Question

As I cannot create a synthesized property in a Category in Objective-C, I do not know how to optimize the following code:

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

@implementation MyClass (Variant)

@dynamic test;

- (NSString *)test {
    NSString *res;
    //do a lot of stuff
    return res;
}

@end

The test-method is called multiple times on runtime and I’m doing a lot of stuff to calculate the result. Normally using a synthesized property I store the value in a IVar _test the first time the method is called, and just returning this IVar next time. How can I optimized the above code?

Answer 1

@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