Dynamically change an object's superclass

Question

Is it possible to change an object's superclass at runtime? If so, how?

Have a look at http://stackoverflow.com/questions/11221110/my-isa-swizzling-breaks-kvo for an example of an isa swizzle. — mttrb, Feb 25 '13 at 05:29
Also, what do mean by "the superclass of an object"? An *object* has no superclass. An object has a class and its class has a superclass. If you, however, alter the superclass of the class of the object, **all members of that class will be affected,** not only that single object. — , Feb 25 '13 at 05:31
why superclass not current class? what happen to the current class? you have to think carefully first — Bryan Chen, Feb 25 '13 at 05:33

vikingosegundo · Accepted Answer · 2013-02-25T16:16:59.823

12

a short question, a short answer: yes, isa swizzling

What Makes Objective C Dynamic?, page 66

An example:

I have a class that handles connections to a REST-API, it is called APIClient. In testing I want to connect to a different server.

In the testing target I subclass APIClient

#import "ApiClient.h"

@interface TestApiClient : ApiClient
//…
@end


@interface TestApiClient ()
@property (nonatomic, strong, readwrite) NSURL *baseURL;

@end


@implementation TestApiClient

- (NSMutableURLRequest *)requestWithMethod:(NSString *)method
                                      path:(NSString *)path
                                parameters:(NSDictionary *)parameters
{
    self.baseURL = [NSURL URLWithString:@"http://localhost:8000/"];
    return [super requestWithMethod:method path:path parameters:parameters];
}

@end

In the Unit test class I do the swizzling #import

@implementation APIUnitTests


+(void)load
{
    client = [[ApiClient alloc ] init];
    object_setClass(client, [TestApiClient class]);
}

//…
@end

This cas is save, as I first created a subclass of an base class and then replaced the latter with the subclass. As the subclass is also a base class, this is valid inheritance.

edited Feb 25 '13 at 16:16

answered Feb 25 '13 at 05:24

vikingosegundo

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This sounds extremely dangerous. Why on earth would you want to? – Richard Brown Feb 25 '13 at 05:35
2

if done right it is very powerful, apple uses it for KVO: http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/KeyValueObserving/Articles/KVOImplementation.html – vikingosegundo Feb 25 '13 at 05:36
I personally use it to inspect code: I subclass and overwrite the methods I am interested in to log some informations and call the original methods on super. – vikingosegundo Feb 25 '13 at 05:40
Thanks for the valid reasons. – Richard Brown Feb 25 '13 at 05:53
@RichardBrown, I added a quick example. – vikingosegundo Feb 25 '13 at 05:59

Dynamically change an object's superclass

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