Bug 1346052 - Part 2: Implement AnimationValue::ComputeDistance. r=birtles

We will use this API to compute the distance between two AnimationValues
in the patch series.

MozReview-Commit-ID: 2Tw9xumuz45

--HG--
extra : rebase_source : 298baaf597a81e03191a0468f9d7509e092a5e78

layout/style/StyleAnimationValue.cpp

@@ -5295,3 +5295,31 @@ AnimationValue::IsInterpolableWith(nsCSSPropertyID aProperty,
   return StyleAnimationValue::Interpolate(
            aProperty, mGecko, aToValue.mGecko, 0.5, dummy);
 }
+
+double
+AnimationValue::ComputeDistance(nsCSSPropertyID aProperty,
+                                const AnimationValue& aOther,
+                                nsStyleContext* aStyleContext) const
+{
+  if (IsNull() || aOther.IsNull()) {
+    return 0.0;
+  }
+
+  MOZ_ASSERT(!mServo != mGecko.IsNull());
+  MOZ_ASSERT(mGecko.IsNull() == aOther.mGecko.IsNull() &&
+             !mServo == !aOther.mServo,
+             "Animation values should have the same style engine");
+
+  if (mServo) {
+    return Servo_AnimationValues_ComputeDistance(mServo, aOther.mServo);
+  }
+
+  double distance = 0.0;
+  return StyleAnimationValue::ComputeDistance(aProperty,
+                                              mGecko,
+                                              aOther.mGecko,
+                                              aStyleContext,
+                                              distance)
+         ? distance
+         : 0.0;
+}

layout/style/StyleAnimationValue.h

@@ -602,6 +602,13 @@ struct AnimationValue
   // Check if |*this| and |aToValue| can be interpolated.
   bool IsInterpolableWith(nsCSSPropertyID aProperty,
                           const AnimationValue& aToValue) const;
+
+  // Compute the distance between *this and aOther.
+  // If |aStyleContext| is nullptr, we will return 0.0 if we have mismatched
+  // transform lists.
+  double ComputeDistance(nsCSSPropertyID aProperty,
+                         const AnimationValue& aOther,
+                         nsStyleContext* aStyleContext) const;
 };
 
 struct PropertyStyleAnimationValuePair