How do I create a smoothly resizable circular UIView?

Question

I\'m trying to create a UIView which shows a semitransparent circle with an opaque border inside its bounds. I want to be able to change the bounds in two ways - inside a <

Answer 1

With many thanks to David, this is the solution I found. In the end what turned out to be the key to it was using the view's -[actionForLayer:forKey:] method, since that's used inside UIView blocks instead of whatever the layer's -[actionForKey] returns.

@implementation SGBRoundView

-(CGFloat)radiusForBounds:(CGRect)bounds
{
    return fminf(bounds.size.width, bounds.size.height) / 2;
}

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        self.backgroundColor = [UIColor clearColor];
        self.opaque = NO;
        self.layer.backgroundColor = [[UIColor purpleColor] CGColor];
        self.layer.borderColor = [[UIColor greenColor] CGColor];
        self.layer.borderWidth = 3;
        self.layer.cornerRadius = [self radiusForBounds:self.bounds];
    }
    return self;
}

-(void)setBounds:(CGRect)bounds
{
    self.layer.cornerRadius = [self radiusForBounds:bounds];
    [super setBounds:bounds];
}

-(id)actionForLayer:(CALayer *)layer forKey:(NSString *)event
{
    id action = [super actionForLayer:layer forKey:event];

    if ([event isEqualToString:@"cornerRadius"])
    {
        CABasicAnimation *boundsAction = (CABasicAnimation *)[self actionForLayer:layer forKey:@"bounds"];
            if ([boundsAction isKindOfClass:[CABasicAnimation class]] && [boundsAction.fromValue isKindOfClass:[NSValue class]])
        {            
            CABasicAnimation *cornerRadiusAction = [CABasicAnimation animationWithKeyPath:@"cornerRadius"];
            cornerRadiusAction.delegate = boundsAction.delegate;
            cornerRadiusAction.duration = boundsAction.duration;
            cornerRadiusAction.fillMode = boundsAction.fillMode;
            cornerRadiusAction.timingFunction = boundsAction.timingFunction;

            CGRect fromBounds = [(NSValue *)boundsAction.fromValue CGRectValue];
            CGFloat fromRadius = [self radiusForBounds:fromBounds];
            cornerRadiusAction.fromValue = [NSNumber numberWithFloat:fromRadius];

            return cornerRadiusAction;
        }
    }

    return action;
}

@end

By using the action that the view provides for the bounds, I was able to get the right duration, fill mode and timing function, and most importantly delegate - without that, the completion block of UIView animations didn't run.

The radius animation follows that of the bounds in almost all circumstances - there are a few edge cases that I'm trying to iron out, but it's basically there. It's also worth mentioning that the pinch gestures are still sometimes jerky - I guess even the accelerated drawing is still costly.