layout/doc/HLD-SpaceManager.html@925c144e1f1f
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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10 <title>Space Manager High Level Design</title>
11 <meta name="author" content="Marc Attinasi (attinasi@netscape.com)">
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13 <body>
15 <h1><font color="#cc0000">Gecko Layout High Level Design Document</font></h1>
17 <h1>Space Manager High Level Design</h1>
18 <br>
20 <h2>Overview</h2>
21 The Space Manager and associated classes and strructures are used by Block
22 and Line layout to manage rectangular regions that are occupied and available,
23 for correct handling of floated elements and the elements that flow around
24 them. When elements are floated to the left or right in a layout, they
25 take up space and influence where other elements can be placed. The
26 Space Manager is responsible for keeping track of where space is taken up
27 and where it is available. This information is used by block layout to correctly
28 compute where other floated elements should be placed, and how much space
29 is available to normal in-flow elements that flow around the floated bits.<br>
30 <br>
31 The Space Manager works in concert with several other classes to do its
32 job. The classes that are considered part of the Space Manager are:<br>
34 <ul>
35 <li>nsSpaceManager</li>
36 <li>nsBandData</li>
37 <li>nsBlockBandData</li>
38 <li>BandRect / BandList (private structs)</li>
39 <li>FrameInfo (private struct)</li>
40 <li>nsBandtrapezoid</li>
42 </ul>
43 Outside of the Space Manager itself, the clients of the Space Manager also
44 play an inportant part in the management of he available and used space.
45 The primary classes that interact with the Space Manager are:<br>
47 <ul>
48 <li>nsBlockReflowState</li>
49 <li>nsBlockFrame</li>
50 <li>nsBoxToBlockAdaptor</li>
52 </ul>
53 The general interaction model is to create a Space Manager for a block
54 frame in the context of a Reflow, and to associate it with the BlockReflowState
55 so it is passed down to child frames' reflow methods. After reflow, the
56 Space Manager is destroyed. During reflow, the space manager stores
57 the space taken up by floats (UpdateSpaceManager in nsBlockFrame) and
58 provides information about the space available for other elements (GetAvailableSpace
59 in nsBlockReflowState). <br>
60 <br>
61 Additionally, there is a need to manage impacts to lines caused by
62 changes to floated elements. This is referred to as Propagation
63 of Float Damage and is handled by the Block Frame, making use of the
64 Space Manager. When a float is incrementally reflowed, the Space
65 Manager is notified if the float's region has changed. If so, the
66 vertical space that has been affected (including both the float's old
67 region and the float's new region) is noted in the internal
68 nsIntervalSet as potential float damage (the method is
69 IncludeInDamage). During the incremental reflow of dirty lines the
70 block frame may encounter lines that are NOT dirty. In this case the
71 Space Manager is also asked if there is any float damage, and
72 if there is then the block further checks to see if that damage
73 intersects the area of the non-dirty line, marking it dirty if there
74 is intersection. Thus, changes to floats on other lines may
75 cause impact to otherwise clean lines, and the Space Manager
76 facilitates the detection of this. <h2>Data Model</h2>
78 <h4>Class/Component Diagram</h4>
80 <blockquote>
81 <div align="Left"><img src="SpaceManagerClasses.png" alt="SpaceManager Class Diagram" idth="500" eight="459" title="Example Class Diagram">
82 <br>
83 </div>
84 </blockquote>
86 <ul>
87 <li>nsSpaceManager: The central point of management of the space taken
88 up by floats in a block</li>
89 <li>nsBandData: Provides information about the frames occupying a band
90 of occupied or available space</li>
91 <li>nsBlockBandData: A specialization of nsBandData that is used by
92 nsBlockReflowState to determine the available space, float impacts, and
93 where floats are cleared. Essentially a CSS-specific wrapper for
94 generic nsBandData.</li>
95 <li>BandRect: Keeps the bounds of a band, along with the frames associated
96 with the band. BandRects are a linked list (provided by PRCListStr
97 super-class) and also provide some geometry-management methods (SplitVertically,
98 SplitHorizontally) and some methods that query or manipulate the frames associated
99 with the band (IsOccupiedBy, AddFrame, RemoveFrame).</li>
100 <li>BandList: A subclass of BandRect that provides a list interface
101 - Head(), Tail(), IsEmpty(), etc.</li>
102 <li>FrameInfo: A structure that keeps information about the rectangle
103 associated with a specific frame, in a linked list.</li>
104 <li>nsBandTrapezoid: Represents the discrete regions within a band that
105 are either Available, Occupied by a single frame, or Occupied by several
106 frames. This is used to communicate information about the space in
107 the band to the clients of the SpaceManager. There is no internal use
108 of the nsBandTrapezoid by the Space Manager, rather it uses its internal
109 BandList to create a BandData collection, which is largely made up of nsTrapezoid
110 data.<br>
111 </li>
113 </ul>
115 <h2>Use Case</h2>
117 <h4>Use Case 1: Space Manager is Created / Destroyed</h4>
118 Space Manager instances are created in the nsBlockFrame's Reflow method.
119 <br>
121 <ul>
122 <li>An instance is created </li>
123 <li>The BlockReflowState's previous Space Manager is saved off.</li>
124 <li>The new Space Manager instance is associated with the BlockReflowState.
125 </li>
126 <li>After the block frame's Reflow has completed, the old Space Manager
127 instance is re-associated with the BlockReflowState</li>
128 <li>The new Space Manager is destroyed.</li>
130 </ul>
131 If the BlockReflowState already had a Space Manager instance associated
132 with it, it is stored off before being replaced, and the returned to the
133 BlockReflowState instance after the new one has been destroyed. Thus,
134 Space Managers are effectively 'nested' during reflow, with each new block
135 introducing its own Space Manager.
137 <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
138 floated block may be added to it. This happens in the method <i>nsBlockReflowState::RecoverFloats</i> and
139 <i>nsBlockReflowState::FlowAndPlaceFloat</i> (formerly this was done in nsBlockFrame::UpdateSpaceManager). <br>
140 <br>
141 The general algorightm in <i>nsBlockReflowState::RecoverFloats</i> is:<br>
143 <ul>
144 <li>For each line in the block, see if it has floated blocks</li>
145 <li>If floats are in the line, iterate over the floats and add each
146 one to the Space Manager via the AddRectRegion method. The actual rect
147 for the frame is cached in an nsFloatCache so it does not have to be recomputed.</li>
148 <li>If the block has any block children, then translate the Space Manager
149 to the child block's origin and update the space manager in the context
150 for the child block, recursively. When done with the child, restore the Space
151 Managers coordinates by translating by the negative of the child block's
152 origin. <br>
153 </li>
155 </ul><br>
157 The general algorightm in <i>nsBlockReflowState::FlowAndPlaceFloat</i> is:<br>
158 <ul>
159 <li>The region that the float currently occupies is recorded.</li>
160 <li>The band of available space is searched (with nsBlockReflowState::GetAvailableSpace);</li>
161 <li>The float frame that is get from the passed nsFloatCache argument is reflowed
162 and its rect is retrieved with GetRect;</li>
163 <li>The floats margins are added;</li>
164 <li>Check if the float can be placed in the acutal band: if not advance to the next band;</li>
165 <li>Check the float type and if it can be added to the space manager;</li>
166 <li>Align the float to its containing block top if rule
167 <a href="http://www.w3.org/TR/REC-CSS2/visuren.html#float-position">CSS2/9.5.1/4</a>
168 is not respected;</li>
169 <li>Add the float using <i>nsSpaceManager::AddRectRegion</i> </li>
170 <li>Compare the area that the float used to occupy with the area that it now occupies: if different,
171 record the vertically affected interval using <i>nsSpaceManager::IncludeInDamage</i></li>
172 </ul>
173 <h4>Use Case 3: Space Manager is used to find available space to reflow
174 into</h4>
175 The nsBlockFrame makes use of the Space Manager indirectly to get the available
176 space to reflow a child block or inline frame into. The block frame uses
177 a helper method on the nsBlockReflowState class to do the actual computation
178 of available space based on the data in the Space Manager. Here is how it
179 works for reflowing an inline frame within a block (this also occurs for
180 reflowing a block frame and, partially, for preparing for a resize reflow).<br>
182 <ul>
183 <li>nsBlockFrame first frees all float information for the line that
184 is being reflowed.</li>
185 <li>GetAvailableSpace is called on the BlockReflowState</li>
186 <li>the BlockReflowState calls GetAvailableSpace on its BlockBandData
187 instance (which was setup in the BlockReflowState's constructor based on
188 the SpaceManager passed in and computed content area).</li>
189 <li>BlockBandData then gets the band data from the space manager via
190 a call to the Space Manager associated with the BlockBandData instance.</li>
191 <li>The BlockBandData then walks the collection of trapezoids that were
192 returned by the SpaceManager method GetBandData (as nsBandData wrappers)
193 and determines the right-most edge of the available space.</li>
194 <li>The BlockReflowState then stores this available space rect for use
195 in the rest of the reflow chain.<br>
196 </li>
198 </ul>
200 <h4>Use Case 4: Propagation of Float Damage: detecting and handling float
201 damage</h4>
202 This process is driven by the Block Frame.<br>
204 <ul>
205 <li>A non-dirty line is encountered by the Block Frame in ReflowDirtyLines</li>
206 <li>Block Frame calls its PropagateFloatDamage method</li>
207 <li>The Space Manager is checked to see if there is any float damage</li>
208 <li>If there is, then the block frame asks the Space Manager if the
209 line in question intersects the float damage</li>
210 <li>If the line does intersect a damage interval, then the line is marked
211 dirty</li>
212 <li>If the line does not intersect a damage interval, it may still be
213 marked dirty if:</li>
215 <ul>
216 <li>it was impacted by floats before, but is not any longer</li>
217 <li>it was not impacted by floats befre, but is now</li>
218 <li><a name="block-line-impact"></a>
219 it is impacted by floats and is a block<br>
220 </li>
222 </ul>
224 </ul>
225 <br>
227 <hr width="100%" size="2"><br>
229 <h1><font color="#ff0000">Problems / bugs found during documentation:</font></h1>
231 <ul>
232 <li>BandRect and BandList are public in nsSpaceManager.h - should be
233 private (compiles fine)</li>
234 <li>nsSpaceManager data members are declared protected, but there are
235 no subclasses. Should be private (compiles fine)</li>
236 <li>nsBlockFrame::Paint is mucking with nsBlockBandData in and #if 0
237 block - remove that and the include (compiles fine)</li>
238 <li>nsSpaceManger has no way of clearing the float damage interval
239 set - this might be needed if the SpaceManager persists beyond a Reflow</li>
240 </ul>
241 <br>
242 <br>
244 </body>
245 </html>
