So I guess you have something like this:
![bad animation]()
(I turned on “Debug > Slow Animations” in the simulator.) And you don't like the way the shadow jumps to its new size. You want this instead:
![good animation]()
You can find my test project in this github repository.
See @horseshoe7's answer for a Swift translation.
It is tricky but not impossible to pick up the animation parameters and add an animation in the table view's animation block. The trickiest part is that you need to update the shadowPath in the layoutSubviews method of the shadowed view itself, or of the shadowed view's immediate superview. In my demo video, that means that the shadowPath needs to be updated by the layoutSubviews method of the green box view or the green box's immediate superview.
I chose to create a ShadowingView class whose only job is to draw and animate the shadow of one of its subviews. Here's the interface:
@interface ShadowingView : UIView
@property (nonatomic, strong) IBOutlet UIView *shadowedView;
@end
To use ShadowingView, I added it to my cell view in my storyboard. Actually it's nested inside a stack view inside the cell. Then I added the green box as a subview of the ShadowingView and connected the shadowedView outlet to the green box.
The ShadowingView implementation has three parts. One is its layoutSubviews method, which sets up the layer shadow properties on its own layer to draw a shadow around its shadowedView subview:
@implementation ShadowingView
- (void)layoutSubviews {
[super layoutSubviews];
CALayer *layer = self.layer;
layer.backgroundColor = nil;
CALayer *shadowedLayer = self.shadowedView.layer;
if (shadowedLayer == nil) {
layer.shadowColor = nil;
return;
}
NSAssert(shadowedLayer.superlayer == layer, @"shadowedView must be my direct subview");
layer.shadowColor = UIColor.blackColor.CGColor;
layer.shadowOffset = CGSizeMake(0, 1);
layer.shadowOpacity = 0.5;
layer.shadowRadius = 3;
layer.masksToBounds = NO;
CGFloat radius = shadowedLayer.cornerRadius;
layer.shadowPath = CGPathCreateWithRoundedRect(shadowedLayer.frame, radius, radius, nil);
}
When this method is run inside an animation block (as is the case when the table view animates a change in the size of a cell), and the method sets shadowPath, Core Animation looks for an “action” to run after updating shadowPath. One of the ways it looks is by sending actionForLayer:forKey: to the layer's delegate, and the delegate is the ShadowingView. So we override actionForLayer:forKey: to provide an action if possible and appropriate. If we can't, we just call super.
It is important to understand that Core Animation asks for the action from inside the shadowPath setter, before actually changing the value of shadowPath.
To provide the action, we make sure the key is @"shadowPath", that there is an existing value for shadowPath, and that there is already an animation on the layer for bounds.size. Why do we look for an existing bounds.size animation? Because that existing animation has the duration and timing function we should use to animate shadowPath. If everything is in order, we grab the existing shadowPath, make a copy of the animation, store them in an action, and return the action:
- (id<CAAction>)actionForLayer:(CALayer *)layer forKey:(NSString *)event {
if (![event isEqualToString:@"shadowPath"]) { return [super actionForLayer:layer forKey:event]; }
CGPathRef priorPath = layer.shadowPath;
if (priorPath == NULL) { return [super actionForLayer:layer forKey:event]; }
CAAnimation *sizeAnimation = [layer animationForKey:@"bounds.size"];
if (![sizeAnimation isKindOfClass:[CABasicAnimation class]]) { return [super actionForLayer:layer forKey:event]; }
CABasicAnimation *animation = [sizeAnimation copy];
animation.keyPath = @"shadowPath";
ShadowingViewAction *action = [[ShadowingViewAction alloc] init];
action.priorPath = priorPath;
action.pendingAnimation = animation;
return action;
}
@end
What does the action look like? Here's the interface:
@interface ShadowingViewAction : NSObject <CAAction>
@property (nonatomic, strong) CABasicAnimation *pendingAnimation;
@property (nonatomic) CGPathRef priorPath;
@end
The implementation requires a runActionForKey:object:arguments: method. In this method, we update the animation that we created in actionForLayer:forKey: using the saved-away old shadowPath and the new shadowPath, and then we add the animation to the layer.
We also need to manage the retain count of the saved path, because ARC doesn't manage CGPath objects.
@implementation ShadowingViewAction
- (void)runActionForKey:(NSString *)event object:(id)anObject arguments:(NSDictionary *)dict {
if (![anObject isKindOfClass:[CALayer class]] || _pendingAnimation == nil) { return; }
CALayer *layer = anObject;
_pendingAnimation.fromValue = (__bridge id)_priorPath;
_pendingAnimation.toValue = (__bridge id)layer.shadowPath;
[layer addAnimation:_pendingAnimation forKey:@"shadowPath"];
}
- (void)setPriorPath:(CGPathRef)priorPath {
CGPathRetain(priorPath);
CGPathRelease(_priorPath);
_priorPath = priorPath;
}
- (void)dealloc {
CGPathRelease(_priorPath);
}
@end
