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diff --git a/dom/svg/DOMSVGAnimatedTransformList.h b/dom/svg/DOMSVGAnimatedTransformList.h
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+++ b/dom/svg/DOMSVGAnimatedTransformList.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef DOM_SVG_DOMSVGANIMATEDTRANSFORMLIST_H_
+#define DOM_SVG_DOMSVGANIMATEDTRANSFORMLIST_H_
+
+#include "nsCycleCollectionParticipant.h"
+#include "SVGElement.h"
+#include "nsWrapperCache.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/RefPtr.h"
+
+namespace mozilla {
+
+class SVGAnimatedTransformList;
+
+namespace dom {
+
+class DOMSVGTransformList;
+
+/**
+ * Class DOMSVGAnimatedTransformList
+ *
+ * This class is used to create the DOM tearoff objects that wrap internal
+ * SVGAnimatedTransformList objects.
+ *
+ * See the architecture comment in DOMSVGAnimatedLengthList.h (that's
+ * LENGTH list). The comment for that class largly applies to this one too
+ * and will go a long way to helping you understand the architecture here.
+ *
+ * This class is strongly intertwined with DOMSVGTransformList and
+ * DOMSVGTransform.
+ * Our DOMSVGTransformList base and anim vals are friends and take care of
+ * nulling out our pointers to them when they die (making our pointers to them
+ * true weak refs).
+ */
+class DOMSVGAnimatedTransformList final : public nsWrapperCache {
+  friend class DOMSVGTransformList;
+
+ public:
+  NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(
+      DOMSVGAnimatedTransformList)
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_NATIVE_CLASS(
+      DOMSVGAnimatedTransformList)
+
+  /**
+   * Factory method to create and return a DOMSVGAnimatedTransformList wrapper
+   * for a given internal SVGAnimatedTransformList object. The factory takes
+   * care of caching the object that it returns so that the same object can be
+   * returned for the given SVGAnimatedTransformList each time it is
+   * requested. The cached object is only removed from the cache when it is
+   * destroyed due to there being no more references to it or to any of its
+   * descendant objects. If that happens, any subsequent call requesting the DOM
+   * wrapper for the SVGAnimatedTransformList will naturally result in a new
+   * DOMSVGAnimatedTransformList being returned.
+   */
+  static already_AddRefed<DOMSVGAnimatedTransformList> GetDOMWrapper(
+      SVGAnimatedTransformList* aList, SVGElement* aElement);
+
+  /**
+   * This method returns the DOMSVGAnimatedTransformList wrapper for an internal
+   * SVGAnimatedTransformList object if it currently has a wrapper. If it does
+   * not, then nullptr is returned.
+   */
+  static DOMSVGAnimatedTransformList* GetDOMWrapperIfExists(
+      SVGAnimatedTransformList* aList);
+
+  /**
+   * Called by internal code to notify us when we need to sync the length of
+   * our baseVal DOM list with its internal list. This is called just prior to
+   * the length of the internal baseVal list being changed so that any DOM list
+   * items that need to be removed from the DOM list can first get their values
+   * from their internal counterpart.
+   *
+   * The only time this method could fail is on OOM when trying to increase the
+   * length of the DOM list. If that happens then this method simply clears the
+   * list and returns. Callers just proceed as normal, and we simply accept
+   * that the DOM list will be empty (until successfully set to a new value).
+   */
+  void InternalBaseValListWillChangeLengthTo(uint32_t aNewLength);
+  void InternalAnimValListWillChangeLengthTo(uint32_t aNewLength);
+
+  /**
+   * Returns true if our attribute is animating (in which case our animVal is
+   * not simply a mirror of our baseVal).
+   */
+  bool IsAnimating() const;
+
+  // WebIDL
+  SVGElement* GetParentObject() const { return mElement; }
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+  // These aren't weak refs because mBaseVal and mAnimVal are weak
+  already_AddRefed<DOMSVGTransformList> BaseVal();
+  already_AddRefed<DOMSVGTransformList> AnimVal();
+
+ private:
+  /**
+   * Only our static GetDOMWrapper() factory method may create objects of our
+   * type.
+   */
+  explicit DOMSVGAnimatedTransformList(SVGElement* aElement)
+      : mBaseVal(nullptr), mAnimVal(nullptr), mElement(aElement) {}
+
+  ~DOMSVGAnimatedTransformList();
+
+  /// Get a reference to this DOM wrapper object's internal counterpart.
+  SVGAnimatedTransformList& InternalAList();
+  const SVGAnimatedTransformList& InternalAList() const;
+
+  // Weak refs to our DOMSVGTransformList baseVal/animVal objects. These objects
+  // are friends and take care of clearing these pointers when they die, making
+  // these true weak references.
+  DOMSVGTransformList* mBaseVal;
+  DOMSVGTransformList* mAnimVal;
+
+  // Strong ref to our element to keep it alive. We hold this not only for
+  // ourself, but also for our base/animVal and all of their items.
+  RefPtr<SVGElement> mElement;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // DOM_SVG_DOMSVGANIMATEDTRANSFORMLIST_H_