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1.7 +
1.8 +<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
1.9 +<html>
1.10 +<head>
1.11 + <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
1.12 + <meta name="GENERATOR" content="Mozilla/4.61 [en] (X11; I; Linux 2.2.5-22 i686) [Netscape]">
1.13 +</head>
1.14 +<body>
1.15 +
1.16 +<h1>
1.17 +<u>Line Layout</u></h1>
1.18 +Line layout is the process of placing inline frames horizontally (left
1.19 +to right or right to left depending on the CSS direction property value).
1.20 +An attempt is made to describe how it works.
1.21 +<p>nsLineLayout is the class that provides support for line layout. The
1.22 +container frames nsBlockFrame and nsInlineFrame use nsLineLayout to perform
1.23 +line layout and span layout. Span layout is a subset of line layout used
1.24 +for inline container classes - for example, the HTML "B" element). Because
1.25 +of spans, nsLineLayout handles the nested nature of line layout.
1.26 +<p>Line layout as a process contains the following steps:
1.27 +<ol>
1.28 +<li>
1.29 +Initialize the nsLineLayout object (done in nsBlockFrame). This prepares
1.30 +the line layout engine for reflow by initializing its internal data structures.</li>
1.31 +
1.32 +<br>
1.33 +<li>
1.34 +Reflowing of inline frames. The block code uses nsLineLayout's <b>ReflowFrame</b>
1.35 +method to reflow each inline frame in a line. This continues until the
1.36 +line runs out of room or the block runs out of frames. The block may be
1.37 +reflowing a span (an instance of nsInlineFrame) which will recursively
1.38 +use nsLineLayout for reflow and placement of the frames in the span.</li>
1.39 +
1.40 +<p><br>Note that the container frames (nsBlockFrame/nsInlineFrame) call
1.41 +nsLineLayout's ReflowFrame method instead of having the line layout code
1.42 +process a list of children. This is done so that the container frames can
1.43 +handle the issues of "pushing" and "pulling" of frames across continuations.
1.44 +Because block and inline maintain different data structures for their child
1.45 +lists, and because we don't want to mandate a common base class, the line
1.46 +layout code doesn't control the "outer loop" of frame reflow.
1.47 +<br>
1.48 +<li>
1.49 +Finish line layout by vertically aligning the frames, horizontally aligning
1.50 +the frames and relatively positioning the frames on the line.</li>
1.51 +</ol>
1.52 +nsLineLayout is also used by nsBlockFrame to construct text-run information;
1.53 +this process is independent of normal line layout is pretty much a hack.
1.54 +<p>When frames are reflowed they return a reflow status. During line layout,
1.55 +there are several additions to the basic reflow status used by most frames:
1.56 +<ul>
1.57 +<li>
1.58 +NS_FRAME_COMPLETE - this is a normal reflow status and indicates that the
1.59 +frame is complete and doesn't need to be continued.</li>
1.60 +
1.61 +<li>
1.62 +NS_FRAME_NOT_COMPLETE - this is another normal reflow status and indicates
1.63 +that the frame is not complete and will need a continuation frame created
1.64 +for it (if it doesn't already have one).</li>
1.65 +
1.66 +<li>
1.67 +NS_INLINE_BREAK - some kind of break has been requested. Breaks types include
1.68 +simple line breaks (like the BR tag in html sometime does) and more complex
1.69 +breaks like page breaks, float breaks, etc. Currently, we only support
1.70 +line breaks, and float clearing breaks. Breaks can occur before the frame
1.71 +(NS_INLINE_IS_BREAK_BEFORE) or after the frame (NS_INLINE_IS_BREAK_AFTER)</li>
1.72 +</ul>
1.73 +The handling of the reflow status is done by the container frame using
1.74 +nsLineLayout.
1.75 +<h3>
1.76 +<u>Line Breaking</u></h3>
1.77 +Another aspect of nsLineLayout is that it supports line breaking. At the
1.78 +highest level, line breaking consists of identifying where it is appropriate
1.79 +to break a line that doesn't fit in the available horizontal space. At
1.80 +a lower level, some frames are breakable (e.g. text) and some frames are
1.81 +not (e.g. images).
1.82 +<p>In order to break text properly, some out-of-band information is needed
1.83 +by the text frame code (nsTextFrame). In particular, because a "word" (a
1.84 +non-breakable unit of text) may span several frames (for example: <b>"<B>H</B>ello
1.85 +there"</b> is breakable after the <b>"o"</b> in "<b>ello</b>" but not after
1.86 +the <b>"H"</b>), text-run information is used to allow the text frame to
1.87 +find adjacent text and look at them to determine where the next breakable
1.88 +point is. nsLineLayout supports this by keeping track of the text-runs
1.89 +as well as both storing and interrogating "word" state.
1.90 +<h3>
1.91 +<u>White-space</u></h3>
1.92 +To support the white-space property, the line layout logic keeps track
1.93 +of the presence of white-space in the line as it told to reflow each inline
1.94 +frame. This allows for the compression of leading whitespace and the compression
1.95 +of adjacent whitespace that is in separate inline elements.
1.96 +<p>As a post-processing step, the TrimTrailingWhiteSpace logic is used
1.97 +to remove those pesky pices of white-space that end up being placed at
1.98 +the end of a line, that shouldn't really be seen.
1.99 +<p>To support pre-formatted text that contains tab characters, the line
1.100 +layout class keeps track of the current column on behalf of the text frame
1.101 +code.
1.102 +<h3>
1.103 +<u>Vertical Alignment</u></h3>
1.104 +Vertical alignment is peformed as a two and a half pass process. The first
1.105 +pass is done during nsInlineFrame reflow: the child frames of the nsInlineFrame
1.106 +are vertically aligned as best as can be done at the time. There are certain
1.107 +values for the vertical-align property that require the alignment be done
1.108 +after the lines entire height is known; those frames are placed during
1.109 +the last half pass.
1.110 +<p>The second pass is done by the block frame when all of the frames for
1.111 +a line are known. This is where the final height of the line
1.112 +<br>(not the line-height property) is known and where the final half pass
1.113 +can be done to place all of the top and bottom aligned elements.
1.114 +<br>
1.115 +<h3>
1.116 +<u>Horizontal Alignment</u></h3>
1.117 +After all frames on a line have been placed vertically, the block code
1.118 +will use nsLineLayout to perform horizontal alignment within the extra
1.119 +space.
1.120 +</body>
1.121 +</html>
