Here's how to create a property on the fly (great for categories)

@dynamic progressView;

- (void)setProgressView:(ACProgressView )progressView { objc_setAssociatedObject(self, ACProgressViewKey, progressView, OBJC_ASSOCIATION_RETAIN_NONATOMIC); }

- (ACProgressView )progressView { return objc_getAssociatedObject(self, ACProgressViewKey); }

    NSObject obj = [NSObject init];
    giveObjectAProperty(obj, @"progressView");
    obj.progressView = [NSNumber 42];

And of course the parent is talking about methods, not properties, but...same argument.

Here's how you'd make that class on the fly:

   Class mySubclass = objc_allocateClassPair([NSObject class], "MySubclass", 0);

   static NSString *Description(id self, SEL _cmd){
        return [NSString stringWithFormat: @"<%@ %p: foo=%@>",[self class], self, [self foo]];
   }

    // add Description to mySubclass
    // grab NSObject's description signature so we can borrow it
    Method description = class_getInstanceMethod([NSObject class],
                                                 @selector(description));
    const char *types = method_getTypeEncoding(description);
    
    // now add
    class_addMethod(mySubclass, @selector(description), (IMP)Description, types);

   class_addIvar(mySubclass, "foo", sizeof(id), rint(log2(sizeof(id))), @encode(id));

    klass = Object.const_set "MySubclass", Class.new(Object)
    
    klass.send(:attr_accessor, :foo)
 
    klass.send(:define_method, "description") {
      "<#{self.class}: foo=#{self.foo}>"
    }

[1]: https://github.com/mikeash/MAObjCRuntime [2]: https://github.com/mikeash/MACollectionUtilities

Edit: I obv. didn't write your parent comment. What I meant was that I'm sure others in my position, having read your parent, would appreciate more elaboration from you as well. :)