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comparison: layout/doc/obsolete/line-layout.html
layout/doc/obsolete/line-layout.html
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- TOR_BUG_3246
- changeset 6
- 8bccb770b82d
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| 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 4.0 transitional//en"> | |
| 6 <html> | |
| 7 <head> | |
| 8 <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> | |
| 9 <meta name="GENERATOR" content="Mozilla/4.61 [en] (X11; I; Linux 2.2.5-22 i686) [Netscape]"> | |
| 10 </head> | |
| 11 <body> | |
| 12 | |
| 13 <h1> | |
| 14 <u>Line Layout</u></h1> | |
| 15 Line layout is the process of placing inline frames horizontally (left | |
| 16 to right or right to left depending on the CSS direction property value). | |
| 17 An attempt is made to describe how it works. | |
| 18 <p>nsLineLayout is the class that provides support for line layout. The | |
| 19 container frames nsBlockFrame and nsInlineFrame use nsLineLayout to perform | |
| 20 line layout and span layout. Span layout is a subset of line layout used | |
| 21 for inline container classes - for example, the HTML "B" element). Because | |
| 22 of spans, nsLineLayout handles the nested nature of line layout. | |
| 23 <p>Line layout as a process contains the following steps: | |
| 24 <ol> | |
| 25 <li> | |
| 26 Initialize the nsLineLayout object (done in nsBlockFrame). This prepares | |
| 27 the line layout engine for reflow by initializing its internal data structures.</li> | |
| 28 | |
| 29 <br> | |
| 30 <li> | |
| 31 Reflowing of inline frames. The block code uses nsLineLayout's <b>ReflowFrame</b> | |
| 32 method to reflow each inline frame in a line. This continues until the | |
| 33 line runs out of room or the block runs out of frames. The block may be | |
| 34 reflowing a span (an instance of nsInlineFrame) which will recursively | |
| 35 use nsLineLayout for reflow and placement of the frames in the span.</li> | |
| 36 | |
| 37 <p><br>Note that the container frames (nsBlockFrame/nsInlineFrame) call | |
| 38 nsLineLayout's ReflowFrame method instead of having the line layout code | |
| 39 process a list of children. This is done so that the container frames can | |
| 40 handle the issues of "pushing" and "pulling" of frames across continuations. | |
| 41 Because block and inline maintain different data structures for their child | |
| 42 lists, and because we don't want to mandate a common base class, the line | |
| 43 layout code doesn't control the "outer loop" of frame reflow. | |
| 44 <br> | |
| 45 <li> | |
| 46 Finish line layout by vertically aligning the frames, horizontally aligning | |
| 47 the frames and relatively positioning the frames on the line.</li> | |
| 48 </ol> | |
| 49 nsLineLayout is also used by nsBlockFrame to construct text-run information; | |
| 50 this process is independent of normal line layout is pretty much a hack. | |
| 51 <p>When frames are reflowed they return a reflow status. During line layout, | |
| 52 there are several additions to the basic reflow status used by most frames: | |
| 53 <ul> | |
| 54 <li> | |
| 55 NS_FRAME_COMPLETE - this is a normal reflow status and indicates that the | |
| 56 frame is complete and doesn't need to be continued.</li> | |
| 57 | |
| 58 <li> | |
| 59 NS_FRAME_NOT_COMPLETE - this is another normal reflow status and indicates | |
| 60 that the frame is not complete and will need a continuation frame created | |
| 61 for it (if it doesn't already have one).</li> | |
| 62 | |
| 63 <li> | |
| 64 NS_INLINE_BREAK - some kind of break has been requested. Breaks types include | |
| 65 simple line breaks (like the BR tag in html sometime does) and more complex | |
| 66 breaks like page breaks, float breaks, etc. Currently, we only support | |
| 67 line breaks, and float clearing breaks. Breaks can occur before the frame | |
| 68 (NS_INLINE_IS_BREAK_BEFORE) or after the frame (NS_INLINE_IS_BREAK_AFTER)</li> | |
| 69 </ul> | |
| 70 The handling of the reflow status is done by the container frame using | |
| 71 nsLineLayout. | |
| 72 <h3> | |
| 73 <u>Line Breaking</u></h3> | |
| 74 Another aspect of nsLineLayout is that it supports line breaking. At the | |
| 75 highest level, line breaking consists of identifying where it is appropriate | |
| 76 to break a line that doesn't fit in the available horizontal space. At | |
| 77 a lower level, some frames are breakable (e.g. text) and some frames are | |
| 78 not (e.g. images). | |
| 79 <p>In order to break text properly, some out-of-band information is needed | |
| 80 by the text frame code (nsTextFrame). In particular, because a "word" (a | |
| 81 non-breakable unit of text) may span several frames (for example: <b>"<B>H</B>ello | |
| 82 there"</b> is breakable after the <b>"o"</b> in "<b>ello</b>" but not after | |
| 83 the <b>"H"</b>), text-run information is used to allow the text frame to | |
| 84 find adjacent text and look at them to determine where the next breakable | |
| 85 point is. nsLineLayout supports this by keeping track of the text-runs | |
| 86 as well as both storing and interrogating "word" state. | |
| 87 <h3> | |
| 88 <u>White-space</u></h3> | |
| 89 To support the white-space property, the line layout logic keeps track | |
| 90 of the presence of white-space in the line as it told to reflow each inline | |
| 91 frame. This allows for the compression of leading whitespace and the compression | |
| 92 of adjacent whitespace that is in separate inline elements. | |
| 93 <p>As a post-processing step, the TrimTrailingWhiteSpace logic is used | |
| 94 to remove those pesky pices of white-space that end up being placed at | |
| 95 the end of a line, that shouldn't really be seen. | |
| 96 <p>To support pre-formatted text that contains tab characters, the line | |
| 97 layout class keeps track of the current column on behalf of the text frame | |
| 98 code. | |
| 99 <h3> | |
| 100 <u>Vertical Alignment</u></h3> | |
| 101 Vertical alignment is peformed as a two and a half pass process. The first | |
| 102 pass is done during nsInlineFrame reflow: the child frames of the nsInlineFrame | |
| 103 are vertically aligned as best as can be done at the time. There are certain | |
| 104 values for the vertical-align property that require the alignment be done | |
| 105 after the lines entire height is known; those frames are placed during | |
| 106 the last half pass. | |
| 107 <p>The second pass is done by the block frame when all of the frames for | |
| 108 a line are known. This is where the final height of the line | |
| 109 <br>(not the line-height property) is known and where the final half pass | |
| 110 can be done to place all of the top and bottom aligned elements. | |
| 111 <br> | |
| 112 <h3> | |
| 113 <u>Horizontal Alignment</u></h3> | |
| 114 After all frames on a line have been placed vertically, the block code | |
| 115 will use nsLineLayout to perform horizontal alignment within the extra | |
| 116 space. | |
| 117 </body> | |
| 118 </html> |
