layout/doc/DD-SpaceManager.html

   1 <!-- This Source Code Form is subject to the terms of the Mozilla Public
   2    - License, v. 2.0. If a copy of the MPL was not distributed with this
   3    - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->
   4
   5 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
   6 <html>
   7 <head>
   8
   9   <meta http-equiv="content-type" content="text/html; charset=ISO-8859-1">
  10   <title>Detailed Design Template</title>
  11 </head>
  12   <body>
  13
  14 <h1><font color="#cc0000">Gecko Layout Detailed Design Document</font></h1>
  15
  16 <h1>Space Manager Detailed Design</h1>
  17
  18 <h2>Overview</h2>
  19 <p>
  20   The Space Manager and related classes and structures are an important of
  21  the Gecko Layout system, specifically Block Layout. &nbsp;See the High Level
  22  Design document for an overview of the Space Manager, and as an introduction
  23  to the classes, structures and algorithms container in this, the Detailed
  24  Design Document.
  25 </p>
  26
  27
  28
  29 <hr width="100%" size="2">
  30 <h2>nsSpaceManager</h2>
  31 <p>
  32    The Space Manager is the central class is a group of classes that manage
  33  the occupied and available space that exists in horizontal bands across
  34 a  canvas. &nbsp;The primary goal of the Space Manager is to provide information
  35  about those bands of space to support the CSS notion of floated elements.
  36 </p>
  37
  38 <p>
  39   There are three important parts to the Space Manager API: the parts that
  40 deal with the coordinate space of the Space Manager, the parts that deal with
  41 the regions managed by the Space Manager, and the parts that manage float
  42 impact intervals.
  43 </p>
  44
  45 <p>
  46   The class nsSpaceManager is declared in the file <a href="http://lxr.mozilla.org/seamonkey/source/layout/base/src/nsSpaceManager.h">
  47   nsSpaceManger.h</a>
  48   . &nbsp;The class is only used in the layout module and cannot be exported
  49  outside of that module (nor does it need to be). &nbsp;It is not a general
  50  purpose class, and is not intended to be subclasses<font color="#cc0000">
  51  .</font>
  52 </p>
  53
  54 <p>
  55   Here is the class declaration, taken from the source file as of 01.08.02
  56 </p>
  57
  58
  59
  60 <pre>/**
  61  * Class for dealing with bands of available space. The space manager
  62  * defines a coordinate space with an origin at (0, 0) that grows down
  63  * and to the right.
  64  */
  65 class nsSpaceManager {
  66 public:
  67   nsSpaceManager(nsIPresShell* aPresShell, nsIFrame* aFrame);
  68   ~nsSpaceManager();
  69
  70   void* operator new(size_t aSize);
  71   void operator delete(void* aPtr, size_t aSize);
  72
  73   static void Shutdown();
  74
  75   /*
  76    * Get the frame that's associated with the space manager. This frame
  77    * created the space manager, and the world coordinate space is
  78    * relative to this frame.
  79    *
  80    * You can use QueryInterface() on this frame to get any additional
  81    * interfaces.
  82    */
  83   nsIFrame* GetFrame() const { return mFrame; }
  84
  85   /**
  86    * Translate the current origin by the specified (dx, dy). This
  87    * creates a new local coordinate space relative to the current
  88    * coordinate space.
  89    */
  90   void Translate(nscoord aDx, nscoord aDy) { mX += aDx; mY += aDy; }
  91
  92   /**
  93    * Returns the current translation from local coordinate space to
  94    * world coordinate space. This represents the accumulated calls to
  95    * Translate().
  96    */
  97   void GetTranslation(nscoord&amp; aX, nscoord&amp; aY) const { aX = mX; aY = mY; }
  98
  99   /**
 100    * Returns the y-most of the bottommost band or 0 if there are no bands.
 101    *
 102    * @return  PR_TRUE if there are bands and PR_FALSE if there are no bands
 103    */
 104   PRBool YMost(nscoord&amp; aYMost) const;
 105
 106   /**
 107    * Returns a band starting at the specified y-offset. The band data
 108    * indicates which parts of the band are available, and which parts
 109    * are unavailable
 110    *
 111    * The band data that is returned is in the coordinate space of the
 112    * local coordinate system.
 113    *
 114    * The local coordinate space origin, the y-offset, and the max size
 115    * describe a rectangle that's used to clip the underlying band of
 116    * available space, i.e.
 117    * {0, aYOffset, aMaxSize.width, aMaxSize.height} in the local
 118    * coordinate space
 119    *
 120    * @param   aYOffset the y-offset of where the band begins. The coordinate is
 121    *            relative to the upper-left corner of the local coordinate space
 122    * @param   aMaxSize the size to use to constrain the band data
 123    * @param   aBandData [in,out] used to return the list of trapezoids that
 124    *            describe the available space and the unavailable space
 125    * @return  NS_OK if successful and NS_ERROR_FAILURE if the band data is not
 126    *            not large enough. The 'count' member of the band data struct
 127    *            indicates how large the array of trapezoids needs to be
 128    */
 129   nsresult GetBandData(nscoord       aYOffset,
 130                        const nsSize&amp; aMaxSize,
 131                        nsBandData&amp;   aBandData) const;
 132
 133   /**
 134    * Add a rectangular region of unavailable space. The space is
 135    * relative to the local coordinate system.
 136    *
 137    * The region is tagged with a frame
 138    *
 139    * @param   aFrame the frame used to identify the region. Must not be NULL
 140    * @param   aUnavailableSpace the bounding rect of the unavailable space
 141    * @return  NS_OK if successful
 142    *          NS_ERROR_FAILURE if there is already a region tagged with aFrame
 143    */
 144   nsresult AddRectRegion(nsIFrame*     aFrame,
 145                          const nsRect&amp; aUnavailableSpace);
 146
 147   /**
 148    * Resize the rectangular region associated with aFrame by the specified
 149    * deltas. The height change always applies to the bottom edge or the existing
 150    * rect. You specify whether the width change applies to the left or right edge
 151    *
 152    * Returns NS_OK if successful, NS_ERROR_INVALID_ARG if there is no region
 153    * tagged with aFrame
 154    */
 155   enum AffectedEdge {LeftEdge, RightEdge};
 156   nsresult ResizeRectRegion(nsIFrame*    aFrame,
 157                             nscoord      aDeltaWidth,
 158                             nscoord      aDeltaHeight,
 159                             AffectedEdge aEdge = RightEdge);
 160
 161   /**
 162    * Offset the region associated with aFrame by the specified amount.
 163    *
 164    * Returns NS_OK if successful, NS_ERROR_INVALID_ARG if there is no region
 165    * tagged with aFrame
 166    */
 167   nsresult OffsetRegion(nsIFrame* aFrame, nscoord dx, nscoord dy);
 168
 169   /**
 170    * Remove the region associated with aFrane.
 171    *
 172    * Returns NS_OK if successful and NS_ERROR_INVALID_ARG if there is no region
 173    * tagged with aFrame
 174    */
 175   nsresult RemoveRegion(nsIFrame* aFrame);
 176
 177   /**
 178    * Clears the list of regions representing the unavailable space.
 179    */
 180   void ClearRegions();
 181
 182   /**
 183    * Methods for dealing with the propagation of float damage during
 184    * reflow.
 185    */
 186   PRBool HasFloatDamage()
 187   {
 188     return !mFloatDamage.IsEmpty();
 189   }
 190
 191   void IncludeInDamage(nscoord aIntervalBegin, nscoord aIntervalEnd)
 192   {
 193     mFloatDamage.IncludeInterval(aIntervalBegin + mY, aIntervalEnd + mY);
 194   }
 195
 196   PRBool IntersectsDamage(nscoord aIntervalBegin, nscoord aIntervalEnd)
 197   {
 198     return mFloatDamage.Intersects(aIntervalBegin + mY, aIntervalEnd + mY);
 199   }
 200
 201 #ifdef DEBUG
 202   /**
 203    * Dump the state of the spacemanager out to a file
 204    */
 205   nsresult List(FILE* out);
 206
 207   void SizeOf(nsISizeOfHandler* aHandler, uint32_t* aResult) const;
 208 #endif
 209
 210 private:
 211   // Structure that maintains information about the region associated
 212   // with a particular frame
 213   struct FrameInfo {
 214     nsIFrame* const mFrame;
 215     nsRect          mRect;       // rectangular region
 216     FrameInfo*      mNext;
 217
 218     FrameInfo(nsIFrame* aFrame, const nsRect&amp; aRect);
 219 #ifdef NS_BUILD_REFCNT_LOGGING
 220     ~FrameInfo();
 221 #endif
 222   };
 223
 224   // Doubly linked list of band rects
 225   struct BandRect : PRCListStr {
 226     nscoord   mLeft, mTop;
 227     nscoord   mRight, mBottom;
 228     int32_t   mNumFrames;    // number of frames occupying this rect
 229     union {
 230       nsIFrame*    mFrame;   // single frame occupying the space
 231       nsVoidArray* mFrames;  // list of frames occupying the space
 232     };
 233
 234     BandRect(nscoord aLeft, nscoord aTop,
 235              nscoord aRight, nscoord aBottom,
 236              nsIFrame*);
 237     BandRect(nscoord aLeft, nscoord aTop,
 238              nscoord aRight, nscoord aBottom,
 239              nsVoidArray*);
 240     ~BandRect();
 241
 242     // List operations
 243     BandRect* Next() const {return (BandRect*)PR_NEXT_LINK(this);}
 244     BandRect* Prev() const {return (BandRect*)PR_PREV_LINK(this);}
 245     void      InsertBefore(BandRect* aBandRect) {PR_INSERT_BEFORE(aBandRect, this);}
 246     void      InsertAfter(BandRect* aBandRect) {PR_INSERT_AFTER(aBandRect, this);}
 247     void      Remove() {PR_REMOVE_LINK(this);}
 248
 249     // Split the band rect into two vertically, with this band rect becoming
 250     // the top part, and a new band rect being allocated and returned for the
 251     // bottom part
 252     //
 253     // Does not insert the new band rect into the linked list
 254     BandRect* SplitVertically(nscoord aBottom);
 255
 256     // Split the band rect into two horizontally, with this band rect becoming
 257     // the left part, and a new band rect being allocated and returned for the
 258     // right part
 259     //
 260     // Does not insert the new band rect into the linked list
 261     BandRect* SplitHorizontally(nscoord aRight);
 262
 263     // Accessor functions
 264     PRBool  IsOccupiedBy(const nsIFrame*) const;
 265     void    AddFrame(const nsIFrame*);
 266     void    RemoveFrame(const nsIFrame*);
 267     PRBool  HasSameFrameList(const BandRect* aBandRect) const;
 268     int32_t Length() const;
 269   };
 270
 271   // Circular linked list of band rects
 272   struct BandList : BandRect {
 273     BandList();
 274
 275     // Accessors
 276     PRBool    IsEmpty() const {return PR_CLIST_IS_EMPTY((PRCListStr*)this);}
 277     BandRect* Head() const {return (BandRect*)PR_LIST_HEAD(this);}
 278     BandRect* Tail() const {return (BandRect*)PR_LIST_TAIL(this);}
 279
 280     // Operations
 281     void      Append(BandRect* aBandRect) {PR_APPEND_LINK(aBandRect, this);}
 282
 283     // Remove and delete all the band rects in the list
 284     void      Clear();
 285   };
 286
 287
 288   FrameInfo* GetFrameInfoFor(nsIFrame* aFrame);
 289   FrameInfo* CreateFrameInfo(nsIFrame* aFrame, const nsRect&amp; aRect);
 290   void       DestroyFrameInfo(FrameInfo*);
 291
 292   void       ClearFrameInfo();
 293   void       ClearBandRects();
 294
 295   BandRect*  GetNextBand(const BandRect* aBandRect) const;
 296   void       DivideBand(BandRect* aBand, nscoord aBottom);
 297   PRBool     CanJoinBands(BandRect* aBand, BandRect* aPrevBand);
 298   PRBool     JoinBands(BandRect* aBand, BandRect* aPrevBand);
 299   void       AddRectToBand(BandRect* aBand, BandRect* aBandRect);
 300   void       InsertBandRect(BandRect* aBandRect);
 301
 302   nsresult   GetBandAvailableSpace(const BandRect* aBand,
 303                                    nscoord         aY,
 304                                    const nsSize&amp;   aMaxSize,
 305                                    nsBandData&amp;     aAvailableSpace) const;
 306
 307   nsIFrame* const mFrame;     // frame associated with the space manager
 308   nscoord         mX, mY;     // translation from local to global coordinate space
 309   BandList        mBandList;  // header/sentinel for circular linked list of band rects
 310   FrameInfo*      mFrameInfoMap;
 311   nsIntervalSet   mFloatDamage;
 312
 313   static int32_t sCachedSpaceManagerCount;
 314   static void* sCachedSpaceManagers[NS_SPACE_MANAGER_CACHE_SIZE];
 315
 316   nsSpaceManager(const nsSpaceManager&amp;);  // no implementation
 317   void operator=(const nsSpaceManager&amp;);  // no implementation
 318 };
 319
 320 </pre>
 321
 322 <h3>Public API</h3>
 323
 324 <h4>Life Cycle:</h4>
 325 <p>
 326   The Constructor requires a Presentation Shell, used for arena allocations
 327  mostly, and a frame that this Space Manager is rooted on. &nbsp;The coordinate
 328  space of this Space Manager is relative to the frame passed in to the constructor.
 329 </p>
 330
 331 <pre>  nsSpaceManager(nsIPresShell* aPresShell, nsIFrame* aFrame);
 332   ~nsSpaceManager();
 333 </pre>
 334 <p>
 335   Operators 'new' and 'delete' are overridden to support a recycler. &nbsp;Space
 336  Manager instances come and go pretty frequently, and this recycler prevents
 337  excessive heap allocations and the performance penalties associated with
 338 it. The #define NS_SPACE_MANAGER_CACHE_SIZE is used to control the number
 339 of Space Manager instances that can be present in the recycler, currently
 340 4. &nbsp;If more than NS_SPACE_MANAGER_CACHE_SIZE are allocated at a time,
 341 then standard allocation is used.
 342 </p>
 343
 344 <pre>
 345   void* operator new(size_t aSize);
 346   void operator delete(void* aPtr, size_t aSize);
 347
 348 </pre>
 349 <p>
 350  A Static method is used to shutdown the Space Manager recycling. &nbsp;This
 351 method deletes all of the Space Mangers inthe recycler,and prevents further
 352 recycling. &nbsp;It is meant to be called only when the layout module is being
 353 terminated.
 354 </p>
 355
 356 <pre>  static void Shutdown();
 357
 358 </pre>
 359
 360 <h4>Origin / Coordinate Space Translation</h4>
 361
 362 <pre>  /**
 363    * Translate the current origin by the specified (dx, dy). This
 364    * creates a new local coordinate space relative to the current
 365    * coordinate space.
 366    */
 367   void Translate(nscoord aDx, nscoord aDy) { mX += aDx; mY += aDy; }
 368
 369   /**
 370    * Returns the current translation from local coordinate space to
 371    * world coordinate space. This represents the accumulated calls to
 372    * Translate().
 373    */
 374   void GetTranslation(nscoord&amp; aX, nscoord&amp; aY) const { aX = mX; aY = mY; }
 375
 376   /**
 377    * Returns the y-most of the bottommost band or 0 if there are no bands.
 378    *
 379    * @return  PR_TRUE if there are bands and PR_FALSE if there are no bands
 380    */
 381   PRBool YMost(nscoord&amp; aYMost) const;
 382 </pre>
 383
 384 <h4>Region Management</h4>
 385
 386 <pre>  /**
 387    * Returns a band starting at the specified y-offset. The band data
 388    * indicates which parts of the band are available, and which parts
 389    * are unavailable
 390    *
 391    * The band data that is returned is in the coordinate space of the
 392    * local coordinate system.
 393    *
 394    * The local coordinate space origin, the y-offset, and the max size
 395    * describe a rectangle that's used to clip the underlying band of
 396    * available space, i.e.
 397    * {0, aYOffset, aMaxSize.width, aMaxSize.height} in the local
 398    * coordinate space
 399    *
 400    * @param   aYOffset the y-offset of where the band begins. The coordinate is
 401    *            relative to the upper-left corner of the local coordinate space
 402    * @param   aMaxSize the size to use to constrain the band data
 403    * @param   aBandData [in,out] used to return the list of trapezoids that
 404    *            describe the available space and the unavailable space
 405    * @return  NS_OK if successful and NS_ERROR_FAILURE if the band data is not
 406    *            not large enough. The 'count' member of the band data struct
 407    *            indicates how large the array of trapezoids needs to be
 408    */
 409   nsresult GetBandData(nscoord       aYOffset,
 410                        const nsSize&amp; aMaxSize,
 411                        nsBandData&amp;   aBandData) const;
 412
 413   /**
 414    * Add a rectangular region of unavailable space. The space is
 415    * relative to the local coordinate system.
 416    *
 417    * The region is tagged with a frame
 418    *
 419    * @param   aFrame the frame used to identify the region. Must not be NULL
 420    * @param   aUnavailableSpace the bounding rect of the unavailable space
 421    * @return  NS_OK if successful
 422    *          NS_ERROR_FAILURE if there is already a region tagged with aFrame
 423    */
 424   nsresult AddRectRegion(nsIFrame*     aFrame,
 425                          const nsRect&amp; aUnavailableSpace);
 426
 427   /**
 428    * Resize the rectangular region associated with aFrame by the specified
 429    * deltas. The height change always applies to the bottom edge or the existing
 430    * rect. You specify whether the width change applies to the left or right edge
 431    *
 432    * Returns NS_OK if successful, NS_ERROR_INVALID_ARG if there is no region
 433    * tagged with aFrame
 434    */
 435   enum AffectedEdge {LeftEdge, RightEdge};
 436   nsresult ResizeRectRegion(nsIFrame*    aFrame,
 437                             nscoord      aDeltaWidth,
 438                             nscoord      aDeltaHeight,
 439                             AffectedEdge aEdge = RightEdge);
 440
 441   /**
 442    * Offset the region associated with aFrame by the specified amount.
 443    *
 444    * Returns NS_OK if successful, NS_ERROR_INVALID_ARG if there is no region
 445    * tagged with aFrame
 446    */
 447   nsresult OffsetRegion(nsIFrame* aFrame, nscoord dx, nscoord dy);
 448
 449   /**
 450    * Remove the region associated with aFrane.
 451    *
 452    * Returns NS_OK if successful and NS_ERROR_INVALID_ARG if there is no region
 453    * tagged with aFrame
 454    */
 455   nsresult RemoveRegion(nsIFrame* aFrame);
 456
 457   /**
 458    * Clears the list of regions representing the unavailable space.
 459    */
 460   void ClearRegions();
 461 </pre>
 462
 463 <h4>Float Impact</h4>
 464
 465 <pre>  /**
 466    * Methods for dealing with the propagation of float damage during
 467    * reflow.
 468    */
 469   PRBool HasFloatDamage()
 470   {
 471     return !mFloatDamage.IsEmpty();
 472   }
 473
 474   void IncludeInDamage(nscoord aIntervalBegin, nscoord aIntervalEnd)
 475   {
 476     mFloatDamage.IncludeInterval(aIntervalBegin + mY, aIntervalEnd + mY);
 477   }
 478
 479   PRBool IntersectsDamage(nscoord aIntervalBegin, nscoord aIntervalEnd)
 480   {
 481     return mFloatDamage.Intersects(aIntervalBegin + mY, aIntervalEnd + mY);
 482   }
 483 </pre>
 484
 485 <h4>Debug Only Methods</h4>
 486
 487 <pre>  /**
 488    * Dump the state of the spacemanager out to a file
 489    */
 490   nsresult List(FILE* out);
 491
 492   void SizeOf(nsISizeOfHandler* aHandler, uint32_t* aResult) const;
 493
 494 </pre>
 495
 496 <h4>Unused / Obsolete Methods</h4>
 497
 498 <pre>  /*
 499    * Get the frame that's associated with the space manager. This frame
 500    * created the space manager, and the world coordinate space is
 501    * relative to this frame.
 502    *
 503    * You can use QueryInterface() on this frame to get any additional
 504    * interfaces.
 505    */
 506    nsIFrame* GetFrame() const { return mFrame; }
 507
 508 </pre>
 509
 510 <h3>Implementation Notes</h3>
 511
 512 <h4></h4>
 513
 514 <h4>Algorithm 1: GetBandData</h4>
 515 <p>
 516 GetBandData is used to provide information to clients about what space if
 517 available and unavailable in a band of space. &nbsp;The client provides a
 518 vertical offset, the yOffset, that corresponds to the band that is of interest.
 519 &nbsp;This will be the y offset of the frame that is being reflowed. &nbsp;The
 520 caller also provides a collection of BandData objects (an array) and the
 521 number of items that the collection can handle. &nbsp;If the collection is
 522 too small, then an error is returned and the count is updated to indicate
 523 the size required.
 524 </p>
 525
 526 <p>
 527 The algorithm to provide the band data is as follows:
 528 </p>
 529 <ul>
 530   <li>Get a &nbsp;vertical offset in world coordinates (instead of frame-relative
 531 coordinates) by adding the y-origin of the SpaceManager to the y offset passed
 532 in</li>
 533   <li>If the (adjusted) y value passed in is greater than the greatest band
 534 being managed, then all space is available so a single trapezoid is returned,
 535 marked as available and sized to the maximum size value (passed in).</li>
 536   <li>If the (adjusted) y offset intersects a band, then gather the band
 537 data:</li>
 538   <ul>
 539     <li>walk the internal bandData list from head to tail</li>
 540     <li>for each band data entry, see if the top of the band is greater than
 541 the (adjusted) y offset requested</li>
 542     <li>if it is, then band is below the offset requested, so the area between
 543 the band and the y offset is available - create a trapezoid with that region
 544 and return it.</li>
 545     <li>if the (adjusted) y offset is between the band top and bottom, then
 546 get the available space for the band by calling GetBandAvailableSpace</li>
 547     <li>otherwise, move to the next band</li>
 548   </ul>
 549 </ul>
 550 <h5>GetBandAvailableSpace:</h5>
 551 This method is called from GetBandData only. It walks all of the bands in
 552 the space manager and determines which bands intersect with the band passed
 553 in, and if within those bands there are regions that are available or occupied.
 554
 555 <ul>
 556   <li>First, walk all of the bands until a band that is to the right of the
 557 desired offset is located</li>
 558   <li>Starting at that band, &nbsp;walk the remaining bands:</li>
 559   <ul>
 560     <li>if the current band is to the right of the requested band, then there
 561 is available space.&nbsp;</li>
 562     <ul>
 563       <li>if there is more room in the bandData collection, then add a trapezoid
 564 to the bandData collection such that it is marked as available and has a
 565 rect that represents the space between the reference band tna dht band being
 566 examined</li>
 567       <li>if there is no more room in the BandData collection, estimate the
 568 number of entries requires as the current count + twice the number of bands
 569 below the reference band, plus two. &nbsp;Return an error code so the caller
 570 can reallocate the collection and try again.</li>
 571     </ul>
 572     <li>check the size of the collection again, if there is no room left
 573 then estimate the number of items requires as the current count + twice the
 574 number of bands below the band in question plus one.&nbsp;</li>
 575     <li>create a new trapezoid in the band collection that has a region corresponding
 576 to the reference band rect, marked as occupied by either a single or multiple
 577 frames.</li>
 578     <li>move to the next band</li>
 579   </ul>
 580   <li>after walking all of the band data, se if we have reached the right
 581 edge of the band.&nbsp;</li>
 582   <ul>
 583     <li>If not, then check for space in the band collection</li>
 584     <ul>
 585       <li>if there is no room left, then set the count to the current count
 586 plus 1 and return an error.</li>
 587       <li>otherwise, create another entry in the band collection, marked
 588 as available, and with a rect corresponding to the area remainin in the band
 589 (eg. from the right edge of the last band rect to the right edge of the band).</li>
 590     </ul>
 591   </ul>
 592 </ul>
 593
 594 <h4>Algorithm 2: AddRectRegion</h4>
 595 Clients call into this method to notify the Space Manager that a new frame
 596 is occupying some space.
 597
 598 <ul>
 599   <li>First, try to get frame info for the frame. If it is found, return
 600 an error since the frame is already associated with a region in the Space
 601 Manager.</li>
 602   <li>Next, create a rect from the occupied space passed in by the caller,
 603 transforming it first to world-coordinates by adding the Space Manager's
 604 offset to the occupied space rect passed in.</li>
 605   <li>Create a new Frame Info instance for the frame and rect, returning
 606 an error if allocation fails.</li>
 607   <li>Check if the occupied space rect (adjusted) is empty, if so, return
 608 an error &nbsp;(<font color="#cc0000">NOTE: this could be done earlier, or
 609 prevented by the caller</font>)</li>
 610   <li>Allocate a new BandRect instance with the rect and frame as constructor
 611 arguments, and insert it into the collection via InsertBandRect</li>
 612 </ul>
 613 <h5>InsertBandRect:</h5>
 614 Internal method to insert a band rect into the BandList in the correct location.
 615 There are several cases it has to handle, as specified in the source file
 616 comments:
 617
 618 <pre>// When comparing a rect to a band there are seven cases to consider.
 619 // 'R' is the rect and 'B' is the band.
 620 //
 621 //      Case 1              Case 2              Case 3              Case 4
 622 //      ------              ------              ------              ------
 623 // +-----+             +-----+                      +-----+             +-----+
 624 // |  R  |             |  R  |  +-----+    +-----+  |     |             |     |
 625 // +-----+             +-----+  |     |    |  R  |  |  B  |             |  B  |
 626 //          +-----+             |  B  |    +-----+  |     |    +-----+  |     |
 627 //          |     |             |     |             +-----+    |  R  |  +-----+
 628 //          |  B  |             +-----+                        +-----+
 629 //          |     |
 630 //          +-----+
 631 //
 632 //
 633 //
 634 //      Case 5              Case 6              Case 7
 635 //      ------              ------              ------
 636 //          +-----+    +-----+  +-----+    +-----+
 637 //          |     |    |  R  |  |  B  |    |     |  +-----+
 638 //          |  B  |    +-----+  +-----+    |  R  |  |  B  |
 639 //          |     |                        |     |  +-----+
 640 //          +-----+                        +-----+
 641 // +-----+
 642 // |  R  |
 643 // +-----+
 644 //
 645 </pre>
 646 <ul>
 647   <li>First, check for the easiest case, where there are no existing band
 648 rects, or the band rect passed in is below the bottommost rect. In this case,
 649 just append the band rect and return.</li>
 650   <li>Starting at the head of the list of bandRects, check for intersection
 651 with the rect passed in:</li>
 652   <ul>
 653     <li>case #1: the rect is totally above the current band rect, so insert
 654 a new band rect before the current bandRect</li>
 655     <li>cases #2 and #7: the rect is partially above the band rect, so it
 656 is divided into two bandRects, one entirely above the band, and one containing
 657 the remainder of the rect. &nbsp;Insert the part that is totally above the
 658 bandRect before the current bandRect, as in case #1 above, and adjust the
 659 other band rect to exclude the part already added.</li>
 660     <li>case #5: the rect is totally below the current bandRect, so just
 661 skip to the next band</li>
 662     <li>case #3 and #4: rect is at least partially intersection with the
 663 bandRect, so divide the current band into two parts, where the top part is
 664 above the current rect. &nbsp;Move to the new band just created, which is
 665 the next band.</li>
 666     <li>case #6: the rect shares the bottom and height with the bandRect,
 667 so just add the rect to the band.</li>
 668     <li>case #4 and #7: create a new rect for the part that overlaps the
 669 bandRect, and add it to the current bandRect (similar to case #6) and then
 670 move on to the next band, removing that part from the rect passed in. &nbsp;If
 671 no more bands, append the rect passed in to the end of the bandRect list.</li>
 672   </ul>
 673 </ul>
 674 <i>This algorithm is pretty confusing - basically what needs to happen is
 675 that rects and bands need to be divided up so that complicated cases like
 676 #2, #4, and #7, are reduced to simpler cases where the rects is totally above,
 677 below, or between a band rect. &nbsp;From the current implementation, it
 678 might be worth verifying that the final result of the inserts is a correctly
 679 ordered liest of bandRects (debug mode only).</i>
 680
 681
 682 <h4>Algorithm 3: RemoveRegion</h4>
 683  When a float is removed, the Space Manager is notified by a call to RemoveRegion,
 684 passing in the frame that is being removed.
 685
 686 <ul>
 687   <li>Get the FrameInfo for the frame passed in. If not found, an error is
 688 returned.</li>
 689   <li>If the rect for the frame is not empty, then visit each band in the
 690 bandList:</li>
 691   <ul>
 692     <li>for each rect in the band:
 693
 694     </li>
 695   </ul>
 696   <ul>
 697     <ul>
 698       <li>if the bandRect is occupied by the frame, either remove the frame
 699 from the bandRect (if there are other frames sharing it) and remember that
 700 it was shared</li>
 701       <li>otherwise simply remove the bandRect (no other frames share it).</li>
 702       <li>if the bandRect was shared, then try to coalesce adjacent bandRects</li>
 703       <ul>
 704         <li>if the previous bandRect is directly next to the current bandRect,
 705 and they have the same frame list, then make the current bandRect cover the
 706 previous bandRect's full region (adjust the left edge to be that of the previous
 707 bandRect) and remove the previous bandRect.</li>
 708       </ul>
 709     </ul>
 710   </ul>
 711   <ul>
 712     <li>if the current band or prior band had a rect occupied byu the frame,
 713 then try to join the two bands via JoinBands</li>
 714   </ul>
 715   <li>Finally, destroy the frameInfo for the frame.
 716
 717   </li>
 718 </ul>
 719
 720 <br>
 721
 722 <hr width="100%" size="2">
 723 <h2>Cross-Component Algorithms</h2>
 724
 725 <br>
 726
 727
 728
 729 <hr width="100%" size="2">
 730 <h2>Tech Notes</h2>
 731 <ul>
 732   <li>
 733
 734   </li>
 735
 736 </ul>
 737
 738
 739
 740
 741
 742 </body>
 743 </html>