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