Result memoization for object values that are both immutable and expensive to compute.

example.m

#import "memoize.h"

@implementation Example

- (id)null;
{
    return NNMemoize(^{
        sleep(1);
        return [NSNull null];
    });
}

- (NSRect)computedFrame;
{
    return [NNMemoize((^{
        NSPoint min = [self.viewCollection lastObject].frame.origin,
                max = [self.viewCollection lastObject].frame.origin;
    
        for (NSView *view in self.viewCollection) {
            min.x = MIN(min.x, window.frame.origin.x);
            min.y = MIN(min.y, window.frame.origin.y);
            max.x = MAX(max.x, window.frame.origin.x + window.frame.size.width);
            max.y = MAX(max.y, window.frame.origin.y + window.frame.size.height);
        }
    
        return [NSValue valueWithRect:(NSRect){.origin = min, .size.width = max.x - min.x, .size.height = max.y - min.y}];
    })) rectValue];
}

@end

memoize.h

#define NNMemoize(block) _NNMemoize(self, _cmd, block)

id _NNMemoize(id self, SEL _cmd, id (^block)());

memoize.m

#include "memoize.h"

id _NNMemoize(id self, SEL _cmd, id (^block)()) {
    id result;
    void *key = (void *)((uintptr_t)(__bridge void *)self ^ (uintptr_t)(void *)_cmd ^ (uintptr_t)&_NNMemoize);
    
    @synchronized(self) {
        result = objc_getAssociatedObject(self, key);
        if (!result) {
            result = block();
            objc_setAssociatedObject(self, key, result, OBJC_ASSOCIATION_COPY_NONATOMIC);
        }
    }
    
    return result;
}