gecko-dev/dom/svg/SVGLengthList.h

/* -*- 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_SVGLENGTHLIST_H_
#define DOM_SVG_SVGLENGTHLIST_H_

#include "nsCOMPtr.h"
#include "nsDebug.h"
#include "nsIContent.h"
#include "nsINode.h"
#include "nsIWeakReferenceUtils.h"
#include "SVGElement.h"
#include "nsTArray.h"
#include "SVGLength.h"
#include "mozilla/dom/SVGLengthBinding.h"

namespace mozilla {

namespace dom {
class DOMSVGLength;
class DOMSVGLengthList;
}  // namespace dom

/**
 * ATTENTION! WARNING! WATCH OUT!!
 *
 * Consumers that modify objects of this type absolutely MUST keep the DOM
 * wrappers for those lists (if any) in sync!! That's why this class is so
 * locked down.
 *
 * The DOM wrapper class for this class is DOMSVGLengthList.
 */
class SVGLengthList {
  friend class dom::DOMSVGLength;
  friend class dom::DOMSVGLengthList;
  friend class SVGAnimatedLengthList;

 public:
  SVGLengthList() = default;
  ~SVGLengthList() = default;

  SVGLengthList& operator=(const SVGLengthList& aOther) {
    mLengths.ClearAndRetainStorage();
    // Best-effort, really.
    Unused << mLengths.AppendElements(aOther.mLengths, fallible);
    return *this;
  }

  SVGLengthList(const SVGLengthList& aOther) { *this = aOther; }

  // Only methods that don't make/permit modification to this list are public.
  // Only our friend classes can access methods that may change us.

  /// This may return an incomplete string on OOM, but that's acceptable.
  void GetValueAsString(nsAString& aValue) const;

  bool IsEmpty() const { return mLengths.IsEmpty(); }

  uint32_t Length() const { return mLengths.Length(); }

  const SVGLength& operator[](uint32_t aIndex) const {
    return mLengths[aIndex];
  }

  bool operator==(const SVGLengthList& rhs) const;

  bool SetCapacity(uint32_t size) {
    return mLengths.SetCapacity(size, fallible);
  }

  void Compact() { mLengths.Compact(); }

  // Access to methods that can modify objects of this type is deliberately
  // limited. This is to reduce the chances of someone modifying objects of
  // this type without taking the necessary steps to keep DOM wrappers in sync.
  // If you need wider access to these methods, consider adding a method to
  // SVGAnimatedLengthList and having that class act as an intermediary so it
  // can take care of keeping DOM wrappers in sync.

 protected:
  /**
   * This may fail on OOM if the internal capacity needs to be increased, in
   * which case the list will be left unmodified.
   */
  nsresult CopyFrom(const SVGLengthList& rhs);

  SVGLength& operator[](uint32_t aIndex) { return mLengths[aIndex]; }

  /**
   * This may fail (return false) on OOM if the internal capacity is being
   * increased, in which case the list will be left unmodified.
   */
  bool SetLength(uint32_t aNumberOfItems) {
    return mLengths.SetLength(aNumberOfItems, fallible);
  }

 private:
  // Marking the following private only serves to show which methods are only
  // used by our friend classes (as opposed to our subclasses) - it doesn't
  // really provide additional safety.

  nsresult SetValueFromString(const nsAString& aValue);

  void Clear() { mLengths.Clear(); }

  bool InsertItem(uint32_t aIndex, const SVGLength& aLength) {
    if (aIndex >= mLengths.Length()) aIndex = mLengths.Length();
    return !!mLengths.InsertElementAt(aIndex, aLength, fallible);
  }

  void ReplaceItem(uint32_t aIndex, const SVGLength& aLength) {
    MOZ_ASSERT(aIndex < mLengths.Length(),
               "DOM wrapper caller should have raised INDEX_SIZE_ERR");
    mLengths[aIndex] = aLength;
  }

  void RemoveItem(uint32_t aIndex) {
    MOZ_ASSERT(aIndex < mLengths.Length(),
               "DOM wrapper caller should have raised INDEX_SIZE_ERR");
    mLengths.RemoveElementAt(aIndex);
  }

  bool AppendItem(SVGLength aLength) {
    return !!mLengths.AppendElement(aLength, fallible);
  }

 protected:
  /* Rationale for using nsTArray<SVGLength> and not nsTArray<SVGLength, 1>:
   *
   * It might seem like we should use AutoTArray<SVGLength, 1> instead of
   * nsTArray<SVGLength>. That would preallocate space for one SVGLength and
   * avoid an extra memory allocation call in the common case of the 'x'
   * and 'y' attributes each containing a single length (and the 'dx' and 'dy'
   * attributes being empty). However, consider this:
   *
   * An empty nsTArray uses sizeof(Header*). An AutoTArray<class E,
   * uint32_t N> on the other hand uses sizeof(Header*) +
   * (2 * sizeof(uint32_t)) + (N * sizeof(E)), which for one SVGLength is
   * sizeof(Header*) + 16 bytes.
   *
   * Now consider that for text elements we have four length list attributes
   * (x, y, dx, dy), each of which can have a baseVal and an animVal list. If
   * we were to go the AutoTArray<SVGLength, 1> route for each of these, we'd
   * end up using at least 160 bytes for these four attributes alone, even
   * though we only need storage for two SVGLengths (16 bytes) in the common
   * case!!
   *
   * A compromise might be to template SVGLengthList to allow
   * SVGAnimatedLengthList to preallocate space for an SVGLength for the
   * baseVal lists only, and that would cut the space used by the four
   * attributes to 96 bytes. Taking that even further and templating
   * SVGAnimatedLengthList too in order to only use nsTArray for 'dx' and 'dy'
   * would reduce the storage further to 64 bytes. Having different types makes
   * things more complicated for code that needs to look at the lists though.
   * In fact it also makes things more complicated when it comes to storing the
   * lists.
   *
   * It may be worth considering using nsAttrValue for length lists instead of
   * storing them directly on the element.
   */
  FallibleTArray<SVGLength> mLengths;
};

/**
 * This SVGLengthList subclass is for SVGLengthListSMILType which needs to know
 * which element and attribute a length list belongs to so that it can convert
 * between unit types if necessary.
 */
class SVGLengthListAndInfo : public SVGLengthList {
 public:
  SVGLengthListAndInfo()
      : mElement(nullptr), mAxis(0), mCanZeroPadList(false) {}

  SVGLengthListAndInfo(dom::SVGElement* aElement, uint8_t aAxis,
                       bool aCanZeroPadList)
      : mElement(do_GetWeakReference(static_cast<nsINode*>(aElement))),
        mAxis(aAxis),
        mCanZeroPadList(aCanZeroPadList) {}

  void SetInfo(dom::SVGElement* aElement, uint8_t aAxis, bool aCanZeroPadList) {
    mElement = do_GetWeakReference(static_cast<nsINode*>(aElement));
    mAxis = aAxis;
    mCanZeroPadList = aCanZeroPadList;
  }

  dom::SVGElement* Element() const {
    nsCOMPtr<nsIContent> e = do_QueryReferent(mElement);
    return static_cast<dom::SVGElement*>(e.get());
  }

  /**
   * Returns true if this object is an "identity" value, from the perspective
   * of SMIL. In other words, returns true until the initial value set up in
   * SVGLengthListSMILType::Init() has been changed with a SetInfo() call.
   */
  bool IsIdentity() const {
    if (!mElement) {
      MOZ_ASSERT(IsEmpty(), "target element propagation failure");
      return true;
    }
    return false;
  }

  uint8_t Axis() const {
    MOZ_ASSERT(mElement, "Axis() isn't valid");
    return mAxis;
  }

  /**
   * The value returned by this function depends on which attribute this object
   * is for. If appending a list of zeros to the attribute's list would have no
   * effect on rendering (e.g. the attributes 'dx' and 'dy' on <text>), then
   * this method will return true. If appending a list of zeros to the
   * attribute's list could *change* rendering (e.g. the attributes 'x' and 'y'
   * on <text>), then this method will return false.
   *
   * The reason that this method exists is because the SMIL code needs to know
   * what to do when it's asked to animate between lists of different length.
   * If this method returns true, then it can zero pad the short list before
   * carrying out its operations. However, in the case of the 'x' and 'y'
   * attributes on <text>, zero would mean "zero in the current coordinate
   * system", whereas we would want to pad shorter lists with the coordinates
   * at which glyphs would otherwise lie, which is almost certainly not zero!
   * Animating from/to zeros in this case would produce terrible results.
   *
   * Currently SVGLengthListSMILType simply disallows (drops) animation between
   * lists of different length if it can't zero pad a list. This is to avoid
   * having some authors create content that depends on undesirable behaviour
   * (which would make it difficult for us to fix the behavior in future). At
   * some point it would be nice to implement a callback to allow this code to
   * determine padding values for lists that can't be zero padded. See
   * https://bugzilla.mozilla.org/show_bug.cgi?id=573431
   */
  bool CanZeroPadList() const {
    // NS_ASSERTION(mElement, "CanZeroPadList() isn't valid");
    return mCanZeroPadList;
  }

  // For the SMIL code. See comment in SVGLengthListSMILType::Add().
  void SetCanZeroPadList(bool aCanZeroPadList) {
    mCanZeroPadList = aCanZeroPadList;
  }

  nsresult CopyFrom(const SVGLengthListAndInfo& rhs) {
    mElement = rhs.mElement;
    mAxis = rhs.mAxis;
    mCanZeroPadList = rhs.mCanZeroPadList;
    return SVGLengthList::CopyFrom(rhs);
  }

  // Instances of this special subclass do not have DOM wrappers that we need
  // to worry about keeping in sync, so it's safe to expose any hidden base
  // class methods required by the SMIL code, as we do below.

  /**
   * Exposed so that SVGLengthList baseVals can be copied to
   * SVGLengthListAndInfo objects. Note that callers should also call
   * SetInfo() when using this method!
   */
  nsresult CopyFrom(const SVGLengthList& rhs) {
    return SVGLengthList::CopyFrom(rhs);
  }
  const SVGLength& operator[](uint32_t aIndex) const {
    return SVGLengthList::operator[](aIndex);
  }
  SVGLength& operator[](uint32_t aIndex) {
    return SVGLengthList::operator[](aIndex);
  }
  bool SetLength(uint32_t aNumberOfItems) {
    return SVGLengthList::SetLength(aNumberOfItems);
  }

 private:
  // We must keep a weak reference to our element because we may belong to a
  // cached baseVal SMILValue. See the comments starting at:
  // https://bugzilla.mozilla.org/show_bug.cgi?id=515116#c15
  // See also https://bugzilla.mozilla.org/show_bug.cgi?id=653497
  nsWeakPtr mElement;
  uint8_t mAxis;
  bool mCanZeroPadList;
};

/**
 * This class wraps SVGLengthList objects to allow frame consumers to process
 * SVGLengthList objects as if they were simply a list of float values in user
 * units. When consumers request the value at a given index, this class
 * dynamically converts the corresponding SVGLength from its actual unit and
 * returns its value in user units.
 *
 * Consumers should check that the user unit values returned are finite. Even
 * if the consumer can guarantee the list's element has a valid viewport
 * ancestor to resolve percentage units against, and a valid presContext and
 * ComputedStyle to resolve absolute and em/ex units against, unit conversions
 * could still overflow. In that case the value returned will be
 * numeric_limits<float>::quiet_NaN().
 */
class MOZ_STACK_CLASS SVGUserUnitList {
 public:
  SVGUserUnitList() : mList(nullptr), mElement(nullptr), mAxis(0) {}

  void Init(const SVGLengthList* aList, dom::SVGElement* aElement,
            uint8_t aAxis) {
    mList = aList;
    mElement = aElement;
    mAxis = aAxis;
  }

  void Clear() { mList = nullptr; }

  bool IsEmpty() const { return !mList || mList->IsEmpty(); }

  uint32_t Length() const { return mList ? mList->Length() : 0; }

  /// This may return a non-finite value
  float operator[](uint32_t aIndex) const {
    return (*mList)[aIndex].GetValueInUserUnits(mElement, mAxis);
  }

  bool HasPercentageValueAt(uint32_t aIndex) const {
    const SVGLength& length = (*mList)[aIndex];
    return length.GetUnit() ==
           dom::SVGLength_Binding::SVG_LENGTHTYPE_PERCENTAGE;
  }

 private:
  const SVGLengthList* mList;
  dom::SVGElement* mElement;
  uint8_t mAxis;
};

}  // namespace mozilla

#endif  // DOM_SVG_SVGLENGTHLIST_H_