.laserUnicorn = [LaserUnicorn new];
NSLog(@"Laser unicorn: %@", myObject.laserUnicorn);

---

Easier syntax

Alternatively you could use @selector(nameOfGetter) instead of creating a static pointer key like so:

- (LaserUnicorn *)laserUnicorn {
    return objc_getAssociatedObject(self, @selector(laserUnicorn));
}

- (void)setLaserUnicorn:(LaserUnicorn *)unicorn {
    objc_setAssociatedObject(self, @selector(laserUnicorn), unicorn, OBJC_ASSOCIATION_RETAIN_NONATOMIC); 
}

For more details see https://stackoverflow.com/a/16020927/202451

@lorean's method will work ^{_{(note: answer is now deleted)}}, but you'd only have a single storage slot. So if you wanted to use this on multiple instances and have each instance compute a distinct value, it wouldn't work.

Fortunately, the Objective-C runtime has this thing called Associated Objects that can do exactly what you're wanting:

#import <objc/runtime.h>

static void *MyClassResultKey;
@implementation MyClass

- (NSString *)test {
  NSString *result = objc_getAssociatedObject(self, &MyClassResultKey);
  if (result == nil) {
    // do a lot of stuff
    result = ...;
    objc_setAssociatedObject(self, &MyClassResultKey, result, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
  }
  return result;
}

@end

Just use libextobjc library:

h-file:

@interface MyClass (Variant)
@property (nonatomic, strong) NSString *test;
@end

m-file:

#import <extobjc.h>
@implementation MyClass (Variant)

@synthesizeAssociation (MyClass, test);

@end

More about @synthesizeAssociation

The given answer works great and my proposal is just an extension to it that avoids writing too much boilerplate code.

In order to avoid writing repeatedly getter and setter methods for category properties this answer introduces macros. Additionally these macros ease the use of primitive type properties such as int or BOOL.

Traditional approach without macros

Traditionally you define a category property like

@interface MyClass (Category)
@property (strong, nonatomic) NSString *text;
@end

Then you need to implement a getter and setter method using an associated object and the get selector as the key (see original answer):

#import <objc/runtime.h>

@implementation MyClass (Category)
- (NSString *)text{
    return objc_getAssociatedObject(self, @selector(text));
}

- (void)setText:(NSString *)text{
    objc_setAssociatedObject(self, @selector(text), text, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
@end

My suggested approach

Now, using a macro you will write instead:

@implementation MyClass (Category)

CATEGORY_PROPERTY_GET_SET(NSString*, text, setText:)

@end

The macros are defined as following:

#import <objc/runtime.h>

#define CATEGORY_PROPERTY_GET(type, property) - (type) property { return objc_getAssociatedObject(self, @selector(property)); }
#define CATEGORY_PROPERTY_SET(type, property, setter) - (void) setter (type) property { objc_setAssociatedObject(self, @selector(property), property, OBJC_ASSOCIATION_RETAIN_NONATOMIC); }
#define CATEGORY_PROPERTY_GET_SET(type, property, setter) CATEGORY_PROPERTY_GET(type, property) CATEGORY_PROPERTY_SET(type, property, setter)

#define CATEGORY_PROPERTY_GET_NSNUMBER_PRIMITIVE(type, property, valueSelector) - (type) property { return [objc_getAssociatedObject(self, @selector(property)) valueSelector]; }
#define CATEGORY_PROPERTY_SET_NSNUMBER_PRIMITIVE(type, property, setter, numberSelector) - (void) setter (type) property { objc_setAssociatedObject(self, @selector(property), [NSNumber numberSelector: property], OBJC_ASSOCIATION_RETAIN_NONATOMIC); }

#define CATEGORY_PROPERTY_GET_UINT(property) CATEGORY_PROPERTY_GET_NSNUMBER_PRIMITIVE(unsigned int, property, unsignedIntValue)
#define CATEGORY_PROPERTY_SET_UINT(property, setter) CATEGORY_PROPERTY_SET_NSNUMBER_PRIMITIVE(unsigned int, property, setter, numberWithUnsignedInt)
#define CATEGORY_PROPERTY_GET_SET_UINT(property, setter) CATEGORY_PROPERTY_GET_UINT(property) CATEGORY_PROPERTY_SET_UINT(property, setter)

The macro CATEGORY_PROPERTY_GET_SET adds a getter and setter for the given property. Read-only or write-only properties will use the CATEGORY_PROPERTY_GET and CATEGORY_PROPERTY_SET macro respectively.

Primitive types need a little more attention

As primitive types are no objects the above macros contain an example for using unsigned int as the property's type. It does so by wrapping the integer value into a NSNumber object. So its usage is analog to the previous example:

@interface ...
@property unsigned int value;
@end

@implementation ...
CATEGORY_PROPERTY_GET_SET_UINT(value, setValue:)
@end

Following this pattern, you can simply add more macros to also support signed int, BOOL, etc...

Limitations

1. All macros are using OBJC_ASSOCIATION_RETAIN_NONATOMIC by default.

2. IDEs like App Code do currently not recognize the setter's name when refactoring the property's name. You would need to rename it by yourself.