Add the property to the ViewController header.

CircleViewController.h

#import <UIKit/UIKit.h>

@interface CircleViewController : UIViewController
@property (nonatomic) int radius; // 1 small circle, 100 big
@end

And synthesize it in our code .m file.

CircleViewController.m

#import "CircleViewController.h"
@implementation CircleViewController
@synthesize radius = _radius;
@end

You can also optionally reset your view for display if you model changes like so:

-(void)setHappiness:(int)happiness
{
    NSLog(@"setHappiness: %@", @"1");
    _happiness = happiness;
    [self.faceView setNeedsDisplay];
}