Dynamically change an object's superclass

Question

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

Answer 1

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

What Makes Objective C Dynamic?, page 66

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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.

Answer 2

It is definitely possible using ObjC runtime, but it will be a bit hairy... This is a link to Apple's docs: Objective-C Runtime and an example of its usage: Objective-C Runtime Programming.