Why I can't get the associated object from another class? - ios

Question

How to get associated object from another class
My code is :
#import <UIKit/UIKit.h>
static char NUMBER ='a';
#interface AppDelegate : UIResponder <UIApplicationDelegate>
#property (strong, nonatomic) UIWindow *window;
#end
#implementation Person
- (instancetype)init
{
self = [super init];
if (self) {
NSNumber *num=#10;
objc_setAssociatedObject(self, &NUMBER, num, OBJC_ASSOCIATION_RETAIN);
}
return self;
}
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
Person *p=[[Person alloc]init];
NSNumber *num=objc_getAssociatedObject(p, &NUMBER);
NSLog(#"%#",num);
}
#end
NSLog(#"%#",num) is null.
Why can't I get the associated object from the above code. Can't we get the associated object from another class ? Thank you!

Answer 1

Problem is in your key. You probably define this classes in different files. Do not use static keyword, static variables can only be accessed within a single translation unit. This means you have new copy of NUMBER for each file. Remove static keyword and add extern declaration in Person.h header:
Person.h:
extern const char NUMBER;
#interface Person : NSObject
#end
Person.m:
#import "Person.h"
#import "objc/runtime.h"
const char NUMBER ='a';
#implementation Person
- (instancetype)init
{
self = [super init];
if (self) {
NSNumber *num = #10;
objc_setAssociatedObject(self, &NUMBER, num, OBJC_ASSOCIATION_RETAIN);
}
return self;
}
#end
ViewController.m:
#import "Person.h"
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
Person *p = [[Person alloc]init];
NSNumber *num = objc_getAssociatedObject(p, &NUMBER);
NSLog(#"%#",num);
}
#end