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| author | Matt A. Tobin <mattatobin@localhost.localdomain> | 2018-02-02 04:16:08 -0500 |
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| committer | Matt A. Tobin <mattatobin@localhost.localdomain> | 2018-02-02 04:16:08 -0500 |
| commit | 5f8de423f190bbb79a62f804151bc24824fa32d8 (patch) | |
| tree | 10027f336435511475e392454359edea8e25895d /layout/doc/HLD-SpaceManager.html | |
| parent | 49ee0794b5d912db1f95dce6eb52d781dc210db5 (diff) | |
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diff --git a/layout/doc/HLD-SpaceManager.html b/layout/doc/HLD-SpaceManager.html new file mode 100644 index 000000000..b67d8d5ef --- /dev/null +++ b/layout/doc/HLD-SpaceManager.html @@ -0,0 +1,245 @@ +<!-- 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/. --> + +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> +<html> +<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. When elements are floated to the left or right in a layout, they +take up space and influence where other elements can be placed. 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. + 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. 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). <br> + <br> + Additionally, there is a need to manage impacts to lines caused by +changes to floated elements. 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 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. 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. 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. 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. This is used to communicate information about the space in +the band to the clients of the SpaceManager. 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. + <br> + + <ul> + <li>An instance is created </li> + <li>The BlockReflowState's previous Space Manager is saved off.</li> + <li>The new Space Manager instance is associated with the BlockReflowState. + </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. 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. This happens in the method <i>nsBlockReflowState::RecoverFloats</i> and +<i>nsBlockReflowState::FlowAndPlaceFloat</i> (formerly this was done in nsBlockFrame::UpdateSpaceManager). <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. 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. <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> + + </body> + </html> |