Bug 1403500, part 1 - Document and do some renaming for SVGObserverUtils' SourceReference. r=longsonr

MozReview-Commit-ID: Hd7uzHltyMl

layout/svg/SVGObserverUtils.cpp

@@ -180,10 +180,10 @@ nsSVGRenderingObserver::ContentRemoved(nsIDocument* aDocument,
 nsSVGIDRenderingObserver::nsSVGIDRenderingObserver(nsIURI* aURI,
                                                    nsIContent* aObservingContent,
                                                    bool aReferenceImage)
-  : mElement(this)
+  : mObservedElementTracker(this)
 {
   // Start watching the target element
-  mElement.Reset(aObservingContent, aURI, true, aReferenceImage);
+  mObservedElementTracker.Reset(aObservingContent, aURI, true, aReferenceImage);
   StartListening();
 }
 
@@ -195,8 +195,8 @@ nsSVGIDRenderingObserver::~nsSVGIDRenderingObserver()
 void
 nsSVGIDRenderingObserver::DoUpdate()
 {
-  if (mElement.get() && mInObserverList) {
-    SVGObserverUtils::RemoveRenderingObserver(mElement.get(), this);
+  if (mObservedElementTracker.get() && mInObserverList) {
+    SVGObserverUtils::RemoveRenderingObserver(mObservedElementTracker.get(), this);
     mInObserverList = false;
   }
 }
@@ -242,12 +242,12 @@ NS_IMPL_CYCLE_COLLECTING_RELEASE(nsSVGFilterReference)
 NS_IMPL_CYCLE_COLLECTION_CLASS(nsSVGFilterReference)
 
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(nsSVGFilterReference)
-  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mElement)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mObservedElementTracker)
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
 
 NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsSVGFilterReference)
   tmp->StopListening();
-  NS_IMPL_CYCLE_COLLECTION_UNLINK(mElement);
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mObservedElementTracker);
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsSVGFilterReference)

layout/svg/SVGObserverUtils.h

@@ -120,21 +120,28 @@ public:
   virtual ~nsSVGIDRenderingObserver();
 
 protected:
-  Element* GetTarget() override { return mElement.get(); }
+  Element* GetTarget() override { return mObservedElementTracker.get(); }
 
   // This is called when the referenced resource changes.
   virtual void DoUpdate() override;
 
-  class SourceReference : public IDTracker
+  /**
+   * Helper that provides a reference to the element with the ID that our
+   * observer wants to observe, and that will invalidate our observer if the
+   * element that that ID identifies changes to a different element (or none).
+   */
+  class ElementTracker final : public IDTracker
   {
   public:
-    explicit SourceReference(nsSVGIDRenderingObserver* aContainer) : mContainer(aContainer) {}
+    explicit ElementTracker(nsSVGIDRenderingObserver* aOwningObserver)
+      : mOwningObserver(aOwningObserver)
+    {}
   protected:
     virtual void ElementChanged(Element* aFrom, Element* aTo) override {
-      mContainer->StopListening();
+      mOwningObserver->StopListening(); // stop observing the old element
       IDTracker::ElementChanged(aFrom, aTo);
-      mContainer->StartListening();
-      mContainer->DoUpdate();
+      mOwningObserver->StartListening(); // start observing the new element
+      mOwningObserver->DoUpdate();
     }
     /**
      * Override IsPersistent because we want to keep tracking the element
@@ -142,10 +149,10 @@ protected:
      */
     virtual bool IsPersistent() override { return true; }
   private:
-    nsSVGIDRenderingObserver* mContainer;
+    nsSVGIDRenderingObserver* mOwningObserver;
   };
 
-  SourceReference mElement;
+  ElementTracker mObservedElementTracker;
 };
 
 struct nsSVGFrameReferenceFromProperty