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authorMatt A. Tobin <mattatobin@localhost.localdomain>2018-02-02 04:16:08 -0500
committerMatt A. Tobin <mattatobin@localhost.localdomain>2018-02-02 04:16:08 -0500
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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 MOZILLA_DOMSVGANIMATEDLENGTHLIST_H__
+#define MOZILLA_DOMSVGANIMATEDLENGTHLIST_H__
+
+#include "nsCOMPtr.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsSVGElement.h"
+#include "mozilla/Attributes.h"
+
+namespace mozilla {
+
+class SVGAnimatedLengthList;
+class SVGLengthList;
+class DOMSVGLengthList;
+
+/**
+ * Class DOMSVGAnimatedLengthList
+ *
+ * This class is used to create the DOM tearoff objects that wrap internal
+ * SVGAnimatedLengthList objects. We have this internal-DOM split because DOM
+ * classes are relatively heavy-weight objects with non-optimal interfaces for
+ * internal code, and they're relatively infrequently used. Having separate
+ * internal and DOM classes does add complexity - especially for lists where
+ * the internal list and DOM lists (and their items) need to be kept in sync -
+ * but it keeps the internal classes light and fast, and in 99% of cases
+ * they're all that's used. DOM wrappers are only instantiated when script
+ * demands it.
+ *
+ * Ownership model:
+ *
+ * The diagram below shows the ownership model between the various DOM objects
+ * in the tree of DOM objects that correspond to an SVG length list attribute.
+ * The angled brackets ">" and "<" denote a reference from one object to
+ * another, where the "!" character denotes a strong reference, and the "~"
+ * character denotes a weak reference.
+ *
+ * .----<!----. .----<!----. .----<!----.
+ * | | | | | |
+ * element ~> DOMSVGAnimatedLengthList ~> DOMSVGLengthList ~> DOMSVGLength
+ *
+ * Rationale:
+ *
+ * The following three paragraphs explain the main three requirements that must
+ * be met by any design. These are followed by an explanation of the rationale
+ * behind our particular design.
+ *
+ * 1: DOMSVGAnimatedLengthList, DOMSVGLengthLists and DOMSVGLength get to their
+ * internal counterparts via their element, and they use their element to send
+ * out appropriate notifications when they change. Because of this, having
+ * their element disappear out from under them would be very bad. To keep their
+ * element alive at least as long as themselves, each of these classes must
+ * contain a _strong_ reference (directly or indirectly) to their element.
+ *
+ * 2: Another central requirement of any design is the SVG specification's
+ * requirement that script must always be given the exact same objects each
+ * time it accesses a given object in a DOM object tree corresponding to an SVG
+ * length list attribute. In practice "always" actually means "whenever script
+ * has kept a references to a DOM object it previously accessed", since a
+ * script will only be able to detect any difference in object identity if it
+ * has a previous reference to compare against.
+ *
+ * 3: The wiggle room in the "same object" requirement leads us to a third
+ * (self imposed) requirement: if script no longer has a reference to a given
+ * DOM object from an object tree corresponding to an SVG length list
+ * attribute, and if that object doesn't currently have any descendants, then
+ * that object should be released to free up memory.
+ *
+ * To help in understanding our current design, consider this BROKEN design:
+ *
+ * .-------------------------------<!-------------------------.
+ * |--------------------<!----------------. |
+ * |----<!----. | |
+ * | | | |
+ * element ~> DOMSVGAnimatedLengthList !> DOMSVGLengthList !> DOMSVGLength
+ *
+ * Having all the objects keep a reference directly to their element like this
+ * would reduce the number of dereferences that they need to make to get their
+ * internal counterpart. Hovewer, this design does not meet the "same object"
+ * requirement of the SVG specification. If script keeps a reference to a
+ * DOMSVGLength or DOMSVGLengthList object, but not to that object's
+ * DOMSVGAnimatedLengthList, then the DOMSVGAnimatedLengthList may be garbage
+ * collected. We'd then have no way to return the same DOMSVGLength /
+ * DOMSVGLengthList object that the script has a reference to if the script
+ * went looking for it via the DOMSVGAnimatedLengthList property on the
+ * element - we'd end up creating a fresh DOMSVGAnimatedLengthList, with no
+ * knowlegde of the existing DOMSVGLengthList or DOMSVGLength object.
+ *
+ * The way we solve this problem is by making sure that parent objects cannot
+ * die until all their children are dead by having child objects hold a strong
+ * reference to their parent object. Note that this design means that the child
+ * objects hold a strong reference to their element too, albeit indirectly via
+ * the strong reference to their parent object:
+ *
+ * .----<!----. .----<!----. .----<!----.
+ * | | | | | |
+ * element ~> DOMSVGAnimatedLengthList ~> DOMSVGLengthList ~> DOMSVGLength
+ *
+ * One drawback of this design is that objects must look up their parent
+ * chain to find their element, but that overhead is relatively small.
+ */
+class DOMSVGAnimatedLengthList final : public nsWrapperCache
+{
+ friend class DOMSVGLengthList;
+
+public:
+ NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(DOMSVGAnimatedLengthList)
+ NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_NATIVE_CLASS(DOMSVGAnimatedLengthList)
+
+ /**
+ * Factory method to create and return a DOMSVGAnimatedLengthList wrapper
+ * for a given internal SVGAnimatedLengthList object. The factory takes care
+ * of caching the object that it returns so that the same object can be
+ * returned for the given SVGAnimatedLengthList 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 SVGAnimatedLengthList will naturally result in a new
+ * DOMSVGAnimatedLengthList being returned.
+ */
+ static already_AddRefed<DOMSVGAnimatedLengthList>
+ GetDOMWrapper(SVGAnimatedLengthList *aList,
+ nsSVGElement *aElement,
+ uint8_t aAttrEnum,
+ uint8_t aAxis);
+
+ /**
+ * This method returns the DOMSVGAnimatedLengthList wrapper for an internal
+ * SVGAnimatedLengthList object if it currently has a wrapper. If it does
+ * not, then nullptr is returned.
+ */
+ static DOMSVGAnimatedLengthList*
+ GetDOMWrapperIfExists(SVGAnimatedLengthList *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 InternalBaseValListWillChangeTo(const SVGLengthList& aNewValue);
+ void InternalAnimValListWillChangeTo(const SVGLengthList& aNewValue);
+
+ /**
+ * Returns true if our attribute is animating (in which case our animVal is
+ * not simply a mirror of our baseVal).
+ */
+ bool IsAnimating() const;
+
+ // WebIDL
+ nsSVGElement* GetParentObject() const { return mElement; }
+ virtual JSObject* WrapObject(JSContext* aCx, JS::Handle<JSObject*> aGivenProto) override;
+ // These aren't weak refs because mBaseVal and mAnimVal are weak
+ already_AddRefed<DOMSVGLengthList> BaseVal();
+ already_AddRefed<DOMSVGLengthList> AnimVal();
+
+private:
+
+ /**
+ * Only our static GetDOMWrapper() factory method may create objects of our
+ * type.
+ */
+ DOMSVGAnimatedLengthList(nsSVGElement *aElement, uint8_t aAttrEnum, uint8_t aAxis)
+ : mBaseVal(nullptr)
+ , mAnimVal(nullptr)
+ , mElement(aElement)
+ , mAttrEnum(aAttrEnum)
+ , mAxis(aAxis)
+ {
+ }
+
+ ~DOMSVGAnimatedLengthList();
+
+ /// Get a reference to this DOM wrapper object's internal counterpart.
+ SVGAnimatedLengthList& InternalAList();
+ const SVGAnimatedLengthList& InternalAList() const;
+
+ // Weak refs to our DOMSVGLengthList baseVal/animVal objects. These objects
+ // are friends and take care of clearing these pointers when they die, making
+ // these true weak references.
+ DOMSVGLengthList *mBaseVal;
+ DOMSVGLengthList *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<nsSVGElement> mElement;
+
+ uint8_t mAttrEnum;
+ uint8_t mAxis;
+};
+
+} // namespace mozilla
+
+#endif // MOZILLA_DOMSVGANIMATEDLENGTHLIST_H__