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Diffstat (limited to 'dom/svg/DOMSVGAnimatedLengthList.h')
| -rw-r--r-- | dom/svg/DOMSVGAnimatedLengthList.h | 204 |
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diff --git a/dom/svg/DOMSVGAnimatedLengthList.h b/dom/svg/DOMSVGAnimatedLengthList.h new file mode 100644 index 000000000..cb502058d --- /dev/null +++ b/dom/svg/DOMSVGAnimatedLengthList.h @@ -0,0 +1,204 @@ +/* -*- 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__ |