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137 lines
6.7 KiB
HTML
137 lines
6.7 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/4.51 [en] (X11; U; 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=+3>Block Layout</font></td>
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</tr>
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</table>
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<p>This document attempts to describe how "block" layout works in the mozilla
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layout engine.
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<p><tt>nsBlockFrame</tt> implements layout behavior that conforms to the
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CSS "block" display property. The primary responsibility of the block code
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is to manage "line layout". Line layout is the process where inline elements
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are placed on a "line" hozironatally (left to right or right to left if
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the CSS direction property is set rtl). In addition to line layout, blocks
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are responsible for placement of left and right floating elements, and
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handling "clear" semantics of child blocks or BR elements.
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<p>To manage the child frames, nsBlockFrame uses a singly linked list of
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nsLineBox's. nsLineBox's contain enough state to find all of the children
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that belong to the block, plus support incremental reflow [Currently there
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is too much state here - we waste memory].
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<br>
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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=+2>Who's Who</font></td>
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</tr>
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</table>
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<p>Here is a list of the various classes involved in block layout:
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<p><b>nsBlockFrame</b>
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<p>The primary culprit.
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<p><b>nsBlockReflowState</b>
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<p>This helper class is used to augment the nsHTMLReflowState with other
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information needed by the block reflow logic during reflow. It is a temporary
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object that is designed to live on the processor stack.
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<p><b>nsBlockBandData</b>
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<p>Another helper class that wraps up management of a space manager (nsISpaceManager,
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nsSpaceManager) and nsBandData. It also assits in management of floating
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elements. While nsSpaceManager is policy free, nsBlockBandData provides
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specific HTML and CSS policy.
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<p><b>nsBlockReflowContext</b>
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<p>A helper class that encapsulates the logic needed to reflow a child
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block frame. This is used by the block code reflow a child block, as well
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as by the inline frame code to reflow an anonymous block, and by the block
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code to reflow floating elements (which are to be treated as blocks according
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to the CSS2 spec).
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<p><b>nsLineBox</b>
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<p>A data class used to store line information for the block frame code.
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Each line has a list of children (though the frames are linked together
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across lines to maintain the sibling list for nsIFrame::FirstChild) and
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some other state used to assit in incremental reflow.
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<p><b>nsLineLayout</b>
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<p>This class is the line layout engine. Its a passive entity in the sense
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that its the responsibility of the block/inline code to use the class (this
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is done so that the line layout engine doesn't have to manage child frame
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lists so that both nsBlockFrame and nsInlineFrame can use the class).
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<p><b>nsTextRun</b>
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<p>This is a data class used to store text run information. Tex turns are
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logically contiguous runs of text (they may or may not be structurally
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contiguous). The block frame stores a pointer to a list of nsTextRun's
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and during line layout provides the list to the nsLineLayout engine so
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that when text is reflowed the text layout code (nsTextFrame) can find
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related text to properly handle word breaking.
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<br><b></b>
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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=+2>Child Frame Management</font></td>
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</tr>
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</table>
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<p>When the blocks child list is modified (AppendFrames, InsertFrames,
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RemoveFrame) the block code updates its nsLineBox list. Since each nsLineBox
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is typed (some are marked "inline" and some are marked "block"), the update
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logic maintains the invaraint of "one block frame per block line". Most
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of the logic is straightforward, however there is some ugly code to manage
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"first-line" frames. First-line frames are used when CSS's ":first-line"
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style is selected for the block. When this is the case, the block code
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creates an "anonymous first line" frame to manage the first inline children
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(all of the inlines that preceed the first child block of the block).
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<p>When structural changes are made to the blocks children (append/insert/remove)
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the block code updates the line's and then marks the affected lines "dirty"
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(each nsLineBox has a dirty bit). After the structural changes are finished
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then the block will generate an incremental reflow command of type "ReflowDirty".
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<br>
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<table BORDER=0 CELLSPACING=0 COLS=1 WIDTH="100%" BGCOLOR="#33CCFF" NOSAVE >
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<tr>
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<td><font size=+2>Basic Reflow Logic</font></td>
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</tr>
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</table>
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<p>The block's reflow engine uses the nsLineBox's dirty bit to determine
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what to reflow. For each line in the block, its dirty bit is examined.
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If the bit is not set then the line is considered "clean" and the block
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determines
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<b>where</b> the line is to be placed. For dirty lines, the
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line is reflowed and when finished, lines affected by the reflow are marked
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dirty. The loop proceeds until either there is no more space for placing
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lines, or there are no more lines (including draining lines from continuations).
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<p>For each line reflowed, there are two types of lines: block lines and
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inline lines. Block lines use ReflowBlockFrame to setup and reflow the
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single block child on the line. Inline lines use nsReflowInlineFrames to
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setup and reflow one or more inline frames on the line.
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<p>nsBlockReflowContext is a helper class used to reflow child block frames.
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nsLineLayout is a helper class used to reflow inline frames. See the document
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on line layout for more detail on how it works.
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<br>
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<table BORDER=0 CELLSPACING=0 COLS=1 WIDTH="100%" BGCOLOR="#33CCFF" NOSAVE >
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<tr>
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<td><font size=+2>Floater Handling</font></td>
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</tr>
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</table>
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<p>When frames are created by the frame construction code, "placeholders"
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are made that remain in-the-flow while the floating element is moved out-of-flow.
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Block frames receive child floating frames and keep them on a floater list.
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Note that if an inline frame contains a floating element it to will recieve
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a placeholder frame, but the floating frame will go to the inline frames
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containing block.
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<p>During the reflow of an inline line, the line layout logic will become
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aware of reflowing a placeholder frame for a floating element. When it
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does, it will inform the containing block of the floater so that the block
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and reflow and then place the floating element. To assist in this process,
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the nsBlockReflowState (misnamed) and the nsBlockBandData classes are used
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by the block code to do the bulk of the work. These classes also assist
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heavily in handling of the CSS "clear" property and HTML BR elements.
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<br>
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<br>
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</body>
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</html>
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