How does objc_setAssociatedObject work? [duplicate]

StackOverflow https://stackoverflow.com/questions/17678298

Question

As we know, we can add a variable in Objective-C using a category and runtime methods like objc_setAssociatedObject and objc_getAssociatedObject. For example:

#import <objc/runtime.h>
@interface Person (EmailAddress)
@property (nonatomic, readwrite, copy) NSString *emailAddress;
@end

@implementation Person (EmailAddress)

static char emailAddressKey;

- (NSString *)emailAddress {
    return objc_getAssociatedObject(self, 
                                    &emailAddressKey);
}

- (void)setEmailAddress:(NSString *)emailAddress {
   objc_setAssociatedObject(self, 
                            &emailAddressKey,
                            emailAddress,
                            OBJC_ASSOCIATION_COPY);
}
@end

But does anybody know what does objc_getAssociatedObject or objc_setAssociatedObject do? I mean, where are the variable we add to the object(here is self) stored? And the relationship between variable and self?

Was it helpful?

Solution

The code for associated objects is in objc-references.mm in the Objective-C runtime.

If I understand it correctly, there is one global hash map (static AssociationsHashMap *_map in class AssociationsManager) that maps the address of an object ("disguised" as uintptr_t) to an ObjectAssociationMap.

ObjectAssociationMap stores all associations for one particular object and is created when 

void objc_setAssociatedObject(id object, void *key, id value, objc_AssociationPolicy policy)

is called the first time for an object.

ObjectAssociationMap is a hash map that maps the key to value and policy.

When an object is deallocated, _object_remove_assocations() removes all associations and releases the values if necessary.

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