mirror of
https://github.com/rn10950/RetroZilla.git
synced 2024-11-10 18:00:15 +01:00
206 lines
11 KiB
HTML
206 lines
11 KiB
HTML
<!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>Block Layout</u></h1>
|
|
This document attempts to describe how "block" layout works in the mozilla
|
|
layout engine.
|
|
<p><tt>nsBlockFrame</tt> implements layout behavior that conforms to the
|
|
CSS "display:block" and "display: list-item" layout. It has several responsibilities:
|
|
<ol>
|
|
<li>
|
|
Line layout. The block is responsible for flowing inline elements
|
|
into "lines" and applying all of the css behavior as one might expect,
|
|
including line-height, vertical-align, relative positioning, etc.</li>
|
|
|
|
<li>
|
|
Float management. The block is responsible for the reflow and placement
|
|
of floating elements.</li>
|
|
|
|
<li>
|
|
Child block management. Blocks can contain inline elements and block elements.
|
|
Hence, blocks are responsible for reflowing child blocks. The majority
|
|
of that logic has been split out into nsBlockReflowContext, but a fair
|
|
amount remains here.</li>
|
|
|
|
<li>
|
|
Supporting table reflow. The block has to carefully compute the "max-element-size"
|
|
information needed by tables. Hence, any time changes are made here one
|
|
should always run the table regression tests because the odds are you broke
|
|
one of them!</li>
|
|
</ol>
|
|
|
|
<h3>
|
|
<u>The Big Picture for Block Reflow</u></h3>
|
|
The block frame uses a list of nsLineBox's to keep track of each "line"
|
|
of frames it manages. There are two types of lines:
|
|
<blockquote>"inline" lines which contain only inline elements
|
|
<br>"block" lines which contain exactly one block element</blockquote>
|
|
Each line has a "dirty" bit which indicates that it needs reflow. Reflow
|
|
consists of identifying which lines need to be marked dirty and then reflowing
|
|
all lines. For lines which are "clean" the reflow logic will endeavor to
|
|
recover the state of reflow <i>as if the line had been reflowed</i>. This
|
|
saves time and allows for a faster incremental reflow. For lines which
|
|
are dirty, the line is reflowed appropriately.
|
|
<p>The only special thing about incremental reflow command handling is
|
|
that it marks lines dirty before proceeding, and keeps track of the child
|
|
frame that is the next frame on the reflow command path.
|
|
<p>Here is a list of the various classes involved in block layout:
|
|
<p><b>nsBlockFrame</b>
|
|
<blockquote>The primary culprit.</blockquote>
|
|
<b>nsBlockReflowState</b>
|
|
<blockquote>This helper class is used to augment the nsHTMLReflowState
|
|
with other information needed by the block reflow logic during reflow.
|
|
It is a temporary object that is designed to live on the processor stack
|
|
and contains "running" state used by the blocks reflow logic.</blockquote>
|
|
<b>nsBlockBandData</b>
|
|
<blockquote>Another helper class that wraps up management of a space manager
|
|
(nsISpaceManager, nsSpaceManager) and nsBandData. It also assists in management
|
|
of floating elements. While nsSpaceManager is policy free, nsBlockBandData
|
|
provides specific HTML and CSS policy.</blockquote>
|
|
<b>nsBlockReflowContext</b>
|
|
<blockquote>A helper class that encapsulates the logic needed to reflow
|
|
a child block frame. This is used by the block code reflow a child block
|
|
and to reflow floating elements (which are to be treated as blocks according
|
|
to the CSS2 spec).</blockquote>
|
|
<b>nsLineBox</b>
|
|
<blockquote>A data class used to store line information for the block frame
|
|
code. Each line has a list of children (though the frames are linked together
|
|
across lines to maintain the sibling list for nsIFrame::FirstChild) and
|
|
some other state used to assist in incremental reflow.</blockquote>
|
|
<b>nsLineLayout</b>
|
|
<blockquote>This class is the line layout engine. Its a passive entity
|
|
in the sense that its the responsibility of the block/inline code to use
|
|
the class (this is done so that the line layout engine doesn't have to
|
|
manage child frame lists so that both nsBlockFrame and nsInlineFrame can
|
|
use the class).</blockquote>
|
|
<b>nsTextRun</b>
|
|
<blockquote>This is a data class used to store text run information. Text
|
|
runs are <i>logically</i> contiguous runs of text (they may or may not
|
|
be structurally contiguous). The block frame stores a pointer to a list
|
|
of nsTextRun's and during line layout provides the list to the nsLineLayout
|
|
engine so that when text is reflowed the text layout code (nsTextFrame)
|
|
can find related text to properly handle word breaking.</blockquote>
|
|
|
|
<h3>
|
|
<u>Frame construction methods</u></h3>
|
|
When the blocks child list is modified (AppendFrames, InsertFrames, RemoveFrame)
|
|
the block code updates its nsLineBox list. Since each nsLineBox is typed
|
|
(some are marked "inline" and some are marked "block"), the update logic
|
|
maintains the invariant of "one block frame per block line".
|
|
<p>When structural changes are made to the blocks children (append/insert/remove)
|
|
the block code updates the line's and then marks the affected lines "dirty"
|
|
(each nsLineBox has a dirty bit). After the structural changes are finished
|
|
then the block will generate an incremental reflow command of type "ReflowDirty".
|
|
<h3>
|
|
<u>Line Layout</u></h3>
|
|
Line layout consists of the placement of inline elements on a line until
|
|
there is no more room on the line. At that point the line is "broken" and
|
|
continued on the next line. This process continues until all inline elements
|
|
have been exhausted. The block code maintains a list of "nsLineBox"'s to
|
|
facilitate this. These are used instead of frames because they use less
|
|
memory and because it allows the block to directly control their behavior.
|
|
<p>The helper class nsLineLayout provides the majority of the line layout
|
|
behavior needed by the block.
|
|
<p>The block does keep "text-run" information around for the nsLineLayout
|
|
logic to use during reflow. Text runs keep track of logically adjacent
|
|
pieces of text within a block. This information is essential for properly
|
|
computing line and word breaking. Why? Well, because in html you can write
|
|
something like this:
|
|
<p> <p>I <b>W</b>as thinking one day</p>
|
|
<p>Notice that the word "Was" is composed of two pieces of text, and that
|
|
they do <i>not</i> have the same parent (content or frame). To properly
|
|
reflow this and not break the word prematurely after the "W", the text-run
|
|
information is used by the text frame code to "look ahead" and prevent
|
|
premature breaking.
|
|
<p>Lines also keep track of the type of "break" that occurred on the line.
|
|
This is used, for example, to support html's "<br clear=left>" behavior.
|
|
<h3>
|
|
<u>Float Management</u></h3>
|
|
Since child block elements are containing blocks for floats, the only
|
|
place where a block frame will see a float is as part of an inline line.
|
|
Consequently, the nsLineBox will only keep track of floats on inline
|
|
lines (saving storage for block lines).
|
|
<p>The nsLineLayout class and the block frame cooperate in the management
|
|
of floats. Since the frame construction code leaves a "placeholder" frame
|
|
in-flow where the float was found, when nsLineLayout reflows a placeholder
|
|
frame it knows to inform the block about it. That triggers the blocks "AddFloat"
|
|
logic which then determines where the float should be placed (on the
|
|
current line or below the current line).
|
|
<p>The block frame uses the space manager to manage the effects of floats,
|
|
namely the consumption of available space. For example, for a left aligned
|
|
floating element, the inline elements must be placed to the right of the
|
|
float. To simplify this process, the spacemanager is used to keep track
|
|
of available and busy space. Floats when placed mark space as busy and
|
|
the spacemanager will them compute the available space. Most of this logic
|
|
is handled by the nsBlockReflowState which uses a helper class, nsBlockBandData,
|
|
in concert with the space manager, to do the available space computations.
|
|
<h3>
|
|
<u>Child Block Placement</u></h3>
|
|
Child block reflow is done primarily by using the nsBlockReflowContext
|
|
code. However, a key detail worth mentioning here is how margins are handled.
|
|
When the nsHTMLReflowState was created, we placed into it the logic for
|
|
computing margins, border and padding (among other things). Unfortunately,
|
|
given the css rules for sibling and generational margin collapsing, the
|
|
nsHTMLReflowState is unable to properly compute top and bottom margins.
|
|
Hence, the block frame and the nsBlockReflowContext code perform that function.
|
|
At the time that the nsBlockReflowContext was designed and implemented
|
|
we thought that it could compute the top-margin itself and then proceed
|
|
to place the child block element. However, that turned out to be wrong
|
|
(oh well) because the correct available space isn't known until <i>after</i>
|
|
the top margin is computed. Hence, there is some unfortunate duplication
|
|
of reflow state calculations present in the block frame code.
|
|
<h3>
|
|
<u>Bullets</u></h3>
|
|
Another type of block frame is the "display: list-item". List-items use
|
|
nsBulletFrame's to manage bullet reflow. However, the block is responsible
|
|
for bullet placement. In most situations, the nsLineLayout class is used
|
|
to do the placement. However, if the first effective child of the block
|
|
is another block, then the block has to do the placement itself.
|
|
<h3>
|
|
<u>Blank lines</u></h3>
|
|
Because our content model contains as much of the original source documents
|
|
content as possible, we end up with a lot of white space that ends up being
|
|
compressed into nothingness. This white space ends up impacting this logic
|
|
in several ways. For example:
|
|
<p> <div>
|
|
<br> <p>abc</p>
|
|
<br> <p>def</p>
|
|
<br> </div>
|
|
<p>In the content model for the above html, there is white space between
|
|
the various block elements (some after the <div>, some after the first
|
|
</p>, again after the second </p>).
|
|
<p>For css margin collapsing to work properly, each of those instances
|
|
of white space has to behave as if they didn't exist. Consequently, there
|
|
is special logic in the inline line reflow code, and in the nsBlockReflowContext
|
|
code and in the GetTopBlockChild method, to basically ignore such lines.
|
|
<h3>
|
|
<u>First-letter style</u></h3>
|
|
The block contributes, in a small way, to first-letter style reflow. The
|
|
frame construction code is responsible for creating the list of child frames
|
|
for all frames, including the block. It manages the creation of letter-frames,
|
|
where appropriate, so that all the block has to do is reflow them almost
|
|
normally like other inline frames.
|
|
<p>There are two things different that the block does:
|
|
<p>It is responsible for calling nsLineLayout::SetFirstLetterStyleOK
|
|
<br>It is responsible for continuing to place frames on a line, even after
|
|
a frame has said "it can't fit". Normally during inline reflow, if a frame
|
|
comes back and says it can't fit, the block will end the line, push all
|
|
remaining frames to the next line and pick up the reflow from there after
|
|
making sure the frame that didn't fit is continued. For letter-frames,
|
|
this would result in the first-letter being on one line with the remaining
|
|
text on subsequent lines. Hence, the block code handles this special case.
|
|
<br>
|
|
<h3>
|
|
<u>First-line style</u></h3>
|
|
First-line is handled entirely by the frame construction code.
|
|
<br>
|
|
<br>
|
|
</body>
|
|
</html>
|