Custom property animation with actionForKey: how do I get the new value for the property? - core-animation

Question

In a CALayer subclass I'm working on I have a custom property that I want to animate automatically, that is, assuming the property is called "myProperty", I want the following code:
[myLayer setMyProperty:newValue];
To cause a smooth animation from the current value to "newValue".
Using the approach of overriding actionForKey: and needsDisplayForKey: (see following code) I was able to get it to run very nicely to simply interpolate between the old and and new value.
My problem is that I want to use a slightly different animation duration or path (or whatever) depending on both the current value and the new value of the property and I wasn't able to figure out how to get the new value from within actionForKey:
Thanks in advance
#interface ERAnimatablePropertyLayer : CALayer {
float myProperty;
}
#property (nonatomic, assign) float myProperty;
#end
#implementation ERAnimatablePropertyLayer
#dynamic myProperty;
- (void)drawInContext:(CGContextRef)ctx {
... some custom drawing code based on "myProperty"
}
- (id <CAAction>)actionForKey:(NSString *)key {
if ([key isEqualToString:#"myProperty"]) {
CABasicAnimation *theAnimation = [CABasicAnimation animationWithKeyPath:key];
theAnimation.fromValue = [[self presentationLayer] valueForKey:key];
... I want to do something special here, depending on both from and to values...
return theAnimation;
}
return [super actionForKey:key];
}
+ (BOOL)needsDisplayForKey:(NSString *)key {
if ([key isEqualToString:#"myProperty"])
return YES;
return [super needsDisplayForKey:key];
}
#end

Answer 1

You need to avoid custom getters and setters for properties you want to animate.
Override the didChangeValueForKey: method. Use it to set the model value for the property you want to animate.
Don't set the toValue on the action animation.
#interface MyLayer: CALayer
#property ( nonatomic ) NSUInteger state;
#end
-
#implementation MyLayer
#dynamic state;
- (id<CAAction>)actionForKey: (NSString *)key {
if( [key isEqualToString: #"state"] )
{
CABasicAnimation * bgAnimation = [CABasicAnimation animationWithKeyPath: #"backgroundColor"];
bgAnimation.fromValue = [self.presentationLayer backgroundColor];
bgAnimation.duration = 0.4;
return bgAnimation;
}
return [super actionForKey: key];
}
- (void)didChangeValueForKey: (NSString *)key {
if( [key isEqualToString: #"state"] )
{
const NSUInteger state = [self valueForKey: key];
UIColor * newBackgroundColor;
switch (state)
{
case 0:
newBackgroundColor = [UIColor redColor];
break;
case 1:
newBackgroundColor = [UIColor blueColor];
break;
case 2:
newBackgroundColor = [UIColor greenColor];
break;
default:
newBackgroundColor = [UIColor purpleColor];
}
self.backgroundColor = newBackgroundColor.CGColor;
}
[super didChangeValueForKey: key];
}
#end

Answer 2

Core Animation calls actionForKey: before updating the property value. It runs the action after updating the property value by sending it runActionForKey:object:arguments:. The CAAnimation implementation of runActionForKey:object:arguments: just calls [object addAnimation:self forKey:key].
Instead of returning the animation from actionForKey:, you can return a CAAction that, when run, creates and installs an animation. Something like this:
#interface MyAction: NSObject <CAAction>
#property (nonatomic, strong) id priorValue;
#end
#implementation MyAction
- (void)runActionForKey:(NSString *)key object:(id)anObject arguments:(NSDictionary *)dict {
ERAnimatablePropertyLayer *layer = anObject;
CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:key];
id newValue = [layer valueForKey:key];
// Set up animation using self.priorValue and newValue to determine
// fromValue and toValue. You could use a CAKeyframeAnimation instead of
// a CABasicAnimation.
[layer addAnimation:animation forKey:key];
}
#end
#implementation ERAnimatablePropertyLayer
- (id <CAAction>)actionForKey:(NSString *)key {
if ([key isEqualToString:#"myProperty"]) {
MyAction *action = [[MyAction alloc] init];
action.priorValue = [self valueForKey:key];
return action;
}
return [super actionForKey:key];
}
You can find a working example of a similar technique (in Swift, animating cornerRadius) in this answer.

Answer 3

You can store the old and new values in CATransaction.
-(void)setMyProperty:(float)value
{
NSNumber *fromValue = [NSNumber numberWithFloat:myProperty];
[CATransaction setValue:fromValue forKey:#"myPropertyFromValue"];
myProperty = value;
NSNumber *toValue = [NSNumber numberWithFloat:myProperty];
[CATransaction setValue:toValue forKey:#"myPropertyToValue"];
}
- (id <CAAction>)actionForKey:(NSString *)key {
if ([key isEqualToString:#"myProperty"]) {
CABasicAnimation *theAnimation = [CABasicAnimation animationWithKeyPath:key];
theAnimation.fromValue = [[self presentationLayer] valueForKey:key];
theAnimation.toValue = [CATransaction objectForKey:#"myPropertyToValue"];
// here you do something special.
}
return [super actionForKey:key];
}