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layout/doc/HLD-SpaceManager.html@925c144e1f1f (annotated)

layout/doc/HLD-SpaceManager.html

Fri, 16 Jan 2015 18:13:44 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Fri, 16 Jan 2015 18:13:44 +0100
branch: TOR_BUG_9701
changeset 14: 925c144e1f1f
permissions: -rw-r--r--

Integrate suggestion from review to improve consistency with existing code.

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 <head>
   <meta http-equiv="content-type" content="text/html; charset=ISO-8859-1">
   <title>Space Manager High Level Design</title>
   <meta name="author" content="Marc Attinasi (attinasi@netscape.com)">
 </head>
   <body>
 <h1><font color="#cc0000">Gecko Layout High Level Design Document</font></h1>
 <h1>Space Manager High Level Design</h1>
  <br>
 <h2>Overview</h2>
   The Space Manager and associated classes and strructures are used by Block
 and Line layout to manage rectangular regions that are occupied and available,
 for correct handling of floated elements and the elements that flow around
 them. &nbsp;When elements are floated to the left or right in a layout, they
 take up space and influence where other elements can be placed. &nbsp;The
 Space Manager is responsible for keeping track of where space is taken up
 and where it is available. This information is used by block layout to correctly
 compute where other floated elements should be placed, and how much space
 is available to normal in-flow elements that flow around the floated bits.<br>
  <br>
   The Space Manager works in concert with several other classes to do its
 job. The classes that are considered part of the Space Manager are:<br>
 <ul>
    <li>nsSpaceManager</li>
    <li>nsBandData</li>
    <li>nsBlockBandData</li>
    <li>BandRect / BandList (private structs)</li>
    <li>FrameInfo (private struct)</li>
    <li>nsBandtrapezoid</li>
 </ul>
   Outside of the Space Manager itself, the clients of the Space Manager also
  play an inportant part in the management of he available and used space.
 &nbsp;The primary classes that interact with the Space Manager are:<br>
 <ul>
    <li>nsBlockReflowState</li>
    <li>nsBlockFrame</li>
    <li>nsBoxToBlockAdaptor</li>
 </ul>
   The general interaction model is to create a Space Manager for a block
 frame  in the context of a Reflow, and to associate it with the BlockReflowState
  so it is passed down to child frames' reflow methods. After reflow, the
 Space  Manager is destroyed. &nbsp;During reflow, the space manager stores
 the space  taken up by floats (UpdateSpaceManager in nsBlockFrame) and
 provides information  about the space available for other elements (GetAvailableSpace
 in nsBlockReflowState).  &nbsp;<br>
  <br>
  Additionally, there is a need to manage impacts to lines caused by
 changes to floated elements. &nbsp;This is referred to as Propagation
 of Float Damage and is handled by the Block Frame, making use of the
 Space Manager. When a float is incrementally reflowed, the Space
 Manager is notified if the float's region has changed. If so, the
 vertical space that has been affected (including both the float's old
 region and the float's new region) is noted in the internal
 nsIntervalSet as potential float damage (the method is
 IncludeInDamage).  During the incremental reflow of dirty lines the
 block frame may encounter lines that are NOT dirty. In this case the
 Space Manager is also asked if &nbsp;there is any float damage, and
 if there is then the block further checks to see if that damage
 intersects the area of the non-dirty line, marking it dirty if there
 is intersection. &nbsp;Thus, changes to floats on other lines may
 cause impact to otherwise clean lines, and the Space Manager
 facilitates the detection of this.  <h2>Data Model</h2>
 <h4>Class/Component Diagram</h4>
 <blockquote>
   <div align="Left"><img src="SpaceManagerClasses.png" alt="SpaceManager Class Diagram" idth="500" eight="459" title="Example Class Diagram">
    <br>
    </div>
    </blockquote>
   <ul>
      <li>nsSpaceManager: The central point of management of the space taken
  up by floats in a block</li>
      <li>nsBandData: Provides information about the frames occupying a band
  of occupied or available space</li>
      <li>nsBlockBandData: A specialization of nsBandData that is used by
 nsBlockReflowState   to determine the available space, float impacts, and
 where floats are  cleared. &nbsp;Essentially a CSS-specific wrapper for
 generic nsBandData.</li>
      <li>BandRect: Keeps the bounds of a band, along with the frames associated
  with the band. &nbsp;BandRects are a linked list (provided by PRCListStr
 super-class) and also provide some geometry-management methods (SplitVertically,
 SplitHorizontally) and some methods that query or manipulate the frames associated
 with the band (IsOccupiedBy, AddFrame, RemoveFrame).</li>
      <li>BandList: A subclass of BandRect that provides a list interface
 -  Head(), Tail(), IsEmpty(), etc.</li>
      <li>FrameInfo: A structure that keeps information about the rectangle
  associated with a specific frame, in a linked list.</li>
      <li>nsBandTrapezoid: Represents the discrete regions within a band that
  are either Available, Occupied by a single frame, or Occupied by several
 frames. &nbsp;This is used to communicate information about the space in
 the band to the clients of the SpaceManager. &nbsp;There is no internal use
 of the nsBandTrapezoid by the Space Manager, rather it uses its internal
 BandList to create a BandData collection, which is largely made up of nsTrapezoid
 data.<br>
      </li>
   </ul>
   <h2>Use Case</h2>
   <h4>Use Case 1: Space Manager is Created / Destroyed</h4>
   Space Manager instances are created in the nsBlockFrame's Reflow method.
  &nbsp;<br>
   <ul>
      <li>An instance is created&nbsp;</li>
      <li>The BlockReflowState's previous Space Manager is saved off.</li>
      <li>The new Space Manager instance is associated with the BlockReflowState.
 &nbsp;</li>
      <li>After the block frame's Reflow has completed, the old Space Manager
 instance is re-associated with the BlockReflowState</li>
      <li>The new Space Manager is destroyed.</li>
   </ul>
  If the BlockReflowState already had a Space Manager instance associated
 with  it, it is stored off before being replaced, and the returned to the
 BlockReflowState  instance after the new one has been destroyed. &nbsp;Thus,
 Space Managers  are effectively 'nested' during reflow, with each new block
 introducing its  own Space Manager.
   <h4>Use Case 2: Float is added to the Space Manager</h4>  After a Space Manager is created for a block context's reflow chain, a
 floated  block may be added to it. &nbsp;This happens in the method <i>nsBlockReflowState::RecoverFloats</i> and
 <i>nsBlockReflowState::FlowAndPlaceFloat</i> (formerly this was done in nsBlockFrame::UpdateSpaceManager). &nbsp;<br>
 <br>
 The general algorightm in <i>nsBlockReflowState::RecoverFloats</i> is:<br>
   <ul>
      <li>For each line in the block, see if it has floated blocks</li>
      <li>If floats are in the line, iterate over the floats and add each
  one to the Space Manager via the AddRectRegion method. &nbsp;The actual rect
  for the frame is cached in an nsFloatCache so it does not have to be recomputed.</li>
      <li>If the block has any block children, then translate the Space Manager
  to the child block's origin and update the space manager in the context
 for the child block, recursively. When done with the child, restore the Space
  Managers coordinates by translating by the negative of the child block's
 origin.&nbsp;       <br>
      </li>
   </ul><br>
 The general algorightm in <i>nsBlockReflowState::FlowAndPlaceFloat</i> is:<br>
 <ul>
   <li>The region that the float currently occupies is recorded.</li>
   <li>The band of available space is searched (with nsBlockReflowState::GetAvailableSpace);</li>
   <li>The float frame that is get from the passed nsFloatCache argument is reflowed
   and its rect is retrieved with GetRect;</li>
   <li>The floats margins are added;</li>
   <li>Check if the float can be placed in the acutal band: if not advance to the next band;</li>
   <li>Check the float type and if it can be added to the space manager;</li>
   <li>Align the float to its containing block top if rule
       <a href="http://www.w3.org/TR/REC-CSS2/visuren.html#float-position">CSS2/9.5.1/4</a>
       is not respected;</li>
   <li>Add the float using <i>nsSpaceManager::AddRectRegion</i> </li>
   <li>Compare the area that the float used to occupy with the area that it now occupies: if different,
       record the vertically affected interval using <i>nsSpaceManager::IncludeInDamage</i></li>
 </ul>
   <h4>Use Case 3: Space Manager is used to find available space to reflow
  into</h4>
   The nsBlockFrame makes use of the Space Manager indirectly to get the available
  space to reflow a child block or inline frame into. The block frame uses
 a helper method on the nsBlockReflowState class to do the actual computation
  of available space based on the data in the Space Manager. Here is how it
 works for reflowing an inline frame within a block (this also occurs for
 reflowing a block frame and, partially, for preparing for a resize reflow).<br>
   <ul>
      <li>nsBlockFrame first frees all float information for the line that
  is being reflowed.</li>
      <li>GetAvailableSpace is called on the BlockReflowState</li>
      <li>the BlockReflowState calls GetAvailableSpace on its BlockBandData
  instance (which was setup in the BlockReflowState's constructor based on
 the SpaceManager passed in and computed content area).</li>
      <li>BlockBandData then gets the band data from the space manager via
 a call to the Space Manager associated with the BlockBandData instance.</li>
      <li>The BlockBandData then walks the collection of trapezoids that were
  returned by the SpaceManager method GetBandData (as nsBandData wrappers)
 and determines the right-most edge of the available space.</li>
      <li>The BlockReflowState then stores this available space rect for use
  in the rest of the reflow chain.<br>
      </li>
   </ul>
   <h4>Use Case 4: Propagation of Float Damage: detecting and handling float
 damage</h4>
   This process is driven by the Block Frame.<br>
   <ul>
      <li>A non-dirty line is encountered by the Block Frame in ReflowDirtyLines</li>
      <li>Block Frame calls its PropagateFloatDamage method</li>
      <li>The Space Manager is checked to see if there is any float damage</li>
      <li>If there is, then the block frame asks the Space Manager if the
 line in question intersects the float damage</li>
      <li>If the line does intersect a damage interval, then the line is marked
 dirty</li>
      <li>If the line does not intersect a damage interval, it may still be
 marked dirty if:</li>
     <ul>
        <li>it was impacted by floats before, but is not any longer</li>
        <li>it was not impacted by floats befre, but is now</li>
        <li><a name="block-line-impact"></a>
  it is impacted by floats and is a block<br>
        </li>
     </ul>
   </ul>
    <br>
   <hr width="100%" size="2"><br>
   <h1><font color="#ff0000">Problems / bugs found during documentation:</font></h1>
   <ul>
      <li>BandRect and BandList are public in nsSpaceManager.h - should be
 private (compiles fine)</li>
      <li>nsSpaceManager data members are declared protected, but there are
  no subclasses. Should be private (compiles fine)</li>
      <li>nsBlockFrame::Paint is mucking with nsBlockBandData in and #if 0
 block - remove that and the include (compiles fine)</li>
      <li>nsSpaceManger has no way of clearing the float damage interval
 set - this might be needed if the SpaceManager persists beyond a Reflow</li>
   </ul>
    <br>
    <br>
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