Dynamically change an object's superclass

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

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 <objc/runtime.h>

@implementation APIUnitTests


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

//…
@end

This case 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.