dom/svg/SVGLengthList.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef DOM_SVG_SVGLENGTHLIST_H_
   8 #define DOM_SVG_SVGLENGTHLIST_H_
   9
  10 #include "nsCOMPtr.h"
  11 #include "nsDebug.h"
  12 #include "nsIContent.h"
  13 #include "nsINode.h"
  14 #include "nsIWeakReferenceUtils.h"
  15 #include "SVGElement.h"
  16 #include "nsTArray.h"
  17 #include "SVGLength.h"
  18 #include "mozilla/dom/SVGLengthBinding.h"
  19
  20 namespace mozilla {
  21
  22 namespace dom {
  23 class DOMSVGLength;
  24 class DOMSVGLengthList;
  25 }  // namespace dom
  26
  27 /**
  28  * ATTENTION! WARNING! WATCH OUT!!
  29  *
  30  * Consumers that modify objects of this type absolutely MUST keep the DOM
  31  * wrappers for those lists (if any) in sync!! That's why this class is so
  32  * locked down.
  33  *
  34  * The DOM wrapper class for this class is DOMSVGLengthList.
  35  */
  36 class SVGLengthList {
  37   friend class dom::DOMSVGLength;
  38   friend class dom::DOMSVGLengthList;
  39   friend class SVGAnimatedLengthList;
  40
  41  public:
  42   SVGLengthList() = default;
  43   ~SVGLengthList() = default;
  44
  45   SVGLengthList& operator=(const SVGLengthList& aOther) {
  46     mLengths.ClearAndRetainStorage();
  47     // Best-effort, really.
  48     Unused << mLengths.AppendElements(aOther.mLengths, fallible);
  49     return *this;
  50   }
  51
  52   SVGLengthList(const SVGLengthList& aOther) { *this = aOther; }
  53
  54   // Only methods that don't make/permit modification to this list are public.
  55   // Only our friend classes can access methods that may change us.
  56
  57   /// This may return an incomplete string on OOM, but that's acceptable.
  58   void GetValueAsString(nsAString& aValue) const;
  59
  60   bool IsEmpty() const { return mLengths.IsEmpty(); }
  61
  62   uint32_t Length() const { return mLengths.Length(); }
  63
  64   const SVGLength& operator[](uint32_t aIndex) const {
  65     return mLengths[aIndex];
  66   }
  67
  68   bool operator==(const SVGLengthList& rhs) const;
  69
  70   bool SetCapacity(uint32_t size) {
  71     return mLengths.SetCapacity(size, fallible);
  72   }
  73
  74   void Compact() { mLengths.Compact(); }
  75
  76   // Access to methods that can modify objects of this type is deliberately
  77   // limited. This is to reduce the chances of someone modifying objects of
  78   // this type without taking the necessary steps to keep DOM wrappers in sync.
  79   // If you need wider access to these methods, consider adding a method to
  80   // SVGAnimatedLengthList and having that class act as an intermediary so it
  81   // can take care of keeping DOM wrappers in sync.
  82
  83  protected:
  84   /**
  85    * This may fail on OOM if the internal capacity needs to be increased, in
  86    * which case the list will be left unmodified.
  87    */
  88   nsresult CopyFrom(const SVGLengthList& rhs);
  89
  90   SVGLength& operator[](uint32_t aIndex) { return mLengths[aIndex]; }
  91
  92   /**
  93    * This may fail (return false) on OOM if the internal capacity is being
  94    * increased, in which case the list will be left unmodified.
  95    */
  96   bool SetLength(uint32_t aNumberOfItems) {
  97     return mLengths.SetLength(aNumberOfItems, fallible);
  98   }
  99
 100  private:
 101   // Marking the following private only serves to show which methods are only
 102   // used by our friend classes (as opposed to our subclasses) - it doesn't
 103   // really provide additional safety.
 104
 105   nsresult SetValueFromString(const nsAString& aValue);
 106
 107   void Clear() { mLengths.Clear(); }
 108
 109   bool InsertItem(uint32_t aIndex, const SVGLength& aLength) {
 110     if (aIndex >= mLengths.Length()) aIndex = mLengths.Length();
 111     return !!mLengths.InsertElementAt(aIndex, aLength, fallible);
 112   }
 113
 114   void ReplaceItem(uint32_t aIndex, const SVGLength& aLength) {
 115     MOZ_ASSERT(aIndex < mLengths.Length(),
 116                "DOM wrapper caller should have raised INDEX_SIZE_ERR");
 117     mLengths[aIndex] = aLength;
 118   }
 119
 120   void RemoveItem(uint32_t aIndex) {
 121     MOZ_ASSERT(aIndex < mLengths.Length(),
 122                "DOM wrapper caller should have raised INDEX_SIZE_ERR");
 123     mLengths.RemoveElementAt(aIndex);
 124   }
 125
 126   bool AppendItem(SVGLength aLength) {
 127     return !!mLengths.AppendElement(aLength, fallible);
 128   }
 129
 130  protected:
 131   /* Rationale for using nsTArray<SVGLength> and not nsTArray<SVGLength, 1>:
 132    *
 133    * It might seem like we should use AutoTArray<SVGLength, 1> instead of
 134    * nsTArray<SVGLength>. That would preallocate space for one SVGLength and
 135    * avoid an extra memory allocation call in the common case of the 'x'
 136    * and 'y' attributes each containing a single length (and the 'dx' and 'dy'
 137    * attributes being empty). However, consider this:
 138    *
 139    * An empty nsTArray uses sizeof(Header*). An AutoTArray<class E,
 140    * uint32_t N> on the other hand uses sizeof(Header*) +
 141    * (2 * sizeof(uint32_t)) + (N * sizeof(E)), which for one SVGLength is
 142    * sizeof(Header*) + 16 bytes.
 143    *
 144    * Now consider that for text elements we have four length list attributes
 145    * (x, y, dx, dy), each of which can have a baseVal and an animVal list. If
 146    * we were to go the AutoTArray<SVGLength, 1> route for each of these, we'd
 147    * end up using at least 160 bytes for these four attributes alone, even
 148    * though we only need storage for two SVGLengths (16 bytes) in the common
 149    * case!!
 150    *
 151    * A compromise might be to template SVGLengthList to allow
 152    * SVGAnimatedLengthList to preallocate space for an SVGLength for the
 153    * baseVal lists only, and that would cut the space used by the four
 154    * attributes to 96 bytes. Taking that even further and templating
 155    * SVGAnimatedLengthList too in order to only use nsTArray for 'dx' and 'dy'
 156    * would reduce the storage further to 64 bytes. Having different types makes
 157    * things more complicated for code that needs to look at the lists though.
 158    * In fact it also makes things more complicated when it comes to storing the
 159    * lists.
 160    *
 161    * It may be worth considering using nsAttrValue for length lists instead of
 162    * storing them directly on the element.
 163    */
 164   FallibleTArray<SVGLength> mLengths;
 165 };
 166
 167 /**
 168  * This SVGLengthList subclass is for SVGLengthListSMILType which needs to know
 169  * which element and attribute a length list belongs to so that it can convert
 170  * between unit types if necessary.
 171  */
 172 class SVGLengthListAndInfo : public SVGLengthList {
 173  public:
 174   SVGLengthListAndInfo()
 175       : mElement(nullptr), mAxis(0), mCanZeroPadList(false) {}
 176
 177   SVGLengthListAndInfo(dom::SVGElement* aElement, uint8_t aAxis,
 178                        bool aCanZeroPadList)
 179       : mElement(do_GetWeakReference(static_cast<nsINode*>(aElement))),
 180         mAxis(aAxis),
 181         mCanZeroPadList(aCanZeroPadList) {}
 182
 183   void SetInfo(dom::SVGElement* aElement, uint8_t aAxis, bool aCanZeroPadList) {
 184     mElement = do_GetWeakReference(static_cast<nsINode*>(aElement));
 185     mAxis = aAxis;
 186     mCanZeroPadList = aCanZeroPadList;
 187   }
 188
 189   dom::SVGElement* Element() const {
 190     nsCOMPtr<nsIContent> e = do_QueryReferent(mElement);
 191     return static_cast<dom::SVGElement*>(e.get());
 192   }
 193
 194   /**
 195    * Returns true if this object is an "identity" value, from the perspective
 196    * of SMIL. In other words, returns true until the initial value set up in
 197    * SVGLengthListSMILType::Init() has been changed with a SetInfo() call.
 198    */
 199   bool IsIdentity() const {
 200     if (!mElement) {
 201       MOZ_ASSERT(IsEmpty(), "target element propagation failure");
 202       return true;
 203     }
 204     return false;
 205   }
 206
 207   uint8_t Axis() const {
 208     MOZ_ASSERT(mElement, "Axis() isn't valid");
 209     return mAxis;
 210   }
 211
 212   /**
 213    * The value returned by this function depends on which attribute this object
 214    * is for. If appending a list of zeros to the attribute's list would have no
 215    * effect on rendering (e.g. the attributes 'dx' and 'dy' on <text>), then
 216    * this method will return true. If appending a list of zeros to the
 217    * attribute's list could *change* rendering (e.g. the attributes 'x' and 'y'
 218    * on <text>), then this method will return false.
 219    *
 220    * The reason that this method exists is because the SMIL code needs to know
 221    * what to do when it's asked to animate between lists of different length.
 222    * If this method returns true, then it can zero pad the short list before
 223    * carrying out its operations. However, in the case of the 'x' and 'y'
 224    * attributes on <text>, zero would mean "zero in the current coordinate
 225    * system", whereas we would want to pad shorter lists with the coordinates
 226    * at which glyphs would otherwise lie, which is almost certainly not zero!
 227    * Animating from/to zeros in this case would produce terrible results.
 228    *
 229    * Currently SVGLengthListSMILType simply disallows (drops) animation between
 230    * lists of different length if it can't zero pad a list. This is to avoid
 231    * having some authors create content that depends on undesirable behaviour
 232    * (which would make it difficult for us to fix the behavior in future). At
 233    * some point it would be nice to implement a callback to allow this code to
 234    * determine padding values for lists that can't be zero padded. See
 235    * https://bugzilla.mozilla.org/show_bug.cgi?id=573431
 236    */
 237   bool CanZeroPadList() const {
 238     // NS_ASSERTION(mElement, "CanZeroPadList() isn't valid");
 239     return mCanZeroPadList;
 240   }
 241
 242   // For the SMIL code. See comment in SVGLengthListSMILType::Add().
 243   void SetCanZeroPadList(bool aCanZeroPadList) {
 244     mCanZeroPadList = aCanZeroPadList;
 245   }
 246
 247   nsresult CopyFrom(const SVGLengthListAndInfo& rhs) {
 248     mElement = rhs.mElement;
 249     mAxis = rhs.mAxis;
 250     mCanZeroPadList = rhs.mCanZeroPadList;
 251     return SVGLengthList::CopyFrom(rhs);
 252   }
 253
 254   // Instances of this special subclass do not have DOM wrappers that we need
 255   // to worry about keeping in sync, so it's safe to expose any hidden base
 256   // class methods required by the SMIL code, as we do below.
 257
 258   /**
 259    * Exposed so that SVGLengthList baseVals can be copied to
 260    * SVGLengthListAndInfo objects. Note that callers should also call
 261    * SetInfo() when using this method!
 262    */
 263   nsresult CopyFrom(const SVGLengthList& rhs) {
 264     return SVGLengthList::CopyFrom(rhs);
 265   }
 266   const SVGLength& operator[](uint32_t aIndex) const {
 267     return SVGLengthList::operator[](aIndex);
 268   }
 269   SVGLength& operator[](uint32_t aIndex) {
 270     return SVGLengthList::operator[](aIndex);
 271   }
 272   bool SetLength(uint32_t aNumberOfItems) {
 273     return SVGLengthList::SetLength(aNumberOfItems);
 274   }
 275
 276  private:
 277   // We must keep a weak reference to our element because we may belong to a
 278   // cached baseVal SMILValue. See the comments starting at:
 279   // https://bugzilla.mozilla.org/show_bug.cgi?id=515116#c15
 280   // See also https://bugzilla.mozilla.org/show_bug.cgi?id=653497
 281   nsWeakPtr mElement;
 282   uint8_t mAxis;
 283   bool mCanZeroPadList;
 284 };
 285
 286 /**
 287  * This class wraps SVGLengthList objects to allow frame consumers to process
 288  * SVGLengthList objects as if they were simply a list of float values in user
 289  * units. When consumers request the value at a given index, this class
 290  * dynamically converts the corresponding SVGLength from its actual unit and
 291  * returns its value in user units.
 292  *
 293  * Consumers should check that the user unit values returned are finite. Even
 294  * if the consumer can guarantee the list's element has a valid viewport
 295  * ancestor to resolve percentage units against, and a valid presContext and
 296  * ComputedStyle to resolve absolute and em/ex units against, unit conversions
 297  * could still overflow. In that case the value returned will be
 298  * numeric_limits<float>::quiet_NaN().
 299  */
 300 class MOZ_STACK_CLASS SVGUserUnitList {
 301  public:
 302   SVGUserUnitList() : mList(nullptr), mElement(nullptr), mAxis(0) {}
 303
 304   void Init(const SVGLengthList* aList, dom::SVGElement* aElement,
 305             uint8_t aAxis) {
 306     mList = aList;
 307     mElement = aElement;
 308     mAxis = aAxis;
 309   }
 310
 311   void Clear() { mList = nullptr; }
 312
 313   bool IsEmpty() const { return !mList || mList->IsEmpty(); }
 314
 315   uint32_t Length() const { return mList ? mList->Length() : 0; }
 316
 317   /// This may return a non-finite value
 318   float operator[](uint32_t aIndex) const {
 319     return (*mList)[aIndex].GetValueInUserUnits(mElement, mAxis);
 320   }
 321
 322   bool HasPercentageValueAt(uint32_t aIndex) const {
 323     const SVGLength& length = (*mList)[aIndex];
 324     return length.GetUnit() ==
 325            dom::SVGLength_Binding::SVG_LENGTHTYPE_PERCENTAGE;
 326   }
 327
 328  private:
 329   const SVGLengthList* mList;
 330   dom::SVGElement* mElement;
 331   uint8_t mAxis;
 332 };
 333
 334 }  // namespace mozilla
 335
 336 #endif  // DOM_SVG_SVGLENGTHLIST_H_