mutable-objects-in-immutable-methods-on-a-mutable-runtime.m

#import <Foundation/Foundation.h>
#import <objc/runtime.h>

@interface Mutable : NSObject

@property (nonatomic, retain) id x;

@end

@implementation Mutable

-init
{
	self = [super init];
	if (!self) return nil;
	NSString *className = [NSString stringWithFormat:@"%@_%@",
		NSStringFromClass([self class]),
		[[NSUUID UUID] UUIDString]];
	Class instanceClass = objc_allocateClassPair([self class], [className UTF8String], 0);
	IMP getter = method_getImplementation(class_getInstanceMethod([self class], @selector(x)));
	class_addMethod(instanceClass, @selector(x), getter, "@@:");
	objc_registerClassPair(instanceClass);
	object_setClass(self, instanceClass);
	return self;
}

-class { return [Mutable class]; }

-x { return nil; }

-(void)setX:nextX
{
	id(^getX)(id self, id alsoSelf) = ^(id self, id alsoSelf){ return nextX; };
	IMP nextGetter = imp_implementationWithBlock(getX);
	Method getterMethod = class_getInstanceMethod(object_getClass(self), @selector(x));
	method_setImplementation(getterMethod, nextGetter);
}

@end

int main(int argc, char *argv[]) {
	@autoreleasepool {
		Mutable *a = [Mutable new];
		Mutable *b = [Mutable new];
		NSLog(@"%@", a.x);
		a.x = @"Hello, world!";
		NSLog(@"%@", a.x);
		NSLog(@"%@", b.x);
	}
}