Line Layout

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.

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.

Line layout as a process contains the following steps:

Initialize the nsLineLayout object (done in nsBlockFrame). This prepares the line layout engine for reflow by initializing its internal data structures.

Reflowing of inline frames. The block code uses nsLineLayout's ReflowFrame 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.

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.

Finish line layout by vertically aligning the frames, horizontally aligning the frames and relatively positioning the frames on the line.

nsLineLayout is also used by nsBlockFrame to construct text-run information; this process is independent of normal line layout is pretty much a hack.

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:

NS_FRAME_COMPLETE - this is a normal reflow status and indicates that the frame is complete and doesn't need to be continued.
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).
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)

The handling of the reflow status is done by the container frame using nsLineLayout.

Line Breaking

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).

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>H</B>ello there" is breakable after the "o" in "ello" but not after the "H"), 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.

White-space

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.

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.

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.

Vertical Alignment

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.

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
(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.

Horizontal Alignment

After all frames on a line have been placed vertically, the block code will use nsLineLayout to perform horizontal alignment within the extra space.