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115 lines
5.6 KiB
HTML
115 lines
5.6 KiB
HTML
<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="GENERATOR" content="Mozilla/4.61 [en] (X11; I; Linux 2.2.5-22 i686) [Netscape]">
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</head>
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<body>
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<h1>
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<u>Line Layout</u></h1>
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Line layout is the process of placing inline frames horizontally (left
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to right or right to left depending on the CSS direction property value).
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An attempt is made to describe how it works.
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<p>nsLineLayout is the class that provides support for line layout. The
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container frames nsBlockFrame and nsInlineFrame use nsLineLayout to perform
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line layout and span layout. Span layout is a subset of line layout used
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for inline container classes - for example, the HTML "B" element). Because
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of spans, nsLineLayout handles the nested nature of line layout.
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<p>Line layout as a process contains the following steps:
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<ol>
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<li>
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Initialize the nsLineLayout object (done in nsBlockFrame). This prepares
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the line layout engine for reflow by initializing its internal data structures.</li>
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<br>
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<li>
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Reflowing of inline frames. The block code uses nsLineLayout's <b>ReflowFrame</b>
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method to reflow each inline frame in a line. This continues until the
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line runs out of room or the block runs out of frames. The block may be
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reflowing a span (an instance of nsInlineFrame) which will recursively
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use nsLineLayout for reflow and placement of the frames in the span.</li>
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<p><br>Note that the container frames (nsBlockFrame/nsInlineFrame) call
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nsLineLayout's ReflowFrame method instead of having the line layout code
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process a list of children. This is done so that the container frames can
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handle the issues of "pushing" and "pulling" of frames across continuations.
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Because block and inline maintain different data structures for their child
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lists, and because we don't want to mandate a common base class, the line
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layout code doesn't control the "outer loop" of frame reflow.
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<br>
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<li>
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Finish line layout by vertically aligning the frames, horizontally aligning
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the frames and relatively positioning the frames on the line.</li>
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</ol>
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nsLineLayout is also used by nsBlockFrame to construct text-run information;
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this process is independent of normal line layout is pretty much a hack.
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<p>When frames are reflowed they return a reflow status. During line layout,
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there are several additions to the basic reflow status used by most frames:
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<ul>
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<li>
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NS_FRAME_COMPLETE - this is a normal reflow status and indicates that the
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frame is complete and doesn't need to be continued.</li>
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<li>
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NS_FRAME_NOT_COMPLETE - this is another normal reflow status and indicates
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that the frame is not complete and will need a continuation frame created
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for it (if it doesn't already have one).</li>
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<li>
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NS_INLINE_BREAK - some kind of break has been requested. Breaks types include
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simple line breaks (like the BR tag in html sometime does) and more complex
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breaks like page breaks, float breaks, etc. Currently, we only support
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line breaks, and float clearing breaks. Breaks can occur before the frame
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(NS_INLINE_IS_BREAK_BEFORE) or after the frame (NS_INLINE_IS_BREAK_AFTER)</li>
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</ul>
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The handling of the reflow status is done by the container frame using
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nsLineLayout.
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<h3>
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<u>Line Breaking</u></h3>
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Another aspect of nsLineLayout is that it supports line breaking. At the
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highest level, line breaking consists of identifying where it is appropriate
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to break a line that doesn't fit in the available horizontal space. At
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a lower level, some frames are breakable (e.g. text) and some frames are
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not (e.g. images).
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<p>In order to break text properly, some out-of-band information is needed
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by the text frame code (nsTextFrame). In particular, because a "word" (a
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non-breakable unit of text) may span several frames (for example: <b>"<B>H</B>ello
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there"</b> is breakable after the <b>"o"</b> in "<b>ello</b>" but not after
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the <b>"H"</b>), text-run information is used to allow the text frame to
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find adjacent text and look at them to determine where the next breakable
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point is. nsLineLayout supports this by keeping track of the text-runs
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as well as both storing and interrogating "word" state.
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<h3>
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<u>White-space</u></h3>
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To support the white-space property, the line layout logic keeps track
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of the presence of white-space in the line as it told to reflow each inline
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frame. This allows for the compression of leading whitespace and the compression
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of adjacent whitespace that is in separate inline elements.
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<p>As a post-processing step, the TrimTrailingWhiteSpace logic is used
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to remove those pesky pices of white-space that end up being placed at
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the end of a line, that shouldn't really be seen.
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<p>To support pre-formatted text that contains tab characters, the line
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layout class keeps track of the current column on behalf of the text frame
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code.
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<h3>
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<u>Vertical Alignment</u></h3>
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Vertical alignment is peformed as a two and a half pass process. The first
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pass is done during nsInlineFrame reflow: the child frames of the nsInlineFrame
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are vertically aligned as best as can be done at the time. There are certain
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values for the vertical-align property that require the alignment be done
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after the lines entire height is known; those frames are placed during
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the last half pass.
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<p>The second pass is done by the block frame when all of the frames for
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a line are known. This is where the final height of the line
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<br>(not the line-height property) is known and where the final half pass
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can be done to place all of the top and bottom aligned elements.
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<br>
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<h3>
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<u>Horizontal Alignment</u></h3>
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After all frames on a line have been placed vertically, the block code
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will use nsLineLayout to perform horizontal alignment within the extra
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space.
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</body>
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</html>
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