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104 lines
4.8 KiB
HTML
104 lines
4.8 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/4.5 [en] (X11; I; Linux 2.0.36 i686) [Netscape]">
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</head>
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<body>
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<table CELLSPACING=0 CELLPADDING=0 COLS=1 WIDTH="100%" BGCOLOR="#33CCFF" NOSAVE >
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<tr>
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<td><font size=+4>Line Layout</font></td>
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</tr>
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</table>
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<p>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 <b>support</b> for line layout.
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The container frames nsBlockFrame and nsInlineFrame use nsLineLayout to
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perform line layout and span layout. Span layout is a subset of line layout
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used for inline container classes - for example, the HTML "B" element).
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Because 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.<br>
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<BR></li>
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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.<br>
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<br>
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Note that the container frames (nsBlockFrame/nsInlineFrame) call nsLineLayout's
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ReflowFrame method instead of having the line layout code process a list
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of children. This is done so that the container frames can handle the issues
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of "pushing" and "pulling" of frames across continuations. Because
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block and inline maintain different data structures for their child lists,
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and because we don't want to mandate a common base class, the line layout
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code doesn't control the "outer loop" of frame reflow.<br>
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<BR></li>
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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, floater breaks, etc. Currently, we only support
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line breaks, and floater 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 <b>using</b>
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nsLineLayout.
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<br>
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<table CELLSPACING=0 CELLPADDING=0 COLS=1 WIDTH="100%" BGCOLOR="#33CCFF" NOSAVE >
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<tr>
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<td><font size=+3>Line Breaking</font></td>
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</tr>
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</table>
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<p>Another aspect of nsLineLayout is that it supports line breaking. At
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the 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 ello 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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<p>In addition, nsLineLayout assists in the compression/expansion of whitespace
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for the CSS white-space property. Compression occurs when neighboring
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whitespace either inside a single piece of text is compressed or when spaces
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between abutting elements is collapsed. Expansion occurs in preformatted
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text when tabs are found. nsLineLayout keeps track of a "column" which
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is used to determine where the next tab "moves" to.
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<br>
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<br>
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<br>
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</body>
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</html>
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