gistfile1.m

typedef void (^pspdf_setter)(id self, SEL _cmd, id value);

static pspdf_setter pspdf_setterForKey(NSString *key) {

    return ^(id self, SEL _cmd, id value) {

        // The ivar name, from the original key: fooBar -> _fooBar
        // It's a convention: don't break it
        const char * ivarName = [[@"_" stringByAppendingString:[key capitalizedString]] UTF8String];

        // Get the ivar, so we can later replace it
        Ivar ivar = class_getInstanceVariable([self class], ivarName);

        id oldValue = object_getIvar(self, ivar);

        // Check it's not the same
        if (oldValue == value) {
            return;
        }

        // Be KVC friently
        [self willChangeValueForKey:key];

        // It's different, set new value
        object_setIvar(self, ivar, value);

        // Be KVC friendly
        [self didChangeValueForKey:key];

        // Do all the common things
        [self setNeedsDisplay];
    };
}

+ (void)load {

    someMagicMethodThatReplaceImplemenationWithBlockForKey(pspdf_setterForKey(@"radius"), @"radius");
    someMagicMethodThatReplaceImplemenationWithBlockForKey(pspdf_setterForKey(@"color"), @"color");
    someMagicMethodThatReplaceImplemenationWithBlockForKey(pspdf_setterForKey(@"secondaryColor"), @"secondaryColor");
    ...
}