Runtime in Objective-C

Using Associated Objects

Associated objects are useful when you want to add functionality to existing classes which requires holding state.

For example, adding a activity indicator to every UIView:

Objective-C Implementation

#import <objc/runtime.h>

static char ActivityIndicatorKey;

@implementation UIView (ActivityIndicator)

- (UIActivityIndicatorView *)activityIndicator {
    return (UIActivityIndicatorView *)objc_getAssociatedObject(self, &ActivityIndicatorKey);
}

- (void)setActivityIndicator: (UIActivityIndicatorView *)activityIndicator {
    objc_setAssociatedObject(self, &ActivityIndicatorKey, activityIndicator, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (void)showActivityIndicator {
    UIActivityIndicatorView *activityIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle: UIActivityIndicatorViewStyleGray];
    
    [self setActivityIndicator:activityIndicator];
    
    activityIndicator.center = self.center;
    activityIndicator.autoresizingMask = UIViewAutoresizingFlexibleTopMargin | UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleRightMargin | UIViewAutoresizingFlexibleBottomMargin;
    
    [activityIndicator startAnimating];
    
    [self addSubview: activityIndicator];
}

- (void)hideActivityIndicator {
    UIActivityIndicatorView * activityIndicator = [self activityIndicator];
    
    if (activityIndicator != nil) {
        [[self activityIndicator] removeFromSuperview];
    }
}

@end

You can also access the Objective-C runtime through Swift:

Swift Code

extension UIView {
    private struct AssociatedKeys {
        static var activityIndicator = "UIView.ActivityIndicatorView"
    }

    private var activityIndicatorView: UIActivityIndicatorView? {
        get {
            return objc_getAssociatedObject(self, &AssociatedKeys.activityIndicator) as? UIActivityIndicatorView
        }
        set (activityIndicatorView) {
            objc_setAssociatedObject(self, &AssociatedKeys.activityIndicator, activityIndicatorView, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }

     func showActivityIndicator() {
        activityIndicatorView = UIActivityIndicatorView(activityIndicatorStyle: .gray)
        activityIndicatorView.center = center
        activityIndicatorView.autoresizingMask = [.flexibleLeftMargin, .flexibleRightMargin, .flexibleTopMargin, .flexibleBottomMargin]
        
        activityIndicatorView.startAnimating()
        
        addSubview(activityIndicatorView)
    }
    
    func hideActivityIndicator() {
        activityIndicatorView.removeFromSuperview()
    }
}