Bug 1166500 - Part 1: Store change hints between from and to for each animation segment. r=dbaron

BuildSegmentsFromValueEntries now needs base nsStyleContext to calculate
the change hints.
If the change hint is not set correctly, we will check it in
CanIgnoreIfNotVisible() introduced in a subsequent patch (part 3).

dom/animation/KeyframeEffect.h

@@ -8,6 +8,7 @@
 #define mozilla_dom_KeyframeEffect_h
 
 #include "nsAutoPtr.h"
+#include "nsChangeHint.h"
 #include "nsCSSProperty.h"
 #include "nsCSSValue.h"
 #include "nsCycleCollectionParticipant.h"
@@ -118,6 +119,8 @@ struct AnimationPropertySegment
   StyleAnimationValue mFromValue, mToValue;
   Maybe<ComputedTimingFunction> mTimingFunction;
 
+  nsChangeHint mChangeHint;
+
   bool operator==(const AnimationPropertySegment& aOther) const {
     return mFromKey == aOther.mFromKey &&
            mToKey == aOther.mToKey &&

dom/animation/KeyframeUtils.cpp

@@ -373,7 +373,8 @@ static bool
 HasValidOffsets(const nsTArray<Keyframe>& aKeyframes);
 
 static void
-BuildSegmentsFromValueEntries(nsTArray<KeyframeValueEntry>& aEntries,
+BuildSegmentsFromValueEntries(nsStyleContext* aStyleContext,
+                              nsTArray<KeyframeValueEntry>& aEntries,
                               nsTArray<AnimationProperty>& aResult);
 
 static void
@@ -550,7 +551,7 @@ KeyframeUtils::GetAnimationPropertiesFromKeyframes(
   }
 
   nsTArray<AnimationProperty> result;
-  BuildSegmentsFromValueEntries(entries, result);
+  BuildSegmentsFromValueEntries(aStyleContext, entries, result);
 
   return result;
 }
@@ -908,12 +909,43 @@ HasValidOffsets(const nsTArray<Keyframe>& aKeyframes)
   return true;
 }
 
+static already_AddRefed<nsStyleContext>
+CreateStyleContextForAnimationValue(nsCSSProperty aProperty,
+                                    StyleAnimationValue aValue,
+                                    nsStyleContext* aBaseStyleContext)
+{
+  MOZ_ASSERT(aBaseStyleContext,
+             "CreateStyleContextForAnimationValue needs to be called "
+             "with a valid nsStyleContext");
+
+  RefPtr<AnimValuesStyleRule> styleRule = new AnimValuesStyleRule();
+  styleRule->AddValue(aProperty, aValue);
+
+  nsCOMArray<nsIStyleRule> rules;
+  rules.AppendObject(styleRule);
+
+  MOZ_ASSERT(aBaseStyleContext->PresContext()->StyleSet()->IsGecko(),
+             "ServoStyleSet should not use StyleAnimationValue for animations");
+  nsStyleSet* styleSet =
+    aBaseStyleContext->PresContext()->StyleSet()->AsGecko();
+
+  RefPtr<nsStyleContext> styleContext =
+    styleSet->ResolveStyleByAddingRules(aBaseStyleContext, rules);
+
+  // We need to call StyleData to generate cached data for the style context.
+  // Otherwise CalcStyleDifference returns no meaningful result.
+  styleContext->StyleData(nsCSSProps::kSIDTable[aProperty]);
+
+  return styleContext.forget();
+}
+
 /**
  * Builds an array of AnimationProperty objects to represent the keyframe
  * animation segments in aEntries.
  */
 static void
-BuildSegmentsFromValueEntries(nsTArray<KeyframeValueEntry>& aEntries,
+BuildSegmentsFromValueEntries(nsStyleContext* aStyleContext,
+                              nsTArray<KeyframeValueEntry>& aEntries,
                               nsTArray<AnimationProperty>& aResult)
 {
   if (aEntries.IsEmpty()) {
@@ -1004,6 +1036,22 @@ BuildSegmentsFromValueEntries(nsTArray<KeyframeValueEntry>& aEntries,
     segment->mToValue   = aEntries[j].mValue;
     segment->mTimingFunction = aEntries[i].mTimingFunction;
 
+    RefPtr<nsStyleContext> fromContext =
+      CreateStyleContextForAnimationValue(animationProperty->mProperty,
+                                          segment->mFromValue, aStyleContext);
+
+    RefPtr<nsStyleContext> toContext =
+      CreateStyleContextForAnimationValue(animationProperty->mProperty,
+                                          segment->mToValue, aStyleContext);
+
+    uint32_t equalStructs = 0;
+    uint32_t samePointerStructs = 0;
+    segment->mChangeHint =
+        fromContext->CalcStyleDifference(toContext,
+          nsChangeHint(0),
+          &equalStructs,
+          &samePointerStructs);
+
     i = j;
   }
 }