Bug 1315874 - Add MightNeedBaseStyle helper method; r=dholbert

We will use this in a subsequent patch in this series to determine if we need to
fetch a style context at the beginning of ComposeAttribute (when we create the
nsISMILAttr) or not.

MozReview-Commit-ID: CFBNmmFNNef

--HG--
extra : rebase_source : 4729b2e1194649f461c353fa74d99b5b7829a077

dom/smil/nsSMILCompositor.cpp

@@ -173,6 +173,24 @@ nsSMILCompositor::GetCSSPropertyToAnimate() const
   return propID;
 }
 
+bool
+nsSMILCompositor::MightNeedBaseStyle() const
+{
+  if (GetCSSPropertyToAnimate() == eCSSProperty_UNKNOWN) {
+    return false;
+  }
+
+  // We should return true if at least one animation function might build on
+  // the base value.
+  for (const nsSMILAnimationFunction* func : mAnimationFunctions) {
+    if (!func->WillReplace()) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
 uint32_t
 nsSMILCompositor::GetFirstFuncToAffectSandwich()
 {

dom/smil/nsSMILCompositor.h

@@ -80,6 +80,16 @@ public:
   // eCSSProperty_UNKNOWN if this compositor does not animate a CSS property.
   nsCSSPropertyID GetCSSPropertyToAnimate() const;
 
+  // Returns true if we might need to refer to base styles (i.e. we are
+  // targeting a CSS property and have one or more animation functions that
+  // don't just replace the underlying value).
+  //
+  // This might return true in some cases where we don't actually need the base
+  // style since it doesn't build up the animation sandwich to check if the
+  // functions that appear to need the base style are actually replaced by
+  // a function further up the stack.
+  bool MightNeedBaseStyle() const;
+
   // Finds the index of the first function that will affect our animation
   // sandwich. Also toggles the 'mForceCompositing' flag if it finds that any
   // (used) functions have changed.