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diff --git a/layout/doc/obsolete/line-layout.html b/layout/doc/obsolete/line-layout.html new file mode 100644 index 000000000..3a1bd17fa --- /dev/null +++ b/layout/doc/obsolete/line-layout.html @@ -0,0 +1,118 @@ +<!-- 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 4.0 transitional//en"> +<html> +<head> + <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> + <meta name="GENERATOR" content="Mozilla/4.61 [en] (X11; I; Linux 2.2.5-22 i686) [Netscape]"> +</head> +<body> + +<h1> +<u>Line Layout</u></h1> +Line layout is the process of placing inline frames horizontally (left +to right or right to left depending on the CSS direction property value). +An attempt is made to describe how it works. +<p>nsLineLayout is the class that provides support for line layout. The +container frames nsBlockFrame and nsInlineFrame use nsLineLayout to perform +line layout and span layout. Span layout is a subset of line layout used +for inline container classes - for example, the HTML "B" element). Because +of spans, nsLineLayout handles the nested nature of line layout. +<p>Line layout as a process contains the following steps: +<ol> +<li> +Initialize the nsLineLayout object (done in nsBlockFrame). This prepares +the line layout engine for reflow by initializing its internal data structures.</li> + +<br> +<li> +Reflowing of inline frames. The block code uses nsLineLayout's <b>ReflowFrame</b> +method to reflow each inline frame in a line. This continues until the +line runs out of room or the block runs out of frames. The block may be +reflowing a span (an instance of nsInlineFrame) which will recursively +use nsLineLayout for reflow and placement of the frames in the span.</li> + +<p><br>Note that the container frames (nsBlockFrame/nsInlineFrame) call +nsLineLayout's ReflowFrame method instead of having the line layout code +process a list of children. This is done so that the container frames can +handle the issues of "pushing" and "pulling" of frames across continuations. +Because block and inline maintain different data structures for their child +lists, and because we don't want to mandate a common base class, the line +layout code doesn't control the "outer loop" of frame reflow. +<br> +<li> +Finish line layout by vertically aligning the frames, horizontally aligning +the frames and relatively positioning the frames on the line.</li> +</ol> +nsLineLayout is also used by nsBlockFrame to construct text-run information; +this process is independent of normal line layout is pretty much a hack. +<p>When frames are reflowed they return a reflow status. During line layout, +there are several additions to the basic reflow status used by most frames: +<ul> +<li> +NS_FRAME_COMPLETE - this is a normal reflow status and indicates that the +frame is complete and doesn't need to be continued.</li> + +<li> +NS_FRAME_NOT_COMPLETE - this is another normal reflow status and indicates +that the frame is not complete and will need a continuation frame created +for it (if it doesn't already have one).</li> + +<li> +NS_INLINE_BREAK - some kind of break has been requested. Breaks types include +simple line breaks (like the BR tag in html sometime does) and more complex +breaks like page breaks, float breaks, etc. Currently, we only support +line breaks, and float clearing breaks. Breaks can occur before the frame +(NS_INLINE_IS_BREAK_BEFORE) or after the frame (NS_INLINE_IS_BREAK_AFTER)</li> +</ul> +The handling of the reflow status is done by the container frame using +nsLineLayout. +<h3> +<u>Line Breaking</u></h3> +Another aspect of nsLineLayout is that it supports line breaking. At the +highest level, line breaking consists of identifying where it is appropriate +to break a line that doesn't fit in the available horizontal space. At +a lower level, some frames are breakable (e.g. text) and some frames are +not (e.g. images). +<p>In order to break text properly, some out-of-band information is needed +by the text frame code (nsTextFrame). In particular, because a "word" (a +non-breakable unit of text) may span several frames (for example: <b>"<B>H</B>ello +there"</b> is breakable after the <b>"o"</b> in "<b>ello</b>" but not after +the <b>"H"</b>), text-run information is used to allow the text frame to +find adjacent text and look at them to determine where the next breakable +point is. nsLineLayout supports this by keeping track of the text-runs +as well as both storing and interrogating "word" state. +<h3> +<u>White-space</u></h3> +To support the white-space property, the line layout logic keeps track +of the presence of white-space in the line as it told to reflow each inline +frame. This allows for the compression of leading whitespace and the compression +of adjacent whitespace that is in separate inline elements. +<p>As a post-processing step, the TrimTrailingWhiteSpace logic is used +to remove those pesky pieces of white-space that end up being placed at +the end of a line, that shouldn't really be seen. +<p>To support pre-formatted text that contains tab characters, the line +layout class keeps track of the current column on behalf of the text frame +code. +<h3> +<u>Vertical Alignment</u></h3> +Vertical alignment is peformed as a two and a half pass process. The first +pass is done during nsInlineFrame reflow: the child frames of the nsInlineFrame +are vertically aligned as best as can be done at the time. There are certain +values for the vertical-align property that require the alignment be done +after the lines entire height is known; those frames are placed during +the last half pass. +<p>The second pass is done by the block frame when all of the frames for +a line are known. This is where the final height of the line +<br>(not the line-height property) is known and where the final half pass +can be done to place all of the top and bottom aligned elements. +<br> +<h3> +<u>Horizontal Alignment</u></h3> +After all frames on a line have been placed vertically, the block code +will use nsLineLayout to perform horizontal alignment within the extra +space. +</body> +</html> |