core graphics - 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 -[UIView animateWithDuration:animations:] block and in a pinch gesture recogniser action which fires several times a second. I've tried three approaches based on answers elsewhere on SO, and none are suitable.

1. Setting the corner radius of the view's layer in layoutSubviews gives smooth translations, but the view doesn't stay circular during animations; it seems that cornerRadius isn't animatable.

2. Drawing the circle in drawRect: gives a consistently circular view, but if the circle gets too big then resizing in the pinch gesture gets choppy because the device is spending too much time redrawing the circle.

3. Adding a CAShapeLayer and setting its path property in layoutSublayersOfLayer, which doesn't animate inside UIView animations since path isn't implicitly animatable.

Is there a way for me to create a view which is consistently circular and smoothly resizable? Is there some other type of layer I could use to take advantage of the hardware acceleration?

UPDATE

A commenter has asked me to expand on what I mean when I say that I want to change the bounds inside a -[UIView animateWithDuration:animations:] block. In my code, I have a view which contains my circle view. The circle view (the version that uses cornerRadius) overrides -[setBounds:] in order to set the corner radius:

-(void)setBounds:(CGRect)bounds
{
    self.layer.cornerRadius = fminf(bounds.size.width, bounds.size.height) / 2.0;
    [super setBounds:bounds];
}

The bounds of the circle view are set in -[layoutSubviews]:

-(void)layoutSubviews
{
    // some other layout is performed and circleRadius and circleCenter are
    // calculated based on the properties and current size of the view.

    self.circleView.bounds = CGRectMake(0, 0, circleRadius*2, circleRadius*2);
    self.circleView.center = circleCenter;
}

The view is sometimes resized in animations, like so:

[UIView animateWithDuration:0.33 animations:^(void) {
    myView.frame = CGRectMake(x, y, w, h);
    [myView setNeedsLayout];
    [myView layoutIfNeeded];
}];

but during these animations, if I draw the circle view using a layer with a cornerRadius, it goes funny shapes. I can't pass the animation duration in to layoutSubviews so I need to add the right animation within -[setBounds].

Answer 1

As the section on Animations in the "View Programming Guide for iOS" says

Both UIKit and Core Animation provide support for animations, but the level of support provided by each technology varies. In UIKit, animations are performed using UIView objects

The full list of properties that you can animate using either the older

[UIView beginAnimations:context:];
[UIView setAnimationDuration:];
// Change properties here...
[UIView commitAnimations];

or the newer

[UIView animateWithDuration:animations:];

(that you are using) are:

• frame
• bounds
• center
• transform (CGAffineTransform, not the CATransform3D)
• alpha
• backgroundColor
• contentStretch

What confuses people is that you can also animate the same properties on the layer inside the UIView animation block, i.e. the frame, bounds, position, opacity, backgroundColor.

The same section goes on to say:

In places where you want to perform more sophisticated animations, or animations not supported by the UIView class, you can use Core Animation and the view’s underlying layer to create the animation. Because view and layer objects are intricately linked together, changes to a view’s layer affect the view itself.

A few lines down you can read the list of Core Animation animatable properties where you see this one:

• The layer’s border (including whether the layer’s corners are rounded)

---

There are at least two good options for achieving the effect that you are after:

• Animating the corner radius
• Using a CAShapeLayer and animating the path

Both of these require that you do the animations with Core Animation. You can create a CAAnimationGroup and add an array of animations to it if you need multiple animations to run as one.

---

Update:

Fixing things with as few code changes as possible would be done by doing the corner radius animation on the layer at the "same time" as the other animations. I put quotations marks around same time since it is not guaranteed that animations that are not in the same group will finish at exactly the same time. Depending on what other animations you are doing it might be better to use only basic animations and animations groups. If you are applying changes to many different views in the same view animation block then maybe you could look into CATransactions.

The below code animates the frame and corner radius much like you describe.

UIView *circle = [[UIView alloc] initWithFrame:CGRectMake(30, 30, 100, 100)];
[[circle layer] setCornerRadius:50];
[[circle layer] setBorderColor:[[UIColor orangeColor] CGColor]];
[[circle layer] setBorderWidth:2.0];
[[circle layer] setBackgroundColor:[[[UIColor orangeColor] colorWithAlphaComponent:0.5] CGColor]];
[[self view] addSubview:circle];

CGFloat animationDuration = 4.0; // Your duration
CGFloat animationDelay = 3.0; // Your delay (if any)

CABasicAnimation *cornerRadiusAnimation = [CABasicAnimation animationWithKeyPath:@"cornerRadius"];
[cornerRadiusAnimation setFromValue:[NSNumber numberWithFloat:50.0]]; // The current value
[cornerRadiusAnimation setToValue:[NSNumber numberWithFloat:10.0]]; // The new value
[cornerRadiusAnimation setDuration:animationDuration];
[cornerRadiusAnimation setBeginTime:CACurrentMediaTime() + animationDelay];

// If your UIView animation uses a timing funcition then your basic animation needs the same one
[cornerRadiusAnimation setTimingFunction:[CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut]];

// This will keep make the animation look as the "from" and "to" values before and after the animation
[cornerRadiusAnimation setFillMode:kCAFillModeBoth];
[[circle layer] addAnimation:cornerRadiusAnimation forKey:@"keepAsCircle"];
[[circle layer] setCornerRadius:10.0]; // Core Animation doesn't change the real value so we have to.

[UIView animateWithDuration:animationDuration
                      delay:animationDelay
                    options:UIViewAnimationOptionCurveEaseInOut 
                 animations:^{
                     [[circle layer] setFrame:CGRectMake(50, 50, 20, 20)]; // Arbitrary frame ...
                     // You other UIView animations in here...
                 } completion:^(BOOL finished) {
                     // Maybe you have your completion in here...
                 }];