Support per property CSS Animation stacks

Source/core/animation/InvalidatableStyleInterpolation.cpp

Index: Source/core/animation/InvalidatableStyleInterpolation.cpp
diff --git a/Source/core/animation/InvalidatableStyleInterpolation.cpp b/Source/core/animation/InvalidatableStyleInterpolation.cpp
index d0f20a686b362d16feadb1ca1e5c59b8d394b7e9..b6856731a3f5701b45becb9f59dfe2f1e96980dc 100644
--- a/Source/core/animation/InvalidatableStyleInterpolation.cpp
+++ b/Source/core/animation/InvalidatableStyleInterpolation.cpp
@@ -120,20 +120,64 @@ void InvalidatableStyleInterpolation::setFlagIfInheritUsed(StyleResolverState& s
     }
 }
 
-void InvalidatableStyleInterpolation::apply(StyleResolverState& state) const
+void InvalidatableStyleInterpolation::applyStack(const ActiveInterpolations& interpolations, StyleResolverState& state)
 {
-    OwnPtr<InterpolationValue> underlyingValue = dependsOnUnderlyingValue() ? maybeConvertUnderlyingValue(state) : nullptr;
-    ensureValidInterpolation(state, underlyingValue.get());
-    if (!m_cachedValue)
-        return;
-    const InterpolableValue* appliedInterpolableValue = &m_cachedValue->interpolableValue();
-    if (underlyingValue && m_cachedValue->type() == underlyingValue->type()) {
-        double underlyingFraction = m_cachedConversion->interpolateUnderlyingFraction(m_startKeyframe->underlyingFraction(), m_endKeyframe->underlyingFraction(), m_currentFraction);
-        underlyingValue->interpolableValue().scaleAndAdd(underlyingFraction, m_cachedValue->interpolableValue());
-        appliedInterpolableValue = &underlyingValue->interpolableValue();
+    ASSERT(interpolations.size() > 0);
+    size_t startingIndex = 0;
+
+    // Compute the underlying value to composite onto.
+    OwnPtr<InterpolationValue> underlyingValueOwner = nullptr;

[dstockwell, 2015/09/07 03:11:37]

Let's see if we can encapsulate this ownptr/rawptr split into a helper.

[alancutter (OOO until 2018), 2015/09/07 03:36:46]

Done.

+    InterpolationValue* underlyingValue = nullptr;
+    const InvalidatableStyleInterpolation& firstInterpolation = toInvalidatableStyleInterpolation(*interpolations.at(startingIndex));
+    if (firstInterpolation.dependsOnUnderlyingValue()) {
+        underlyingValueOwner = firstInterpolation.maybeConvertUnderlyingValue(state);
+        underlyingValue = underlyingValueOwner.get();
+    } else {
+        firstInterpolation.ensureValidInterpolation(state, nullptr);

[dstockwell, 2015/09/07 03:11:37]

Should ensureValidInterpolation just return the m_cachedValue? It seems we
always use it that way.

[alancutter (OOO until 2018), 2015/09/07 03:36:46]

Good idea, done.

+        // Fast path for replace interpolations that are the last to apply.
+        if (interpolations.size() == 1) {
+            const InterpolationValue* firstValue = firstInterpolation.m_cachedValue.get();
+            if (firstValue) {
+                firstInterpolation.setFlagIfInheritUsed(state);
+                firstValue->type().apply(firstValue->interpolableValue(), firstValue->nonInterpolableValue(), state);
+            }
+            return;
+        }
+        underlyingValue = firstInterpolation.m_cachedValue.get();
+        startingIndex++;
+    }
+
+    // Composite interpolations onto the underlying value.
+    bool shouldApply = false;
+    for (size_t i = startingIndex; i < interpolations.size(); i++) {
+        const InvalidatableStyleInterpolation& currentInterpolation = toInvalidatableStyleInterpolation(*interpolations.at(i));
+        ASSERT(currentInterpolation.dependsOnUnderlyingValue());
+        currentInterpolation.ensureValidInterpolation(state, underlyingValue);
+        const InterpolationValue* currentValue = currentInterpolation.m_cachedValue.get();
+        if (!currentValue)
+            continue;
+        shouldApply = true;
+        currentInterpolation.setFlagIfInheritUsed(state);
+        if (!underlyingValue || underlyingValue->type() != currentValue->type()) {
+            // We keep track of the current value as the new underlying value but do not mutate it, instead perform copy on write.
+            underlyingValue = const_cast<InterpolationValue*>(currentValue);
+            underlyingValueOwner.clear();
+        } else {
+            double underlyingFraction = currentInterpolation.m_cachedConversion->interpolateUnderlyingFraction(
+                currentInterpolation.m_startKeyframe->underlyingFraction(),
+                currentInterpolation.m_endKeyframe->underlyingFraction(),
+                currentInterpolation.m_currentFraction);
+            if (!underlyingValueOwner) {
+                underlyingValueOwner = underlyingValue->clone();
+                underlyingValue = underlyingValueOwner.get();
+            }
+            underlyingValue->interpolableValue().scaleAndAdd(underlyingFraction, currentInterpolation.m_cachedValue->interpolableValue());
+        }
+    }
+
+    if (shouldApply && underlyingValue) {
+        underlyingValue->type().apply(underlyingValue->interpolableValue(), underlyingValue->nonInterpolableValue(), state);
     }
-    m_cachedValue->type().apply(*appliedInterpolableValue, m_cachedValue->nonInterpolableValue(), state);
-    setFlagIfInheritUsed(state);
 }
 
 } // namespace blink