Check for divide-by-zero before calculating the initial velocity of spring animations. (#75)

This improves the robustness of MDMConfigureAnimation while enabling us to call the method without having to check whether the from/to value are equal.

Author: jverkoey

src/private/CABasicAnimation+MotionAnimator.h

@@ -33,6 +33,4 @@ FOUNDATION_EXPORT BOOL MDMCanAnimationBeAdditive(NSString *keyPath, id toValue);
 //
 // Not all animation value types support being additive. If an animation's value type was not
 // supported, the animation's values will not be modified.
-//
-// If the from and to values of the animation match then the behavior is undefined.
 FOUNDATION_EXPORT void MDMConfigureAnimation(CABasicAnimation *animation, MDMMotionTiming timing);

src/private/CABasicAnimation+MotionAnimator.m

@@ -180,7 +180,9 @@ void MDMConfigureAnimation(CABasicAnimation *animation, MDMMotionTiming timing)
       // As for our sign, if absoluteInitialVelocity matches the direction of displacement, then our
       // sign will be positive. Otherwise, our sign will be negative, as expected by Core Animation.
 
-      springAnimation.initialVelocity = absoluteInitialVelocity / displacement;
+      if (fabs(displacement) > 0.00001) {
+        springAnimation.initialVelocity = absoluteInitialVelocity / displacement;
+      }
     }
 
   } else if (IsCGSizeType(animation.toValue)) {
@@ -211,7 +213,9 @@ void MDMConfigureAnimation(CABasicAnimation *animation, MDMMotionTiming timing)
       CGFloat displacement = -biggestDelta;
       CGFloat absoluteInitialVelocity =
           timing.curve.data[MDMSpringMotionCurveDataIndexInitialVelocity];
-      springAnimation.initialVelocity = absoluteInitialVelocity / displacement;
+      if (fabs(displacement) > 0.00001) {
+        springAnimation.initialVelocity = absoluteInitialVelocity / displacement;
+      }
     }
 
   } else if (IsCGPointType(animation.toValue)) {
@@ -242,7 +246,9 @@ void MDMConfigureAnimation(CABasicAnimation *animation, MDMMotionTiming timing)
       CGFloat displacement = -biggestDelta;
       CGFloat absoluteInitialVelocity =
           timing.curve.data[MDMSpringMotionCurveDataIndexInitialVelocity];
-      springAnimation.initialVelocity = absoluteInitialVelocity / displacement;
+      if (fabs(displacement) > 0.00001) {
+        springAnimation.initialVelocity = absoluteInitialVelocity / displacement;
+      }
     }
   }