Update Core Animation engine to support negative speed values (airbnb…

…#1660)

Co-authored-by: Cal Stephens <cal.stephens@airbnb.com>

Example/iOS/ViewControllers/AnimationPreviewViewController.swift

@@ -107,6 +107,7 @@ class AnimationPreviewViewController: UIViewController {
   private var displayLink: CADisplayLink?
 
   private var loopMode = LottieLoopMode.autoReverse
+  private var speed: CGFloat = 1
   private var fromProgress: AnimationProgressTime = 0
   private var toProgress: AnimationProgressTime = 1
 
@@ -142,6 +143,39 @@ class AnimationPreviewViewController: UIViewController {
               }),
           ]),
 
+        UIMenu(
+          title: "Speed",
+          children: [
+            UIAction(
+              title: "-100%",
+              state: speed == -1 ? .on : .off,
+              handler: { [unowned self] _ in
+                speed = -1
+                updateAnimation()
+              }),
+            UIAction(
+              title: "-50%",
+              state: speed == -0.5 ? .on : .off,
+              handler: { [unowned self] _ in
+                speed = -0.5
+                updateAnimation()
+              }),
+            UIAction(
+              title: "50%",
+              state: speed == 0.5 ? .on : .off,
+              handler: { [unowned self] _ in
+                speed = 0.5
+                updateAnimation()
+              }),
+            UIAction(
+              title: "100%",
+              state: speed == 1 ? .on : .off,
+              handler: { [unowned self] _ in
+                speed = 1
+                updateAnimation()
+              }),
+          ]),
+
         UIMenu(
           title: "From Progress...",
           children: [
@@ -205,6 +239,7 @@ class AnimationPreviewViewController: UIViewController {
 
   private func updateAnimation() {
     animationView.play(fromProgress: fromProgress, toProgress: toProgress, loopMode: loopMode)
+    animationView.animationSpeed = speed
     configureSettingsMenu()
   }
 

Sources/Public/Animation/AnimationView.swift

@@ -1210,10 +1210,19 @@ final public class AnimationView: AnimationViewBase {
 
     if let coreAnimationLayer = animationlayer as? CoreAnimationLayer {
       var animationContext = animationContext
+
+      // Core Animation doesn't natively support negative speed values,
+      // so instead we can swap `playFrom` / `playTo`
+      if animationSpeed < 0 {
+        let temp = animationContext.playFrom
+        animationContext.playFrom = animationContext.playTo
+        animationContext.playTo = temp
+      }
+
       var timingConfiguration = CoreAnimationLayer.CAMediaTimingConfiguration(
         autoreverses: loopMode.caAnimationConfiguration.autoreverses,
         repeatCount: loopMode.caAnimationConfiguration.repeatCount,
-        speed: Float(animationSpeed))
+        speed: abs(Float(animationSpeed)))
 
       // The animation should start playing from the `currentFrame`,
       // if `currentFrame` is included in the time range being played.
@@ -1234,7 +1243,11 @@ final public class AnimationView: AnimationViewBase {
         // the duration of the _first_ loop. Instead of setting `playFrom`, we just add a `timeOffset`
         // so the first loop begins at `currentTime` but all subsequent loops are the standard duration.
         default:
-          timingConfiguration.timeOffset = currentTime - animation.time(forFrame: animationContext.playFrom)
+          if animationSpeed < 0 {
+            timingConfiguration.timeOffset = animation.time(forFrame: animationContext.playFrom) - currentTime
+          } else {
+            timingConfiguration.timeOffset = currentTime - animation.time(forFrame: animationContext.playFrom)
+          }
         }
       }