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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* 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__
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